blob: cf77cf70e32326c3d3058a257d2191a6269f1f2e [file] [log] [blame]
///===- SimpleLoopUnswitch.cpp - Hoist loop-invariant control flow ---------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
#include "llvm/Transforms/Scalar/SimpleLoopUnswitch.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/Sequence.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/Twine.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/CFG.h"
#include "llvm/Analysis/CodeMetrics.h"
#include "llvm/Analysis/GuardUtils.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/LoopAnalysisManager.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopIterator.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/MemorySSA.h"
#include "llvm/Analysis/MemorySSAUpdater.h"
#include "llvm/Analysis/MustExecute.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/IR/Use.h"
#include "llvm/IR/Value.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/GenericDomTree.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar/SimpleLoopUnswitch.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Cloning.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
#include "llvm/Transforms/Utils/ValueMapper.h"
#include <algorithm>
#include <cassert>
#include <iterator>
#include <numeric>
#include <utility>
#define DEBUG_TYPE "simple-loop-unswitch"
using namespace llvm;
using namespace llvm::PatternMatch;
STATISTIC(NumBranches, "Number of branches unswitched");
STATISTIC(NumSwitches, "Number of switches unswitched");
STATISTIC(NumGuards, "Number of guards turned into branches for unswitching");
STATISTIC(NumTrivial, "Number of unswitches that are trivial");
"Number of unswitch candidates that had their cost multiplier skipped");
static cl::opt<bool> EnableNonTrivialUnswitch(
"enable-nontrivial-unswitch", cl::init(false), cl::Hidden,
cl::desc("Forcibly enables non-trivial loop unswitching rather than "
"following the configuration passed into the pass."));
static cl::opt<int>
UnswitchThreshold("unswitch-threshold", cl::init(50), cl::Hidden,
cl::desc("The cost threshold for unswitching a loop."));
static cl::opt<bool> EnableUnswitchCostMultiplier(
"enable-unswitch-cost-multiplier", cl::init(true), cl::Hidden,
cl::desc("Enable unswitch cost multiplier that prohibits exponential "
"explosion in nontrivial unswitch."));
static cl::opt<int> UnswitchSiblingsToplevelDiv(
"unswitch-siblings-toplevel-div", cl::init(2), cl::Hidden,
cl::desc("Toplevel siblings divisor for cost multiplier."));
static cl::opt<int> UnswitchNumInitialUnscaledCandidates(
"unswitch-num-initial-unscaled-candidates", cl::init(8), cl::Hidden,
cl::desc("Number of unswitch candidates that are ignored when calculating "
"cost multiplier."));
static cl::opt<bool> UnswitchGuards(
"simple-loop-unswitch-guards", cl::init(true), cl::Hidden,
cl::desc("If enabled, simple loop unswitching will also consider "
"llvm.experimental.guard intrinsics as unswitch candidates."));
static cl::opt<bool> DropNonTrivialImplicitNullChecks(
cl::init(false), cl::Hidden,
cl::desc("If enabled, drop make.implicit metadata in unswitched implicit "
"null checks to save time analyzing if we can keep it."));
/// Collect all of the loop invariant input values transitively used by the
/// homogeneous instruction graph from a given root.
/// This essentially walks from a root recursively through loop variant operands
/// which have the exact same opcode and finds all inputs which are loop
/// invariant. For some operations these can be re-associated and unswitched out
/// of the loop entirely.
static TinyPtrVector<Value *>
collectHomogenousInstGraphLoopInvariants(Loop &L, Instruction &Root,
LoopInfo &LI) {
assert(!L.isLoopInvariant(&Root) &&
"Only need to walk the graph if root itself is not invariant.");
TinyPtrVector<Value *> Invariants;
bool IsRootAnd = match(&Root, m_LogicalAnd());
bool IsRootOr = match(&Root, m_LogicalOr());
// Build a worklist and recurse through operators collecting invariants.
SmallVector<Instruction *, 4> Worklist;
SmallPtrSet<Instruction *, 8> Visited;
do {
Instruction &I = *Worklist.pop_back_val();
for (Value *OpV : I.operand_values()) {
// Skip constants as unswitching isn't interesting for them.
if (isa<Constant>(OpV))
// Add it to our result if loop invariant.
if (L.isLoopInvariant(OpV)) {
// If not an instruction with the same opcode, nothing we can do.
Instruction *OpI = dyn_cast<Instruction>(OpV);
if (OpI && ((IsRootAnd && match(OpI, m_LogicalAnd())) ||
(IsRootOr && match(OpI, m_LogicalOr())))) {
// Visit this operand.
if (Visited.insert(OpI).second)
} while (!Worklist.empty());
return Invariants;
static void replaceLoopInvariantUses(Loop &L, Value *Invariant,
Constant &Replacement) {
assert(!isa<Constant>(Invariant) && "Why are we unswitching on a constant?");
// Replace uses of LIC in the loop with the given constant.
// We use make_early_inc_range as set invalidates the iterator.
for (Use &U : llvm::make_early_inc_range(Invariant->uses())) {
Instruction *UserI = dyn_cast<Instruction>(U.getUser());
// Replace this use within the loop body.
if (UserI && L.contains(UserI))
/// Check that all the LCSSA PHI nodes in the loop exit block have trivial
/// incoming values along this edge.
static bool areLoopExitPHIsLoopInvariant(Loop &L, BasicBlock &ExitingBB,
BasicBlock &ExitBB) {
for (Instruction &I : ExitBB) {
auto *PN = dyn_cast<PHINode>(&I);
if (!PN)
// No more PHIs to check.
return true;
// If the incoming value for this edge isn't loop invariant the unswitch
// won't be trivial.
if (!L.isLoopInvariant(PN->getIncomingValueForBlock(&ExitingBB)))
return false;
llvm_unreachable("Basic blocks should never be empty!");
/// Insert code to test a set of loop invariant values, and conditionally branch
/// on them.
static void buildPartialUnswitchConditionalBranch(BasicBlock &BB,
ArrayRef<Value *> Invariants,
bool Direction,
BasicBlock &UnswitchedSucc,
BasicBlock &NormalSucc) {
IRBuilder<> IRB(&BB);
Value *Cond = Direction ? IRB.CreateOr(Invariants) :
IRB.CreateCondBr(Cond, Direction ? &UnswitchedSucc : &NormalSucc,
Direction ? &NormalSucc : &UnswitchedSucc);
/// Rewrite the PHI nodes in an unswitched loop exit basic block.
/// Requires that the loop exit and unswitched basic block are the same, and
/// that the exiting block was a unique predecessor of that block. Rewrites the
/// PHI nodes in that block such that what were LCSSA PHI nodes become trivial
/// PHI nodes from the old preheader that now contains the unswitched
/// terminator.
static void rewritePHINodesForUnswitchedExitBlock(BasicBlock &UnswitchedBB,
BasicBlock &OldExitingBB,
BasicBlock &OldPH) {
for (PHINode &PN : UnswitchedBB.phis()) {
// When the loop exit is directly unswitched we just need to update the
// incoming basic block. We loop to handle weird cases with repeated
// incoming blocks, but expect to typically only have one operand here.
for (auto i : seq<int>(0, PN.getNumOperands())) {
assert(PN.getIncomingBlock(i) == &OldExitingBB &&
"Found incoming block different from unique predecessor!");
PN.setIncomingBlock(i, &OldPH);
/// Rewrite the PHI nodes in the loop exit basic block and the split off
/// unswitched block.
/// Because the exit block remains an exit from the loop, this rewrites the
/// LCSSA PHI nodes in it to remove the unswitched edge and introduces PHI
/// nodes into the unswitched basic block to select between the value in the
/// old preheader and the loop exit.
static void rewritePHINodesForExitAndUnswitchedBlocks(BasicBlock &ExitBB,
BasicBlock &UnswitchedBB,
BasicBlock &OldExitingBB,
BasicBlock &OldPH,
bool FullUnswitch) {
assert(&ExitBB != &UnswitchedBB &&
"Must have different loop exit and unswitched blocks!");
Instruction *InsertPt = &*UnswitchedBB.begin();
for (PHINode &PN : ExitBB.phis()) {
auto *NewPN = PHINode::Create(PN.getType(), /*NumReservedValues*/ 2,
PN.getName() + ".split", InsertPt);
// Walk backwards over the old PHI node's inputs to minimize the cost of
// removing each one. We have to do this weird loop manually so that we
// create the same number of new incoming edges in the new PHI as we expect
// each case-based edge to be included in the unswitched switch in some
// cases.
// FIXME: This is really, really gross. It would be much cleaner if LLVM
// allowed us to create a single entry for a predecessor block without
// having separate entries for each "edge" even though these edges are
// required to produce identical results.
for (int i = PN.getNumIncomingValues() - 1; i >= 0; --i) {
if (PN.getIncomingBlock(i) != &OldExitingBB)
Value *Incoming = PN.getIncomingValue(i);
if (FullUnswitch)
// No more edge from the old exiting block to the exit block.
NewPN->addIncoming(Incoming, &OldPH);
// Now replace the old PHI with the new one and wire the old one in as an
// input to the new one.
NewPN->addIncoming(&PN, &ExitBB);
/// Hoist the current loop up to the innermost loop containing a remaining exit.
/// Because we've removed an exit from the loop, we may have changed the set of
/// loops reachable and need to move the current loop up the loop nest or even
/// to an entirely separate nest.
static void hoistLoopToNewParent(Loop &L, BasicBlock &Preheader,
DominatorTree &DT, LoopInfo &LI,
MemorySSAUpdater *MSSAU, ScalarEvolution *SE) {
// If the loop is already at the top level, we can't hoist it anywhere.
Loop *OldParentL = L.getParentLoop();
if (!OldParentL)
SmallVector<BasicBlock *, 4> Exits;
Loop *NewParentL = nullptr;
for (auto *ExitBB : Exits)
if (Loop *ExitL = LI.getLoopFor(ExitBB))
if (!NewParentL || NewParentL->contains(ExitL))
NewParentL = ExitL;
if (NewParentL == OldParentL)
// The new parent loop (if different) should always contain the old one.
if (NewParentL)
assert(NewParentL->contains(OldParentL) &&
"Can only hoist this loop up the nest!");
// The preheader will need to move with the body of this loop. However,
// because it isn't in this loop we also need to update the primary loop map.
assert(OldParentL == LI.getLoopFor(&Preheader) &&
"Parent loop of this loop should contain this loop's preheader!");
LI.changeLoopFor(&Preheader, NewParentL);
// Remove this loop from its old parent.
// Add the loop either to the new parent or as a top-level loop.
if (NewParentL)
// Remove this loops blocks from the old parent and every other loop up the
// nest until reaching the new parent. Also update all of these
// no-longer-containing loops to reflect the nesting change.
for (Loop *OldContainingL = OldParentL; OldContainingL != NewParentL;
OldContainingL = OldContainingL->getParentLoop()) {
[&](const BasicBlock *BB) {
return BB == &Preheader || L.contains(BB);
for (BasicBlock *BB : L.blocks())
// Because we just hoisted a loop out of this one, we have essentially
// created new exit paths from it. That means we need to form LCSSA PHI
// nodes for values used in the no-longer-nested loop.
formLCSSA(*OldContainingL, DT, &LI, SE);
// We shouldn't need to form dedicated exits because the exit introduced
// here is the (just split by unswitching) preheader. However, after trivial
// unswitching it is possible to get new non-dedicated exits out of parent
// loop so let's conservatively form dedicated exit blocks and figure out
// if we can optimize later.
formDedicatedExitBlocks(OldContainingL, &DT, &LI, MSSAU,
/*PreserveLCSSA*/ true);
// Return the top-most loop containing ExitBB and having ExitBB as exiting block
// or the loop containing ExitBB, if there is no parent loop containing ExitBB
// as exiting block.
static Loop *getTopMostExitingLoop(BasicBlock *ExitBB, LoopInfo &LI) {
Loop *TopMost = LI.getLoopFor(ExitBB);
Loop *Current = TopMost;
while (Current) {
if (Current->isLoopExiting(ExitBB))
TopMost = Current;
Current = Current->getParentLoop();
return TopMost;
/// Unswitch a trivial branch if the condition is loop invariant.
/// This routine should only be called when loop code leading to the branch has
/// been validated as trivial (no side effects). This routine checks if the
/// condition is invariant and one of the successors is a loop exit. This
/// allows us to unswitch without duplicating the loop, making it trivial.
/// If this routine fails to unswitch the branch it returns false.
/// If the branch can be unswitched, this routine splits the preheader and
/// hoists the branch above that split. Preserves loop simplified form
/// (splitting the exit block as necessary). It simplifies the branch within
/// the loop to an unconditional branch but doesn't remove it entirely. Further
/// cleanup can be done with some simplify-cfg like pass.
/// If `SE` is not null, it will be updated based on the potential loop SCEVs
/// invalidated by this.
static bool unswitchTrivialBranch(Loop &L, BranchInst &BI, DominatorTree &DT,
LoopInfo &LI, ScalarEvolution *SE,
MemorySSAUpdater *MSSAU) {
assert(BI.isConditional() && "Can only unswitch a conditional branch!");
LLVM_DEBUG(dbgs() << " Trying to unswitch branch: " << BI << "\n");
// The loop invariant values that we want to unswitch.
TinyPtrVector<Value *> Invariants;
// When true, we're fully unswitching the branch rather than just unswitching
// some input conditions to the branch.
bool FullUnswitch = false;
if (L.isLoopInvariant(BI.getCondition())) {
FullUnswitch = true;
} else {
if (auto *CondInst = dyn_cast<Instruction>(BI.getCondition()))
Invariants = collectHomogenousInstGraphLoopInvariants(L, *CondInst, LI);
if (Invariants.empty())
// Couldn't find invariant inputs!
return false;
// Check that one of the branch's successors exits, and which one.
bool ExitDirection = true;
int LoopExitSuccIdx = 0;
auto *LoopExitBB = BI.getSuccessor(0);
if (L.contains(LoopExitBB)) {
ExitDirection = false;
LoopExitSuccIdx = 1;
LoopExitBB = BI.getSuccessor(1);
if (L.contains(LoopExitBB))
return false;
auto *ContinueBB = BI.getSuccessor(1 - LoopExitSuccIdx);
auto *ParentBB = BI.getParent();
if (!areLoopExitPHIsLoopInvariant(L, *ParentBB, *LoopExitBB))
return false;
// When unswitching only part of the branch's condition, we need the exit
// block to be reached directly from the partially unswitched input. This can
// be done when the exit block is along the true edge and the branch condition
// is a graph of `or` operations, or the exit block is along the false edge
// and the condition is a graph of `and` operations.
if (!FullUnswitch) {
if (ExitDirection) {
if (!match(BI.getCondition(), m_LogicalOr()))
return false;
} else {
if (!match(BI.getCondition(), m_LogicalAnd()))
return false;
dbgs() << " unswitching trivial invariant conditions for: " << BI
<< "\n";
for (Value *Invariant : Invariants) {
dbgs() << " " << *Invariant << " == true";
if (Invariant != Invariants.back())
dbgs() << " ||";
dbgs() << "\n";
// If we have scalar evolutions, we need to invalidate them including this
// loop, the loop containing the exit block and the topmost parent loop
// exiting via LoopExitBB.
if (SE) {
if (Loop *ExitL = getTopMostExitingLoop(LoopExitBB, LI))
// Forget the entire nest as this exits the entire nest.
if (MSSAU && VerifyMemorySSA)
// Split the preheader, so that we know that there is a safe place to insert
// the conditional branch. We will change the preheader to have a conditional
// branch on LoopCond.
BasicBlock *OldPH = L.getLoopPreheader();
BasicBlock *NewPH = SplitEdge(OldPH, L.getHeader(), &DT, &LI, MSSAU);
// Now that we have a place to insert the conditional branch, create a place
// to branch to: this is the exit block out of the loop that we are
// unswitching. We need to split this if there are other loop predecessors.
// Because the loop is in simplified form, *any* other predecessor is enough.
BasicBlock *UnswitchedBB;
if (FullUnswitch && LoopExitBB->getUniquePredecessor()) {
assert(LoopExitBB->getUniquePredecessor() == BI.getParent() &&
"A branch's parent isn't a predecessor!");
UnswitchedBB = LoopExitBB;
} else {
UnswitchedBB =
SplitBlock(LoopExitBB, &LoopExitBB->front(), &DT, &LI, MSSAU);
if (MSSAU && VerifyMemorySSA)
// Actually move the invariant uses into the unswitched position. If possible,
// we do this by moving the instructions, but when doing partial unswitching
// we do it by building a new merge of the values in the unswitched position.
if (FullUnswitch) {
// If fully unswitching, we can use the existing branch instruction.
// Splice it into the old PH to gate reaching the new preheader and re-point
// its successors.
OldPH->getInstList().splice(OldPH->end(), BI.getParent()->getInstList(),
if (MSSAU) {
// Temporarily clone the terminator, to make MSSA update cheaper by
// separating "insert edge" updates from "remove edge" ones.
} else {
// Create a new unconditional branch that will continue the loop as a new
// terminator.
BranchInst::Create(ContinueBB, ParentBB);
BI.setSuccessor(LoopExitSuccIdx, UnswitchedBB);
BI.setSuccessor(1 - LoopExitSuccIdx, NewPH);
} else {
// Only unswitching a subset of inputs to the condition, so we will need to
// build a new branch that merges the invariant inputs.
if (ExitDirection)
assert(match(BI.getCondition(), m_LogicalOr()) &&
"Must have an `or` of `i1`s or `select i1 X, true, Y`s for the "
assert(match(BI.getCondition(), m_LogicalAnd()) &&
"Must have an `and` of `i1`s or `select i1 X, Y, false`s for the"
" condition!");
buildPartialUnswitchConditionalBranch(*OldPH, Invariants, ExitDirection,
*UnswitchedBB, *NewPH);
// Update the dominator tree with the added edge.
DT.insertEdge(OldPH, UnswitchedBB);
// After the dominator tree was updated with the added edge, update MemorySSA
// if available.
if (MSSAU) {
SmallVector<CFGUpdate, 1> Updates;
Updates.push_back({cfg::UpdateKind::Insert, OldPH, UnswitchedBB});
MSSAU->applyInsertUpdates(Updates, DT);
// Finish updating dominator tree and memory ssa for full unswitch.
if (FullUnswitch) {
if (MSSAU) {
// Remove the cloned branch instruction.
// Create unconditional branch now.
BranchInst::Create(ContinueBB, ParentBB);
MSSAU->removeEdge(ParentBB, LoopExitBB);
DT.deleteEdge(ParentBB, LoopExitBB);
if (MSSAU && VerifyMemorySSA)
// Rewrite the relevant PHI nodes.
if (UnswitchedBB == LoopExitBB)
rewritePHINodesForUnswitchedExitBlock(*UnswitchedBB, *ParentBB, *OldPH);
rewritePHINodesForExitAndUnswitchedBlocks(*LoopExitBB, *UnswitchedBB,
*ParentBB, *OldPH, FullUnswitch);
// The constant we can replace all of our invariants with inside the loop
// body. If any of the invariants have a value other than this the loop won't
// be entered.
ConstantInt *Replacement = ExitDirection
? ConstantInt::getFalse(BI.getContext())
: ConstantInt::getTrue(BI.getContext());
// Since this is an i1 condition we can also trivially replace uses of it
// within the loop with a constant.
for (Value *Invariant : Invariants)
replaceLoopInvariantUses(L, Invariant, *Replacement);
// If this was full unswitching, we may have changed the nesting relationship
// for this loop so hoist it to its correct parent if needed.
if (FullUnswitch)
hoistLoopToNewParent(L, *NewPH, DT, LI, MSSAU, SE);
if (MSSAU && VerifyMemorySSA)
LLVM_DEBUG(dbgs() << " done: unswitching trivial branch...\n");
return true;
/// Unswitch a trivial switch if the condition is loop invariant.
/// This routine should only be called when loop code leading to the switch has
/// been validated as trivial (no side effects). This routine checks if the
/// condition is invariant and that at least one of the successors is a loop
/// exit. This allows us to unswitch without duplicating the loop, making it
/// trivial.
/// If this routine fails to unswitch the switch it returns false.
/// If the switch can be unswitched, this routine splits the preheader and
/// copies the switch above that split. If the default case is one of the
/// exiting cases, it copies the non-exiting cases and points them at the new
/// preheader. If the default case is not exiting, it copies the exiting cases
/// and points the default at the preheader. It preserves loop simplified form
/// (splitting the exit blocks as necessary). It simplifies the switch within
/// the loop by removing now-dead cases. If the default case is one of those
/// unswitched, it replaces its destination with a new basic block containing
/// only unreachable. Such basic blocks, while technically loop exits, are not
/// considered for unswitching so this is a stable transform and the same
/// switch will not be revisited. If after unswitching there is only a single
/// in-loop successor, the switch is further simplified to an unconditional
/// branch. Still more cleanup can be done with some simplify-cfg like pass.
/// If `SE` is not null, it will be updated based on the potential loop SCEVs
/// invalidated by this.
static bool unswitchTrivialSwitch(Loop &L, SwitchInst &SI, DominatorTree &DT,
LoopInfo &LI, ScalarEvolution *SE,
MemorySSAUpdater *MSSAU) {
LLVM_DEBUG(dbgs() << " Trying to unswitch switch: " << SI << "\n");
Value *LoopCond = SI.getCondition();
// If this isn't switching on an invariant condition, we can't unswitch it.
if (!L.isLoopInvariant(LoopCond))
return false;
auto *ParentBB = SI.getParent();
// The same check must be used both for the default and the exit cases. We
// should never leave edges from the switch instruction to a basic block that
// we are unswitching, hence the condition used to determine the default case
// needs to also be used to populate ExitCaseIndices, which is then used to
// remove cases from the switch.
auto IsTriviallyUnswitchableExitBlock = [&](BasicBlock &BBToCheck) {
// BBToCheck is not an exit block if it is inside loop L.
if (L.contains(&BBToCheck))
return false;
// BBToCheck is not trivial to unswitch if its phis aren't loop invariant.
if (!areLoopExitPHIsLoopInvariant(L, *ParentBB, BBToCheck))
return false;
// We do not unswitch a block that only has an unreachable statement, as
// it's possible this is a previously unswitched block. Only unswitch if
// either the terminator is not unreachable, or, if it is, it's not the only
// instruction in the block.
auto *TI = BBToCheck.getTerminator();
bool isUnreachable = isa<UnreachableInst>(TI);
return !isUnreachable ||
(isUnreachable && (BBToCheck.getFirstNonPHIOrDbg() != TI));
SmallVector<int, 4> ExitCaseIndices;
for (auto Case : SI.cases())
if (IsTriviallyUnswitchableExitBlock(*Case.getCaseSuccessor()))
BasicBlock *DefaultExitBB = nullptr;
SwitchInstProfUpdateWrapper::CaseWeightOpt DefaultCaseWeight =
SwitchInstProfUpdateWrapper::getSuccessorWeight(SI, 0);
if (IsTriviallyUnswitchableExitBlock(*SI.getDefaultDest())) {
DefaultExitBB = SI.getDefaultDest();
} else if (ExitCaseIndices.empty())
return false;
LLVM_DEBUG(dbgs() << " unswitching trivial switch...\n");
if (MSSAU && VerifyMemorySSA)
// We may need to invalidate SCEVs for the outermost loop reached by any of
// the exits.
Loop *OuterL = &L;
if (DefaultExitBB) {
// Clear out the default destination temporarily to allow accurate
// predecessor lists to be examined below.
// Check the loop containing this exit.
Loop *ExitL = LI.getLoopFor(DefaultExitBB);
if (!ExitL || ExitL->contains(OuterL))
OuterL = ExitL;
// Store the exit cases into a separate data structure and remove them from
// the switch.
SmallVector<std::tuple<ConstantInt *, BasicBlock *,
4> ExitCases;
SwitchInstProfUpdateWrapper SIW(SI);
// We walk the case indices backwards so that we remove the last case first
// and don't disrupt the earlier indices.
for (unsigned Index : reverse(ExitCaseIndices)) {
auto CaseI = SI.case_begin() + Index;
// Compute the outer loop from this exit.
Loop *ExitL = LI.getLoopFor(CaseI->getCaseSuccessor());
if (!ExitL || ExitL->contains(OuterL))
OuterL = ExitL;
// Save the value of this case.
auto W = SIW.getSuccessorWeight(CaseI->getSuccessorIndex());
ExitCases.emplace_back(CaseI->getCaseValue(), CaseI->getCaseSuccessor(), W);
// Delete the unswitched cases.
if (SE) {
if (OuterL)
// Check if after this all of the remaining cases point at the same
// successor.
BasicBlock *CommonSuccBB = nullptr;
if (SI.getNumCases() > 0 &&
all_of(drop_begin(SI.cases()), [&SI](const SwitchInst::CaseHandle &Case) {
return Case.getCaseSuccessor() == SI.case_begin()->getCaseSuccessor();
CommonSuccBB = SI.case_begin()->getCaseSuccessor();
if (!DefaultExitBB) {
// If we're not unswitching the default, we need it to match any cases to
// have a common successor or if we have no cases it is the common
// successor.
if (SI.getNumCases() == 0)
CommonSuccBB = SI.getDefaultDest();
else if (SI.getDefaultDest() != CommonSuccBB)
CommonSuccBB = nullptr;
// Split the preheader, so that we know that there is a safe place to insert
// the switch.
BasicBlock *OldPH = L.getLoopPreheader();
BasicBlock *NewPH = SplitEdge(OldPH, L.getHeader(), &DT, &LI, MSSAU);
// Now add the unswitched switch.
auto *NewSI = SwitchInst::Create(LoopCond, NewPH, ExitCases.size(), OldPH);
SwitchInstProfUpdateWrapper NewSIW(*NewSI);
// Rewrite the IR for the unswitched basic blocks. This requires two steps.
// First, we split any exit blocks with remaining in-loop predecessors. Then
// we update the PHIs in one of two ways depending on if there was a split.
// We walk in reverse so that we split in the same order as the cases
// appeared. This is purely for convenience of reading the resulting IR, but
// it doesn't cost anything really.
SmallPtrSet<BasicBlock *, 2> UnswitchedExitBBs;
SmallDenseMap<BasicBlock *, BasicBlock *, 2> SplitExitBBMap;
// Handle the default exit if necessary.
// FIXME: It'd be great if we could merge this with the loop below but LLVM's
// ranges aren't quite powerful enough yet.
if (DefaultExitBB) {
if (pred_empty(DefaultExitBB)) {
rewritePHINodesForUnswitchedExitBlock(*DefaultExitBB, *ParentBB, *OldPH);
} else {
auto *SplitBB =
SplitBlock(DefaultExitBB, &DefaultExitBB->front(), &DT, &LI, MSSAU);
rewritePHINodesForExitAndUnswitchedBlocks(*DefaultExitBB, *SplitBB,
*ParentBB, *OldPH,
/*FullUnswitch*/ true);
DefaultExitBB = SplitExitBBMap[DefaultExitBB] = SplitBB;
// Note that we must use a reference in the for loop so that we update the
// container.
for (auto &ExitCase : reverse(ExitCases)) {
// Grab a reference to the exit block in the pair so that we can update it.
BasicBlock *ExitBB = std::get<1>(ExitCase);
// If this case is the last edge into the exit block, we can simply reuse it
// as it will no longer be a loop exit. No mapping necessary.
if (pred_empty(ExitBB)) {
// Only rewrite once.
if (UnswitchedExitBBs.insert(ExitBB).second)
rewritePHINodesForUnswitchedExitBlock(*ExitBB, *ParentBB, *OldPH);
// Otherwise we need to split the exit block so that we retain an exit
// block from the loop and a target for the unswitched condition.
BasicBlock *&SplitExitBB = SplitExitBBMap[ExitBB];
if (!SplitExitBB) {
// If this is the first time we see this, do the split and remember it.
SplitExitBB = SplitBlock(ExitBB, &ExitBB->front(), &DT, &LI, MSSAU);
rewritePHINodesForExitAndUnswitchedBlocks(*ExitBB, *SplitExitBB,
*ParentBB, *OldPH,
/*FullUnswitch*/ true);
// Update the case pair to point to the split block.
std::get<1>(ExitCase) = SplitExitBB;
// Now add the unswitched cases. We do this in reverse order as we built them
// in reverse order.
for (auto &ExitCase : reverse(ExitCases)) {
ConstantInt *CaseVal = std::get<0>(ExitCase);
BasicBlock *UnswitchedBB = std::get<1>(ExitCase);
NewSIW.addCase(CaseVal, UnswitchedBB, std::get<2>(ExitCase));
// If the default was unswitched, re-point it and add explicit cases for
// entering the loop.
if (DefaultExitBB) {
NewSIW.setSuccessorWeight(0, DefaultCaseWeight);
// We removed all the exit cases, so we just copy the cases to the
// unswitched switch.
for (const auto &Case : SI.cases())
NewSIW.addCase(Case.getCaseValue(), NewPH,
} else if (DefaultCaseWeight) {
// We have to set branch weight of the default case.
uint64_t SW = *DefaultCaseWeight;
for (const auto &Case : SI.cases()) {
auto W = SIW.getSuccessorWeight(Case.getSuccessorIndex());
assert(W &&
"case weight must be defined as default case weight is defined");
SW += *W;
NewSIW.setSuccessorWeight(0, SW);
// If we ended up with a common successor for every path through the switch
// after unswitching, rewrite it to an unconditional branch to make it easy
// to recognize. Otherwise we potentially have to recognize the default case
// pointing at unreachable and other complexity.
if (CommonSuccBB) {
BasicBlock *BB = SI.getParent();
// We may have had multiple edges to this common successor block, so remove
// them as predecessors. We skip the first one, either the default or the
// actual first case.
bool SkippedFirst = DefaultExitBB == nullptr;
for (auto Case : SI.cases()) {
assert(Case.getCaseSuccessor() == CommonSuccBB &&
"Non-common successor!");
if (!SkippedFirst) {
SkippedFirst = true;
/*KeepOneInputPHIs*/ true);
// Now nuke the switch and replace it with a direct branch.
BranchInst::Create(CommonSuccBB, BB);
} else if (DefaultExitBB) {
assert(SI.getNumCases() > 0 &&
"If we had no cases we'd have a common successor!");
// Move the last case to the default successor. This is valid as if the
// default got unswitched it cannot be reached. This has the advantage of
// being simple and keeping the number of edges from this switch to
// successors the same, and avoiding any PHI update complexity.
auto LastCaseI = std::prev(SI.case_end());
0, SIW.getSuccessorWeight(LastCaseI->getSuccessorIndex()));
// Walk the unswitched exit blocks and the unswitched split blocks and update
// the dominator tree based on the CFG edits. While we are walking unordered
// containers here, the API for applyUpdates takes an unordered list of
// updates and requires them to not contain duplicates.
SmallVector<DominatorTree::UpdateType, 4> DTUpdates;
for (auto *UnswitchedExitBB : UnswitchedExitBBs) {
DTUpdates.push_back({DT.Delete, ParentBB, UnswitchedExitBB});
DTUpdates.push_back({DT.Insert, OldPH, UnswitchedExitBB});
for (auto SplitUnswitchedPair : SplitExitBBMap) {
DTUpdates.push_back({DT.Delete, ParentBB, SplitUnswitchedPair.first});
DTUpdates.push_back({DT.Insert, OldPH, SplitUnswitchedPair.second});
if (MSSAU) {
MSSAU->applyUpdates(DTUpdates, DT, /*UpdateDT=*/true);
if (VerifyMemorySSA)
} else {
// We may have changed the nesting relationship for this loop so hoist it to
// its correct parent if needed.
hoistLoopToNewParent(L, *NewPH, DT, LI, MSSAU, SE);
if (MSSAU && VerifyMemorySSA)
LLVM_DEBUG(dbgs() << " done: unswitching trivial switch...\n");
return true;
/// This routine scans the loop to find a branch or switch which occurs before
/// any side effects occur. These can potentially be unswitched without
/// duplicating the loop. If a branch or switch is successfully unswitched the
/// scanning continues to see if subsequent branches or switches have become
/// trivial. Once all trivial candidates have been unswitched, this routine
/// returns.
/// The return value indicates whether anything was unswitched (and therefore
/// changed).
/// If `SE` is not null, it will be updated based on the potential loop SCEVs
/// invalidated by this.
static bool unswitchAllTrivialConditions(Loop &L, DominatorTree &DT,
LoopInfo &LI, ScalarEvolution *SE,
MemorySSAUpdater *MSSAU) {
bool Changed = false;
// If loop header has only one reachable successor we should keep looking for
// trivial condition candidates in the successor as well. An alternative is
// to constant fold conditions and merge successors into loop header (then we
// only need to check header's terminator). The reason for not doing this in
// LoopUnswitch pass is that it could potentially break LoopPassManager's
// invariants. Folding dead branches could either eliminate the current loop
// or make other loops unreachable. LCSSA form might also not be preserved
// after deleting branches. The following code keeps traversing loop header's
// successors until it finds the trivial condition candidate (condition that
// is not a constant). Since unswitching generates branches with constant
// conditions, this scenario could be very common in practice.
BasicBlock *CurrentBB = L.getHeader();
SmallPtrSet<BasicBlock *, 8> Visited;
do {
// Check if there are any side-effecting instructions (e.g. stores, calls,
// volatile loads) in the part of the loop that the code *would* execute
// without unswitching.
if (MSSAU) // Possible early exit with MSSA
if (auto *Defs = MSSAU->getMemorySSA()->getBlockDefs(CurrentBB))
if (!isa<MemoryPhi>(*Defs->begin()) || (++Defs->begin() != Defs->end()))
return Changed;
if (llvm::any_of(*CurrentBB,
[](Instruction &I) { return I.mayHaveSideEffects(); }))
return Changed;
Instruction *CurrentTerm = CurrentBB->getTerminator();
if (auto *SI = dyn_cast<SwitchInst>(CurrentTerm)) {
// Don't bother trying to unswitch past a switch with a constant
// condition. This should be removed prior to running this pass by
// simplify-cfg.
if (isa<Constant>(SI->getCondition()))
return Changed;
if (!unswitchTrivialSwitch(L, *SI, DT, LI, SE, MSSAU))
// Couldn't unswitch this one so we're done.
return Changed;
// Mark that we managed to unswitch something.
Changed = true;
// If unswitching turned the terminator into an unconditional branch then
// we can continue. The unswitching logic specifically works to fold any
// cases it can into an unconditional branch to make it easier to
// recognize here.
auto *BI = dyn_cast<BranchInst>(CurrentBB->getTerminator());
if (!BI || BI->isConditional())
return Changed;
CurrentBB = BI->getSuccessor(0);
auto *BI = dyn_cast<BranchInst>(CurrentTerm);
if (!BI)
// We do not understand other terminator instructions.
return Changed;
// Don't bother trying to unswitch past an unconditional branch or a branch
// with a constant value. These should be removed by simplify-cfg prior to
// running this pass.
if (!BI->isConditional() || isa<Constant>(BI->getCondition()))
return Changed;
// Found a trivial condition candidate: non-foldable conditional branch. If
// we fail to unswitch this, we can't do anything else that is trivial.
if (!unswitchTrivialBranch(L, *BI, DT, LI, SE, MSSAU))
return Changed;
// Mark that we managed to unswitch something.
Changed = true;
// If we only unswitched some of the conditions feeding the branch, we won't
// have collapsed it to a single successor.
BI = cast<BranchInst>(CurrentBB->getTerminator());
if (BI->isConditional())
return Changed;
// Follow the newly unconditional branch into its successor.
CurrentBB = BI->getSuccessor(0);
// When continuing, if we exit the loop or reach a previous visited block,
// then we can not reach any trivial condition candidates (unfoldable
// branch instructions or switch instructions) and no unswitch can happen.
} while (L.contains(CurrentBB) && Visited.insert(CurrentBB).second);
return Changed;
/// Build the cloned blocks for an unswitched copy of the given loop.
/// The cloned blocks are inserted before the loop preheader (`LoopPH`) and
/// after the split block (`SplitBB`) that will be used to select between the
/// cloned and original loop.
/// This routine handles cloning all of the necessary loop blocks and exit
/// blocks including rewriting their instructions and the relevant PHI nodes.
/// Any loop blocks or exit blocks which are dominated by a different successor
/// than the one for this clone of the loop blocks can be trivially skipped. We
/// use the `DominatingSucc` map to determine whether a block satisfies that
/// property with a simple map lookup.
/// It also correctly creates the unconditional branch in the cloned
/// unswitched parent block to only point at the unswitched successor.
/// This does not handle most of the necessary updates to `LoopInfo`. Only exit
/// block splitting is correctly reflected in `LoopInfo`, essentially all of
/// the cloned blocks (and their loops) are left without full `LoopInfo`
/// updates. This also doesn't fully update `DominatorTree`. It adds the cloned
/// blocks to them but doesn't create the cloned `DominatorTree` structure and
/// instead the caller must recompute an accurate DT. It *does* correctly
/// update the `AssumptionCache` provided in `AC`.
static BasicBlock *buildClonedLoopBlocks(
Loop &L, BasicBlock *LoopPH, BasicBlock *SplitBB,
ArrayRef<BasicBlock *> ExitBlocks, BasicBlock *ParentBB,
BasicBlock *UnswitchedSuccBB, BasicBlock *ContinueSuccBB,
const SmallDenseMap<BasicBlock *, BasicBlock *, 16> &DominatingSucc,
ValueToValueMapTy &VMap,
SmallVectorImpl<DominatorTree::UpdateType> &DTUpdates, AssumptionCache &AC,
DominatorTree &DT, LoopInfo &LI, MemorySSAUpdater *MSSAU) {
SmallVector<BasicBlock *, 4> NewBlocks;
NewBlocks.reserve(L.getNumBlocks() + ExitBlocks.size());
// We will need to clone a bunch of blocks, wrap up the clone operation in
// a helper.
auto CloneBlock = [&](BasicBlock *OldBB) {
// Clone the basic block and insert it before the new preheader.
BasicBlock *NewBB = CloneBasicBlock(OldBB, VMap, ".us", OldBB->getParent());
// Record this block and the mapping.
VMap[OldBB] = NewBB;
return NewBB;
// We skip cloning blocks when they have a dominating succ that is not the
// succ we are cloning for.
auto SkipBlock = [&](BasicBlock *BB) {
auto It = DominatingSucc.find(BB);
return It != DominatingSucc.end() && It->second != UnswitchedSuccBB;
// First, clone the preheader.
auto *ClonedPH = CloneBlock(LoopPH);
// Then clone all the loop blocks, skipping the ones that aren't necessary.
for (auto *LoopBB : L.blocks())
if (!SkipBlock(LoopBB))
// Split all the loop exit edges so that when we clone the exit blocks, if
// any of the exit blocks are *also* a preheader for some other loop, we
// don't create multiple predecessors entering the loop header.
for (auto *ExitBB : ExitBlocks) {
if (SkipBlock(ExitBB))
// When we are going to clone an exit, we don't need to clone all the
// instructions in the exit block and we want to ensure we have an easy
// place to merge the CFG, so split the exit first. This is always safe to
// do because there cannot be any non-loop predecessors of a loop exit in
// loop simplified form.
auto *MergeBB = SplitBlock(ExitBB, &ExitBB->front(), &DT, &LI, MSSAU);
// Rearrange the names to make it easier to write test cases by having the
// exit block carry the suffix rather than the merge block carrying the
// suffix.
ExitBB->setName(Twine(MergeBB->getName()) + ".split");
// Now clone the original exit block.
auto *ClonedExitBB = CloneBlock(ExitBB);
assert(ClonedExitBB->getTerminator()->getNumSuccessors() == 1 &&
"Exit block should have been split to have one successor!");
assert(ClonedExitBB->getTerminator()->getSuccessor(0) == MergeBB &&
"Cloned exit block has the wrong successor!");
// Remap any cloned instructions and create a merge phi node for them.
for (auto ZippedInsts : llvm::zip_first(
llvm::make_range(ExitBB->begin(), std::prev(ExitBB->end())),
std::prev(ClonedExitBB->end())))) {
Instruction &I = std::get<0>(ZippedInsts);
Instruction &ClonedI = std::get<1>(ZippedInsts);
// The only instructions in the exit block should be PHI nodes and
// potentially a landing pad.
(isa<PHINode>(I) || isa<LandingPadInst>(I) || isa<CatchPadInst>(I)) &&
"Bad instruction in exit block!");
// We should have a value map between the instruction and its clone.
assert(VMap.lookup(&I) == &ClonedI && "Mismatch in the value map!");
auto *MergePN =
PHINode::Create(I.getType(), /*NumReservedValues*/ 2, ".us-phi",
MergePN->addIncoming(&I, ExitBB);
MergePN->addIncoming(&ClonedI, ClonedExitBB);
// Rewrite the instructions in the cloned blocks to refer to the instructions
// in the cloned blocks. We have to do this as a second pass so that we have
// everything available. Also, we have inserted new instructions which may
// include assume intrinsics, so we update the assumption cache while
// processing this.
for (auto *ClonedBB : NewBlocks)
for (Instruction &I : *ClonedBB) {
RemapInstruction(&I, VMap,
RF_NoModuleLevelChanges | RF_IgnoreMissingLocals);
if (auto *II = dyn_cast<IntrinsicInst>(&I))
if (II->getIntrinsicID() == Intrinsic::assume)
// Update any PHI nodes in the cloned successors of the skipped blocks to not
// have spurious incoming values.
for (auto *LoopBB : L.blocks())
if (SkipBlock(LoopBB))
for (auto *SuccBB : successors(LoopBB))
if (auto *ClonedSuccBB = cast_or_null<BasicBlock>(VMap.lookup(SuccBB)))
for (PHINode &PN : ClonedSuccBB->phis())
PN.removeIncomingValue(LoopBB, /*DeletePHIIfEmpty*/ false);
// Remove the cloned parent as a predecessor of any successor we ended up
// cloning other than the unswitched one.
auto *ClonedParentBB = cast<BasicBlock>(VMap.lookup(ParentBB));
for (auto *SuccBB : successors(ParentBB)) {
if (SuccBB == UnswitchedSuccBB)
auto *ClonedSuccBB = cast_or_null<BasicBlock>(VMap.lookup(SuccBB));
if (!ClonedSuccBB)
/*KeepOneInputPHIs*/ true);
// Replace the cloned branch with an unconditional branch to the cloned
// unswitched successor.
auto *ClonedSuccBB = cast<BasicBlock>(VMap.lookup(UnswitchedSuccBB));
Instruction *ClonedTerminator = ClonedParentBB->getTerminator();
// Trivial Simplification. If Terminator is a conditional branch and
// condition becomes dead - erase it.
Value *ClonedConditionToErase = nullptr;
if (auto *BI = dyn_cast<BranchInst>(ClonedTerminator))
ClonedConditionToErase = BI->getCondition();
else if (auto *SI = dyn_cast<SwitchInst>(ClonedTerminator))
ClonedConditionToErase = SI->getCondition();
BranchInst::Create(ClonedSuccBB, ClonedParentBB);
if (ClonedConditionToErase)
RecursivelyDeleteTriviallyDeadInstructions(ClonedConditionToErase, nullptr,
// If there are duplicate entries in the PHI nodes because of multiple edges
// to the unswitched successor, we need to nuke all but one as we replaced it
// with a direct branch.
for (PHINode &PN : ClonedSuccBB->phis()) {
bool Found = false;
// Loop over the incoming operands backwards so we can easily delete as we
// go without invalidating the index.
for (int i = PN.getNumOperands() - 1; i >= 0; --i) {
if (PN.getIncomingBlock(i) != ClonedParentBB)
if (!Found) {
Found = true;
PN.removeIncomingValue(i, /*DeletePHIIfEmpty*/ false);
// Record the domtree updates for the new blocks.
SmallPtrSet<BasicBlock *, 4> SuccSet;
for (auto *ClonedBB : NewBlocks) {
for (auto *SuccBB : successors(ClonedBB))
if (SuccSet.insert(SuccBB).second)
DTUpdates.push_back({DominatorTree::Insert, ClonedBB, SuccBB});
return ClonedPH;
/// Recursively clone the specified loop and all of its children.
/// The target parent loop for the clone should be provided, or can be null if
/// the clone is a top-level loop. While cloning, all the blocks are mapped
/// with the provided value map. The entire original loop must be present in
/// the value map. The cloned loop is returned.
static Loop *cloneLoopNest(Loop &OrigRootL, Loop *RootParentL,
const ValueToValueMapTy &VMap, LoopInfo &LI) {
auto AddClonedBlocksToLoop = [&](Loop &OrigL, Loop &ClonedL) {
assert(ClonedL.getBlocks().empty() && "Must start with an empty loop!");
for (auto *BB : OrigL.blocks()) {
auto *ClonedBB = cast<BasicBlock>(VMap.lookup(BB));
if (LI.getLoopFor(BB) == &OrigL)
LI.changeLoopFor(ClonedBB, &ClonedL);
// We specially handle the first loop because it may get cloned into
// a different parent and because we most commonly are cloning leaf loops.
Loop *ClonedRootL = LI.AllocateLoop();
if (RootParentL)
AddClonedBlocksToLoop(OrigRootL, *ClonedRootL);
if (OrigRootL.isInnermost())
return ClonedRootL;
// If we have a nest, we can quickly clone the entire loop nest using an
// iterative approach because it is a tree. We keep the cloned parent in the
// data structure to avoid repeatedly querying through a map to find it.
SmallVector<std::pair<Loop *, Loop *>, 16> LoopsToClone;
// Build up the loops to clone in reverse order as we'll clone them from the
// back.
for (Loop *ChildL : llvm::reverse(OrigRootL))
LoopsToClone.push_back({ClonedRootL, ChildL});
do {
Loop *ClonedParentL, *L;
std::tie(ClonedParentL, L) = LoopsToClone.pop_back_val();
Loop *ClonedL = LI.AllocateLoop();
AddClonedBlocksToLoop(*L, *ClonedL);
for (Loop *ChildL : llvm::reverse(*L))
LoopsToClone.push_back({ClonedL, ChildL});
} while (!LoopsToClone.empty());
return ClonedRootL;
/// Build the cloned loops of an original loop from unswitching.
/// Because unswitching simplifies the CFG of the loop, this isn't a trivial
/// operation. We need to re-verify that there even is a loop (as the backedge
/// may not have been cloned), and even if there are remaining backedges the
/// backedge set may be different. However, we know that each child loop is
/// undisturbed, we only need to find where to place each child loop within
/// either any parent loop or within a cloned version of the original loop.
/// Because child loops may end up cloned outside of any cloned version of the
/// original loop, multiple cloned sibling loops may be created. All of them
/// are returned so that the newly introduced loop nest roots can be
/// identified.
static void buildClonedLoops(Loop &OrigL, ArrayRef<BasicBlock *> ExitBlocks,
const ValueToValueMapTy &VMap, LoopInfo &LI,
SmallVectorImpl<Loop *> &NonChildClonedLoops) {
Loop *ClonedL = nullptr;
auto *OrigPH = OrigL.getLoopPreheader();
auto *OrigHeader = OrigL.getHeader();
auto *ClonedPH = cast<BasicBlock>(VMap.lookup(OrigPH));
auto *ClonedHeader = cast<BasicBlock>(VMap.lookup(OrigHeader));
// We need to know the loops of the cloned exit blocks to even compute the
// accurate parent loop. If we only clone exits to some parent of the
// original parent, we want to clone into that outer loop. We also keep track
// of the loops that our cloned exit blocks participate in.
Loop *ParentL = nullptr;
SmallVector<BasicBlock *, 4> ClonedExitsInLoops;
SmallDenseMap<BasicBlock *, Loop *, 16> ExitLoopMap;
for (auto *ExitBB : ExitBlocks)
if (auto *ClonedExitBB = cast_or_null<BasicBlock>(VMap.lookup(ExitBB)))
if (Loop *ExitL = LI.getLoopFor(ExitBB)) {
ExitLoopMap[ClonedExitBB] = ExitL;
if (!ParentL || (ParentL != ExitL && ParentL->contains(ExitL)))
ParentL = ExitL;
assert((!ParentL || ParentL == OrigL.getParentLoop() ||
ParentL->contains(OrigL.getParentLoop())) &&
"The computed parent loop should always contain (or be) the parent of "
"the original loop.");
// We build the set of blocks dominated by the cloned header from the set of
// cloned blocks out of the original loop. While not all of these will
// necessarily be in the cloned loop, it is enough to establish that they
// aren't in unreachable cycles, etc.
SmallSetVector<BasicBlock *, 16> ClonedLoopBlocks;
for (auto *BB : OrigL.blocks())
if (auto *ClonedBB = cast_or_null<BasicBlock>(VMap.lookup(BB)))
// Rebuild the set of blocks that will end up in the cloned loop. We may have
// skipped cloning some region of this loop which can in turn skip some of
// the backedges so we have to rebuild the blocks in the loop based on the
// backedges that remain after cloning.
SmallVector<BasicBlock *, 16> Worklist;
SmallPtrSet<BasicBlock *, 16> BlocksInClonedLoop;
for (auto *Pred : predecessors(ClonedHeader)) {
// The only possible non-loop header predecessor is the preheader because
// we know we cloned the loop in simplified form.
if (Pred == ClonedPH)
// Because the loop was in simplified form, the only non-loop predecessor
// should be the preheader.
assert(ClonedLoopBlocks.count(Pred) && "Found a predecessor of the loop "
"header other than the preheader "
"that is not part of the loop!");
// Insert this block into the loop set and on the first visit (and if it
// isn't the header we're currently walking) put it into the worklist to
// recurse through.
if (BlocksInClonedLoop.insert(Pred).second && Pred != ClonedHeader)
// If we had any backedges then there *is* a cloned loop. Put the header into
// the loop set and then walk the worklist backwards to find all the blocks
// that remain within the loop after cloning.
if (!BlocksInClonedLoop.empty()) {
while (!Worklist.empty()) {
BasicBlock *BB = Worklist.pop_back_val();
assert(BlocksInClonedLoop.count(BB) &&
"Didn't put block into the loop set!");
// Insert any predecessors that are in the possible set into the cloned
// set, and if the insert is successful, add them to the worklist. Note
// that we filter on the blocks that are definitely reachable via the
// backedge to the loop header so we may prune out dead code within the
// cloned loop.
for (auto *Pred : predecessors(BB))
if (ClonedLoopBlocks.count(Pred) &&
ClonedL = LI.AllocateLoop();
if (ParentL) {
ParentL->addBasicBlockToLoop(ClonedPH, LI);
} else {
// We don't want to just add the cloned loop blocks based on how we
// discovered them. The original order of blocks was carefully built in
// a way that doesn't rely on predecessor ordering. Rather than re-invent
// that logic, we just re-walk the original blocks (and those of the child
// loops) and filter them as we add them into the cloned loop.
for (auto *BB : OrigL.blocks()) {
auto *ClonedBB = cast_or_null<BasicBlock>(VMap.lookup(BB));
if (!ClonedBB || !BlocksInClonedLoop.count(ClonedBB))
// Directly add the blocks that are only in this loop.
if (LI.getLoopFor(BB) == &OrigL) {
ClonedL->addBasicBlockToLoop(ClonedBB, LI);
// We want to manually add it to this loop and parents.
// Registering it with LoopInfo will happen when we clone the top
// loop for this block.
for (Loop *PL = ClonedL; PL; PL = PL->getParentLoop())
// Now add each child loop whose header remains within the cloned loop. All
// of the blocks within the loop must satisfy the same constraints as the
// header so once we pass the header checks we can just clone the entire
// child loop nest.
for (Loop *ChildL : OrigL) {
auto *ClonedChildHeader =
if (!ClonedChildHeader || !BlocksInClonedLoop.count(ClonedChildHeader))
#ifndef NDEBUG
// We should never have a cloned child loop header but fail to have
// all of the blocks for that child loop.
for (auto *ChildLoopBB : ChildL->blocks())
cast<BasicBlock>(VMap.lookup(ChildLoopBB))) &&
"Child cloned loop has a header within the cloned outer "
"loop but not all of its blocks!");
cloneLoopNest(*ChildL, ClonedL, VMap, LI);
// Now that we've handled all the components of the original loop that were
// cloned into a new loop, we still need to handle anything from the original
// loop that wasn't in a cloned loop.
// Figure out what blocks are left to place within any loop nest containing
// the unswitched loop. If we never formed a loop, the cloned PH is one of
// them.
SmallPtrSet<BasicBlock *, 16> UnloopedBlockSet;
if (BlocksInClonedLoop.empty())
for (auto *ClonedBB : ClonedLoopBlocks)
if (!BlocksInClonedLoop.count(ClonedBB))
// Copy the cloned exits and sort them in ascending loop depth, we'll work
// backwards across these to process them inside out. The order shouldn't
// matter as we're just trying to build up the map from inside-out; we use
// the map in a more stably ordered way below.
auto OrderedClonedExitsInLoops = ClonedExitsInLoops;
llvm::sort(OrderedClonedExitsInLoops, [&](BasicBlock *LHS, BasicBlock *RHS) {
return ExitLoopMap.lookup(LHS)->getLoopDepth() <
// Populate the existing ExitLoopMap with everything reachable from each
// exit, starting from the inner most exit.
while (!UnloopedBlockSet.empty() && !OrderedClonedExitsInLoops.empty()) {
assert(Worklist.empty() && "Didn't clear worklist!");
BasicBlock *ExitBB = OrderedClonedExitsInLoops.pop_back_val();
Loop *ExitL = ExitLoopMap.lookup(ExitBB);
// Walk the CFG back until we hit the cloned PH adding everything reachable
// and in the unlooped set to this exit block's loop.
do {
BasicBlock *BB = Worklist.pop_back_val();
// We can stop recursing at the cloned preheader (if we get there).
if (BB == ClonedPH)
for (BasicBlock *PredBB : predecessors(BB)) {
// If this pred has already been moved to our set or is part of some
// (inner) loop, no update needed.
if (!UnloopedBlockSet.erase(PredBB)) {
(BlocksInClonedLoop.count(PredBB) || ExitLoopMap.count(PredBB)) &&
"Predecessor not mapped to a loop!");
// We just insert into the loop set here. We'll add these blocks to the
// exit loop after we build up the set in an order that doesn't rely on
// predecessor order (which in turn relies on use list order).
bool Inserted = ExitLoopMap.insert({PredBB, ExitL}).second;
assert(Inserted && "Should only visit an unlooped block once!");
// And recurse through to its predecessors.
} while (!Worklist.empty());
// Now that the ExitLoopMap gives as mapping for all the non-looping cloned
// blocks to their outer loops, walk the cloned blocks and the cloned exits
// in their original order adding them to the correct loop.
// We need a stable insertion order. We use the order of the original loop
// order and map into the correct parent loop.
for (auto *BB : llvm::concat<BasicBlock *const>(
makeArrayRef(ClonedPH), ClonedLoopBlocks, ClonedExitsInLoops))
if (Loop *OuterL = ExitLoopMap.lookup(BB))
OuterL->addBasicBlockToLoop(BB, LI);
#ifndef NDEBUG
for (auto &BBAndL : ExitLoopMap) {
auto *BB = BBAndL.first;
auto *OuterL = BBAndL.second;
assert(LI.getLoopFor(BB) == OuterL &&
"Failed to put all blocks into outer loops!");
// Now that all the blocks are placed into the correct containing loop in the
// absence of child loops, find all the potentially cloned child loops and
// clone them into whatever outer loop we placed their header into.
for (Loop *ChildL : OrigL) {
auto *ClonedChildHeader =
if (!ClonedChildHeader || BlocksInClonedLoop.count(ClonedChildHeader))
#ifndef NDEBUG
for (auto *ChildLoopBB : ChildL->blocks())
assert(VMap.count(ChildLoopBB) &&
"Cloned a child loop header but not all of that loops blocks!");
*ChildL, ExitLoopMap.lookup(ClonedChildHeader), VMap, LI));
static void
deleteDeadClonedBlocks(Loop &L, ArrayRef<BasicBlock *> ExitBlocks,
ArrayRef<std::unique_ptr<ValueToValueMapTy>> VMaps,
DominatorTree &DT, MemorySSAUpdater *MSSAU) {
// Find all the dead clones, and remove them from their successors.
SmallVector<BasicBlock *, 16> DeadBlocks;
for (BasicBlock *BB : llvm::concat<BasicBlock *const>(L.blocks(), ExitBlocks))
for (auto &VMap : VMaps)
if (BasicBlock *ClonedBB = cast_or_null<BasicBlock>(VMap->lookup(BB)))
if (!DT.isReachableFromEntry(ClonedBB)) {
for (BasicBlock *SuccBB : successors(ClonedBB))
// Remove all MemorySSA in the dead blocks
if (MSSAU) {
SmallSetVector<BasicBlock *, 8> DeadBlockSet(DeadBlocks.begin(),
// Drop any remaining references to break cycles.
for (BasicBlock *BB : DeadBlocks)
// Erase them from the IR.
for (BasicBlock *BB : DeadBlocks)
static void deleteDeadBlocksFromLoop(Loop &L,
SmallVectorImpl<BasicBlock *> &ExitBlocks,
DominatorTree &DT, LoopInfo &LI,
MemorySSAUpdater *MSSAU) {
// Find all the dead blocks tied to this loop, and remove them from their
// successors.
SmallSetVector<BasicBlock *, 8> DeadBlockSet;
// Start with loop/exit blocks and get a transitive closure of reachable dead
// blocks.
SmallVector<BasicBlock *, 16> DeathCandidates(ExitBlocks.begin(),
DeathCandidates.append(L.blocks().begin(), L.blocks().end());
while (!DeathCandidates.empty()) {
auto *BB = DeathCandidates.pop_back_val();
if (!DeadBlockSet.count(BB) && !DT.isReachableFromEntry(BB)) {
for (BasicBlock *SuccBB : successors(BB)) {
// Remove all MemorySSA in the dead blocks
if (MSSAU)
// Filter out the dead blocks from the exit blocks list so that it can be
// used in the caller.
[&](BasicBlock *BB) { return DeadBlockSet.count(BB); });
// Walk from this loop up through its parents removing all of the dead blocks.
for (Loop *ParentL = &L; ParentL; ParentL = ParentL->getParentLoop()) {
for (auto *BB : DeadBlockSet)
[&](BasicBlock *BB) { return DeadBlockSet.count(BB); });
// Now delete the dead child loops. This raw delete will clear them
// recursively.
llvm::erase_if(L.getSubLoopsVector(), [&](Loop *ChildL) {
if (!DeadBlockSet.count(ChildL->getHeader()))
return false;
[&](BasicBlock *ChildBB) {
return DeadBlockSet.count(ChildBB);
}) &&
"If the child loop header is dead all blocks in the child loop must "
"be dead as well!");
return true;
// Remove the loop mappings for the dead blocks and drop all the references
// from these blocks to others to handle cyclic references as we start
// deleting the blocks themselves.
for (auto *BB : DeadBlockSet) {
// Check that the dominator tree has already been updated.
assert(!DT.getNode(BB) && "Should already have cleared domtree!");
LI.changeLoopFor(BB, nullptr);
// Drop all uses of the instructions to make sure we won't have dangling
// uses in other blocks.
for (auto &I : *BB)
if (!I.use_empty())
// Actually delete the blocks now that they've been fully unhooked from the
// IR.
for (auto *BB : DeadBlockSet)
/// Recompute the set of blocks in a loop after unswitching.
/// This walks from the original headers predecessors to rebuild the loop. We
/// take advantage of the fact that new blocks can't have been added, and so we
/// filter by the original loop's blocks. This also handles potentially
/// unreachable code that we don't want to explore but might be found examining
/// the predecessors of the header.
/// If the original loop is no longer a loop, this will return an empty set. If
/// it remains a loop, all the blocks within it will be added to the set
/// (including those blocks in inner loops).
static SmallPtrSet<const BasicBlock *, 16> recomputeLoopBlockSet(Loop &L,
LoopInfo &LI) {
SmallPtrSet<const BasicBlock *, 16> LoopBlockSet;
auto *PH = L.getLoopPreheader();
auto *Header = L.getHeader();
// A worklist to use while walking backwards from the header.
SmallVector<BasicBlock *, 16> Worklist;
// First walk the predecessors of the header to find the backedges. This will
// form the basis of our walk.
for (auto *Pred : predecessors(Header)) {
// Skip the preheader.
if (Pred == PH)
// Because the loop was in simplified form, the only non-loop predecessor
// is the preheader.
assert(L.contains(Pred) && "Found a predecessor of the loop header other "
"than the preheader that is not part of the "
// Insert this block into the loop set and on the first visit and, if it
// isn't the header we're currently walking, put it into the worklist to
// recurse through.
if (LoopBlockSet.insert(Pred).second && Pred != Header)
// If no backedges were found, we're done.
if (LoopBlockSet.empty())
return LoopBlockSet;
// We found backedges, recurse through them to identify the loop blocks.
while (!Worklist.empty()) {
BasicBlock *BB = Worklist.pop_back_val();
assert(LoopBlockSet.count(BB) && "Didn't put block into the loop set!");
// No need to walk past the header.
if (BB == Header)
// Because we know the inner loop structure remains valid we can use the
// loop structure to jump immediately across the entire nested loop.
// Further, because it is in loop simplified form, we can directly jump
// to its preheader afterward.
if (Loop *InnerL = LI.getLoopFor(BB))
if (InnerL != &L) {
assert(L.contains(InnerL) &&
"Should not reach a loop *outside* this loop!");
// The preheader is the only possible predecessor of the loop so
// insert it into the set and check whether it was already handled.
auto *InnerPH = InnerL->getLoopPreheader();
assert(L.contains(InnerPH) && "Cannot contain an inner loop block "
"but not contain the inner loop "
if (!LoopBlockSet.insert(InnerPH).second)
// The only way to reach the preheader is through the loop body
// itself so if it has been visited the loop is already handled.
// Insert all of the blocks (other than those already present) into
// the loop set. We expect at least the block that led us to find the
// inner loop to be in the block set, but we may also have other loop
// blocks if they were already enqueued as predecessors of some other
// outer loop block.
for (auto *InnerBB : InnerL->blocks()) {
if (InnerBB == BB) {
assert(LoopBlockSet.count(InnerBB) &&
"Block should already be in the set!");
// Add the preheader to the worklist so we will continue past the
// loop body.
// Insert any predecessors that were in the original loop into the new
// set, and if the insert is successful, add them to the worklist.
for (auto *Pred : predecessors(BB))
if (L.contains(Pred) && LoopBlockSet.insert(Pred).second)
assert(LoopBlockSet.count(Header) && "Cannot fail to add the header!");
// We've found all the blocks participating in the loop, return our completed
// set.
return LoopBlockSet;
/// Rebuild a loop after unswitching removes some subset of blocks and edges.
/// The removal may have removed some child loops entirely but cannot have
/// disturbed any remaining child loops. However, they may need to be hoisted
/// to the parent loop (or to be top-level loops). The original loop may be
/// completely removed.
/// The sibling loops resulting from this update are returned. If the original
/// loop remains a valid loop, it will be the first entry in this list with all
/// of the newly sibling loops following it.
/// Returns true if the loop remains a loop after unswitching, and false if it
/// is no longer a loop after unswitching (and should not continue to be
/// referenced).
static bool rebuildLoopAfterUnswitch(Loop &L, ArrayRef<BasicBlock *> ExitBlocks,
LoopInfo &LI,
SmallVectorImpl<Loop *> &HoistedLoops) {
auto *PH = L.getLoopPreheader();
// Compute the actual parent loop from the exit blocks. Because we may have
// pruned some exits the loop may be different from the original parent.
Loop *ParentL = nullptr;
SmallVector<Loop *, 4> ExitLoops;
SmallVector<BasicBlock *, 4> ExitsInLoops;
for (auto *ExitBB : ExitBlocks)
if (Loop *ExitL = LI.getLoopFor(ExitBB)) {
if (!ParentL || (ParentL != ExitL && ParentL->contains(ExitL)))
ParentL = ExitL;
// Recompute the blocks participating in this loop. This may be empty if it
// is no longer a loop.
auto LoopBlockSet = recomputeLoopBlockSet(L, LI);
// If we still have a loop, we need to re-set the loop's parent as the exit
// block set changing may have moved it within the loop nest. Note that this
// can only happen when this loop has a parent as it can only hoist the loop
// *up* the nest.
if (!LoopBlockSet.empty() && L.getParentLoop() != ParentL) {
// Remove this loop's (original) blocks from all of the intervening loops.
for (Loop *IL = L.getParentLoop(); IL != ParentL;
IL = IL->getParentLoop()) {
for (auto *BB : L.blocks())
llvm::erase_if(IL->getBlocksVector(), [&](BasicBlock *BB) {
return BB == PH || L.contains(BB);
LI.changeLoopFor(PH, ParentL);
if (ParentL)
// Now we update all the blocks which are no longer within the loop.
auto &Blocks = L.getBlocksVector();
auto BlocksSplitI =
? Blocks.begin()
: std::stable_partition(
Blocks.begin(), Blocks.end(),
[&](BasicBlock *BB) { return LoopBlockSet.count(BB); });
// Before we erase the list of unlooped blocks, build a set of them.
SmallPtrSet<BasicBlock *, 16> UnloopedBlocks(BlocksSplitI, Blocks.end());
if (LoopBlockSet.empty())
// Now erase these blocks from the loop.
for (auto *BB : make_range(BlocksSplitI, Blocks.end()))
Blocks.erase(BlocksSplitI, Blocks.end());
// Sort the exits in ascending loop depth, we'll work backwards across these
// to process them inside out.
llvm::stable_sort(ExitsInLoops, [&](BasicBlock *LHS, BasicBlock *RHS) {
return LI.getLoopDepth(LHS) < LI.getLoopDepth(RHS);
// We'll build up a set for each exit loop.
SmallPtrSet<BasicBlock *, 16> NewExitLoopBlocks;
Loop *PrevExitL = L.getParentLoop(); // The deepest possible exit loop.
auto RemoveUnloopedBlocksFromLoop =
[](Loop &L, SmallPtrSetImpl<BasicBlock *> &UnloopedBlocks) {
for (auto *BB : UnloopedBlocks)
llvm::erase_if(L.getBlocksVector(), [&](BasicBlock *BB) {
return UnloopedBlocks.count(BB);
SmallVector<BasicBlock *, 16> Worklist;
while (!UnloopedBlocks.empty() && !ExitsInLoops.empty()) {
assert(Worklist.empty() && "Didn't clear worklist!");
assert(NewExitLoopBlocks.empty() && "Didn't clear loop set!");
// Grab the next exit block, in decreasing loop depth order.
BasicBlock *ExitBB = ExitsInLoops.pop_back_val();
Loop &ExitL = *LI.getLoopFor(ExitBB);
assert(ExitL.contains(&L) && "Exit loop must contain the inner loop!");
// Erase all of the unlooped blocks from the loops between the previous
// exit loop and this exit loop. This works because the ExitInLoops list is
// sorted in increasing order of loop depth and thus we visit loops in
// decreasing order of loop depth.
for (; PrevExitL != &ExitL; PrevExitL = PrevExitL->getParentLoop())
RemoveUnloopedBlocksFromLoop(*PrevExitL, UnloopedBlocks);
// Walk the CFG back until we hit the cloned PH adding everything reachable
// and in the unlooped set to this exit block's loop.
do {
BasicBlock *BB = Worklist.pop_back_val();
// We can stop recursing at the cloned preheader (if we get there).
if (BB == PH)
for (BasicBlock *PredBB : predecessors(BB)) {
// If this pred has already been moved to our set or is part of some
// (inner) loop, no update needed.
if (!UnloopedBlocks.erase(PredBB)) {
assert((NewExitLoopBlocks.count(PredBB) ||
ExitL.contains(LI.getLoopFor(PredBB))) &&
"Predecessor not in a nested loop (or already visited)!");
// We just insert into the loop set here. We'll add these blocks to the
// exit loop after we build up the set in a deterministic order rather
// than the predecessor-influenced visit order.
bool Inserted = NewExitLoopBlocks.insert(PredBB).second;
assert(Inserted && "Should only visit an unlooped block once!");
// And recurse through to its predecessors.
} while (!Worklist.empty());
// If blocks in this exit loop were directly part of the original loop (as
// opposed to a child loop) update the map to point to this exit loop. This
// just updates a map and so the fact that the order is unstable is fine.
for (auto *BB : NewExitLoopBlocks)
if (Loop *BBL = LI.getLoopFor(BB))
if (BBL == &L || !L.contains(BBL))
LI.changeLoopFor(BB, &ExitL);
// We will remove the remaining unlooped blocks from this loop in the next
// iteration or below.
// Any remaining unlooped blocks are no longer part of any loop unless they
// are part of some child loop.
for (; PrevExitL; PrevExitL = PrevExitL->getParentLoop())
RemoveUnloopedBlocksFromLoop(*PrevExitL, UnloopedBlocks);
for (auto *BB : UnloopedBlocks)
if (Loop *BBL = LI.getLoopFor(BB))
if (BBL == &L || !L.contains(BBL))
LI.changeLoopFor(BB, nullptr);
// Sink all the child loops whose headers are no longer in the loop set to
// the parent (or to be top level loops). We reach into the loop and directly
// update its subloop vector to make this batch update efficient.
auto &SubLoops = L.getSubLoopsVector();
auto SubLoopsSplitI =
? SubLoops.begin()
: std::stable_partition(
SubLoops.begin(), SubLoops.end(), [&](Loop *SubL) {
return LoopBlockSet.count(SubL->getHeader());
for (auto *HoistedL : make_range(SubLoopsSplitI, SubLoops.end())) {
// To compute the new parent of this hoisted loop we look at where we
// placed the preheader above. We can't lookup the header itself because we
// retained the mapping from the header to the hoisted loop. But the
// preheader and header should have the exact same new parent computed
// based on the set of exit blocks from the original loop as the preheader
// is a predecessor of the header and so reached in the reverse walk. And
// because the loops were all in simplified form the preheader of the
// hoisted loop can't be part of some *other* loop.
if (auto *NewParentL = LI.getLoopFor(HoistedL->getLoopPreheader()))
SubLoops.erase(SubLoopsSplitI, SubLoops.end());
// Actually delete the loop if nothing remained within it.
if (Blocks.empty()) {
assert(SubLoops.empty() &&
"Failed to remove all subloops from the original loop!");
if (Loop *ParentL = L.getParentLoop())
ParentL->removeChildLoop(llvm::find(*ParentL, &L));
LI.removeLoop(llvm::find(LI, &L));
return false;
return true;
/// Helper to visit a dominator subtree, invoking a callable on each node.
/// Returning false at any point will stop walking past that node of the tree.
template <typename CallableT>
void visitDomSubTree(DominatorTree &DT, BasicBlock *BB, CallableT Callable) {
SmallVector<DomTreeNode *, 4> DomWorklist;
#ifndef NDEBUG
SmallPtrSet<DomTreeNode *, 4> Visited;
do {
DomTreeNode *N = DomWorklist.pop_back_val();
// Visit this node.
if (!Callable(N->getBlock()))
// Accumulate the child nodes.
for (DomTreeNode *ChildN : *N) {
assert(Visited.insert(ChildN).second &&
"Cannot visit a node twice when walking a tree!");
} while (!DomWorklist.empty());
static void unswitchNontrivialInvariants(
Loop &L, Instruction &TI, ArrayRef<Value *> Invariants,
SmallVectorImpl<BasicBlock *> &ExitBlocks, DominatorTree &DT, LoopInfo &LI,
AssumptionCache &AC, function_ref<void(bool, ArrayRef<Loop *>)> UnswitchCB,
ScalarEvolution *SE, MemorySSAUpdater *MSSAU) {
auto *ParentBB = TI.getParent();
BranchInst *BI = dyn_cast<BranchInst>(&TI);
SwitchInst *SI = BI ? nullptr : cast<SwitchInst>(&TI);
// We can only unswitch switches, conditional branches with an invariant
// condition, or combining invariant conditions with an instruction.
assert((SI || (BI && BI->isConditional())) &&
"Can only unswitch switches and conditional branch!");
bool FullUnswitch = SI || BI->getCondition() == Invariants[0];
if (FullUnswitch)
assert(Invariants.size() == 1 &&
"Cannot have other invariants with full unswitching!");
assert(isa<Instruction>(BI->getCondition()) &&
"Partial unswitching requires an instruction as the condition!");
if (MSSAU && VerifyMemorySSA)
// Constant and BBs tracking the cloned and continuing successor. When we are
// unswitching the entire condition, this can just be trivially chosen to
// unswitch towards `true`. However, when we are unswitching a set of
// invariants combined with `and` or `or`, the combining operation determines
// the best direction to unswitch: we want to unswitch the direction that will
// collapse the branch.
bool Direction = true;
int ClonedSucc = 0;
if (!FullUnswitch) {
if (!match(BI->getCondition(), m_LogicalOr())) {
assert(match(BI->getCondition(), m_LogicalAnd()) &&
"Only `or`, `and`, an `select` instructions can combine "
"invariants being unswitched.");
Direction = false;
ClonedSucc = 1;
BasicBlock *RetainedSuccBB =
BI ? BI->getSuccessor(1 - ClonedSucc) : SI->getDefaultDest();
SmallSetVector<BasicBlock *, 4> UnswitchedSuccBBs;
if (BI)
for (auto Case : SI->cases())
if (Case.getCaseSuccessor() != RetainedSuccBB)
assert(!UnswitchedSuccBBs.count(RetainedSuccBB) &&
"Should not unswitch the same successor we are retaining!");
// The branch should be in this exact loop. Any inner loop's invariant branch
// should be handled by unswitching that inner loop. The caller of this
// routine should filter out any candidates that remain (but were skipped for
// whatever reason).
assert(LI.getLoopFor(ParentBB) == &L && "Branch in an inner loop!");
// Compute the parent loop now before we start hacking on things.
Loop *ParentL = L.getParentLoop();
// Get blocks in RPO order for MSSA update, before changing the CFG.
LoopBlocksRPO LBRPO(&L);
if (MSSAU)
// Compute the outer-most loop containing one of our exit blocks. This is the
// furthest up our loopnest which can be mutated, which we will use below to
// update things.
Loop *OuterExitL = &L;
for (auto *ExitBB : ExitBlocks) {
Loop *NewOuterExitL = LI.getLoopFor(ExitBB);
if (!NewOuterExitL) {
// We exited the entire nest with this block, so we're done.
OuterExitL = nullptr;
if (NewOuterExitL != OuterExitL && NewOuterExitL->contains(OuterExitL))
OuterExitL = NewOuterExitL;
// At this point, we're definitely going to unswitch something so invalidate
// any cached information in ScalarEvolution for the outer most loop
// containing an exit block and all nested loops.
if (SE) {
if (OuterExitL)
// If the edge from this terminator to a successor dominates that successor,
// store a map from each block in its dominator subtree to it. This lets us
// tell when cloning for a particular successor if a block is dominated by
// some *other* successor with a single data structure. We use this to
// significantly reduce cloning.
SmallDenseMap<BasicBlock *, BasicBlock *, 16> DominatingSucc;
for (auto *SuccBB : llvm::concat<BasicBlock *const>(
makeArrayRef(RetainedSuccBB), UnswitchedSuccBBs))
if (SuccBB->getUniquePredecessor() ||
llvm::all_of(predecessors(SuccBB), [&](BasicBlock *PredBB) {
return PredBB == ParentBB || DT.dominates(SuccBB, PredBB);
visitDomSubTree(DT, SuccBB, [&](BasicBlock *BB) {
DominatingSucc[BB] = SuccBB;
return true;
// Split the preheader, so that we know that there is a safe place to insert
// the conditional branch. We will change the preheader to have a conditional
// branch on LoopCond. The original preheader will become the split point
// between the unswitched versions, and we will have a new preheader for the
// original loop.
BasicBlock *SplitBB = L.getLoopPreheader();
BasicBlock *LoopPH = SplitEdge(SplitBB, L.getHeader(), &DT, &LI, MSSAU);
// Keep track of the dominator tree updates needed.
SmallVector<DominatorTree::UpdateType, 4> DTUpdates;
// Clone the loop for each unswitched successor.
SmallVector<std::unique_ptr<ValueToValueMapTy>, 4> VMaps;
SmallDenseMap<BasicBlock *, BasicBlock *, 4> ClonedPHs;
for (auto *SuccBB : UnswitchedSuccBBs) {
VMaps.emplace_back(new ValueToValueMapTy());
ClonedPHs[SuccBB] = buildClonedLoopBlocks(
L, LoopPH, SplitBB, ExitBlocks, ParentBB, SuccBB, RetainedSuccBB,
DominatingSucc, *VMaps.back(), DTUpdates, AC, DT, LI, MSSAU);
// Drop metadata if we may break its semantics by moving this instr into the
// split block.
if (TI.getMetadata(LLVMContext::MD_make_implicit)) {
if (DropNonTrivialImplicitNullChecks)
// Do not spend time trying to understand if we can keep it, just drop it
// to save compile time.
TI.setMetadata(LLVMContext::MD_make_implicit, nullptr);
else {
// It is only legal to preserve make.implicit metadata if we are
// guaranteed no reach implicit null check after following this branch.
ICFLoopSafetyInfo SafetyInfo;
if (!SafetyInfo.isGuaranteedToExecute(TI, &DT, &L))
TI.setMetadata(LLVMContext::MD_make_implicit, nullptr);
// The stitching of the branched code back together depends on whether we're
// doing full unswitching or not with the exception that we always want to
// nuke the initial terminator placed in the split block.
if (FullUnswitch) {
// Splice the terminator from the original loop and rewrite its
// successors.
SplitBB->getInstList().splice(SplitBB->end(), ParentBB->getInstList(), TI);
// Keep a clone of the terminator for MSSA updates.
Instruction *NewTI = TI.clone();
// First wire up the moved terminator to the preheaders.
if (BI) {
BasicBlock *ClonedPH = ClonedPHs.begin()->second;
BI->setSuccessor(ClonedSucc, ClonedPH);
BI->setSuccessor(1 - ClonedSucc, LoopPH);
DTUpdates.push_back({DominatorTree::Insert, SplitBB, ClonedPH});
} else {
assert(SI && "Must either be a branch or switch!");
// Walk the cases and directly update their successors.
assert(SI->getDefaultDest() == RetainedSuccBB &&
"Not retaining default successor!");
for (auto &Case : SI->cases())
if (Case.getCaseSuccessor() == RetainedSuccBB)
// We need to use the set to populate domtree updates as even when there
// are multiple cases pointing at the same successor we only want to
// remove and insert one edge in the domtree.
for (BasicBlock *SuccBB : UnswitchedSuccBBs)
{DominatorTree::Insert, SplitBB, ClonedPHs.find(SuccBB)->second});
if (MSSAU) {
// Remove all but one edge to the retained block and all unswitched
// blocks. This is to avoid having duplicate entries in the cloned Phis,
// when we know we only keep a single edge for each case.
MSSAU->removeDuplicatePhiEdgesBetween(ParentBB, RetainedSuccBB);
for (BasicBlock *SuccBB : UnswitchedSuccBBs)
MSSAU->removeDuplicatePhiEdgesBetween(ParentBB, SuccBB);
for (auto &VMap : VMaps)
MSSAU->updateForClonedLoop(LBRPO, ExitBlocks, *VMap,
MSSAU->updateExitBlocksForClonedLoop(ExitBlocks, VMaps, DT);
// Remove all edges to unswitched blocks.
for (BasicBlock *SuccBB : UnswitchedSuccBBs)
MSSAU->removeEdge(ParentBB, SuccBB);
// Now unhook the successor relationship as we'll be replacing
// the terminator with a direct branch. This is much simpler for branches
// than switches so we handle those first.
if (BI) {
// Remove the parent as a predecessor of the unswitched successor.
assert(UnswitchedSuccBBs.size() == 1 &&
"Only one possible unswitched block for a branch!");
BasicBlock *UnswitchedSuccBB = *UnswitchedSuccBBs.begin();
/*KeepOneInputPHIs*/ true);
DTUpdates.push_back({DominatorTree::Delete, ParentBB, UnswitchedSuccBB});
} else {
// Note that we actually want to remove the parent block as a predecessor
// of *every* case successor. The case successor is either unswitched,
// completely eliminating an edge from the parent to that successor, or it
// is a duplicate edge to the retained successor as the retained successor
// is always the default successor and as we'll replace this with a direct
// branch we no longer need the duplicate entries in the PHI nodes.
SwitchInst *NewSI = cast<SwitchInst>(NewTI);
assert(NewSI->getDefaultDest() == RetainedSuccBB &&
"Not retaining default successor!");
for (auto &Case : NewSI->cases())
/*KeepOneInputPHIs*/ true);
// We need to use the set to populate domtree updates as even when there
// are multiple cases pointing at the same successor we only want to
// remove and insert one edge in the domtree.
for (BasicBlock *SuccBB : UnswitchedSuccBBs)
DTUpdates.push_back({DominatorTree::Delete, ParentBB, SuccBB});
// After MSSAU update, remove the cloned terminator instruction NewTI.
// Create a new unconditional branch to the continuing block (as opposed to
// the one cloned).
BranchInst::Create(RetainedSuccBB, ParentBB);
} else {
assert(BI && "Only branches have partial unswitching.");
assert(UnswitchedSuccBBs.size() == 1 &&
"Only one possible unswitched block for a branch!");
BasicBlock *ClonedPH = ClonedPHs.begin()->second;
// When doing a partial unswitch, we have to do a bit more work to build up
// the branch in the split block.
buildPartialUnswitchConditionalBranch(*SplitBB, Invariants, Direction,
*ClonedPH, *LoopPH);
DTUpdates.push_back({DominatorTree::Insert, SplitBB, ClonedPH});
if (MSSAU) {
// Perform MSSA cloning updates.
for (auto &VMap : VMaps)
MSSAU->updateForClonedLoop(LBRPO, ExitBlocks, *VMap,
MSSAU->updateExitBlocksForClonedLoop(ExitBlocks, VMaps, DT);
// Apply the updates accumulated above to get an up-to-date dominator tree.
// Now that we have an accurate dominator tree, first delete the dead cloned
// blocks so that we can accurately build any cloned loops. It is important to
// not delete the blocks from the original loop yet because we still want to
// reference the original loop to understand the cloned loop's structure.
deleteDeadClonedBlocks(L, ExitBlocks, VMaps, DT, MSSAU);
// Build the cloned loop structure itself. This may be substantially
// different from the original structure due to the simplified CFG. This also
// handles inserting all the cloned blocks into the correct loops.
SmallVector<Loop *, 4> NonChildClonedLoops;
for (std::unique_ptr<ValueToValueMapTy> &VMap : VMaps)
buildClonedLoops(L, ExitBlocks, *VMap, LI, NonChildClonedLoops);
// Now that our cloned loops have been built, we can update the original loop.
// First we delete the dead blocks from it and then we rebuild the loop
// structure taking these deletions into account.
deleteDeadBlocksFromLoop(L, ExitBlocks, DT, LI, MSSAU);
if (MSSAU && VerifyMemorySSA)
SmallVector<Loop *, 4> HoistedLoops;
bool IsStillLoop = rebuildLoopAfterUnswitch(L, ExitBlocks, LI, HoistedLoops);
if (MSSAU && VerifyMemorySSA)
// This transformation has a high risk of corrupting the dominator tree, and
// the below steps to rebuild loop structures will result in hard to debug
// errors in that case so verify that the dominator tree is sane first.
// FIXME: Remove this when the bugs stop showing up and rely on existing
// verification steps.
if (BI) {
// If we unswitched a branch which collapses the condition to a known
// constant we want to replace all the uses of the invariants within both
// the original and cloned blocks. We do this here so that we can use the
// now updated dominator tree to identify which side the users are on.
assert(UnswitchedSuccBBs.size() == 1 &&
"Only one possible unswitched block for a branch!");
BasicBlock *ClonedPH = ClonedPHs.begin()->second;
// When considering multiple partially-unswitched invariants
// we cant just go replace them with constants in both branches.
// For 'AND' we infer that true branch ("continue") means true
// for each invariant operand.
// For 'OR' we can infer that false branch ("continue") means false
// for each invariant operand.
// So it happens that for multiple-partial case we dont replace
// in the unswitched branch.
bool ReplaceUnswitched = FullUnswitch || (Invariants.size() == 1);
ConstantInt *UnswitchedReplacement =
Direction ? ConstantInt::getTrue(BI->getContext())
: ConstantInt::getFalse(BI->getContext());
ConstantInt *ContinueReplacement =
Direction ? ConstantInt::getFalse(BI->getContext())
: ConstantInt::getTrue(BI->getContext());
for (Value *Invariant : Invariants)
// Use make_early_inc_range here as set invalidates the iterator.
for (Use &U : llvm::make_early_inc_range(Invariant->uses())) {
Instruction *UserI = dyn_cast<Instruction>(U.getUser());
if (!UserI)
// Replace it with the 'continue' side if in the main loop body, and the
// unswitched if in the cloned blocks.
if (DT.dominates(LoopPH, UserI->getParent()))
else if (ReplaceUnswitched &&
DT.dominates(ClonedPH, UserI->getParent()))
// We can change which blocks are exit blocks of all the cloned sibling
// loops, the current loop, and any parent loops which shared exit blocks
// with the current loop. As a consequence, we need to re-form LCSSA for
// them. But we shouldn't need to re-form LCSSA for any child loops.
// FIXME: This could be made more efficient by tracking which exit blocks are
// new, and focusing on them, but that isn't likely to be necessary.
// In order to reasonably rebuild LCSSA we need to walk inside-out across the
// loop nest and update every loop that could have had its exits changed. We
// also need to cover any intervening loops. We add all of these loops to
// a list and sort them by loop depth to achieve this without updating
// unnecessary loops.
auto UpdateLoop = [&](Loop &UpdateL) {
#ifndef NDEBUG
for (Loop *ChildL : UpdateL) {
assert(ChildL->isRecursivelyLCSSAForm(DT, LI) &&
"Perturbed a child loop's LCSSA form!");
// First build LCSSA for this loop so that we can preserve it when
// forming dedicated exits. We don't want to perturb some other loop's
// LCSSA while doing that CFG edit.
formLCSSA(UpdateL, DT, &LI, SE);
// For loops reached by this loop's original exit blocks we may
// introduced new, non-dedicated exits. At least try to re-form dedicated
// exits for these loops. This may fail if they couldn't have dedicated
// exits to start with.
formDedicatedExitBlocks(&UpdateL, &DT, &LI, MSSAU, /*PreserveLCSSA*/ true);
// For non-child cloned loops and hoisted loops, we just need to update LCSSA
// and we can do it in any order as they don't nest relative to each other.
// Also check if any of the loops we have updated have become top-level loops
// as that will necessitate widening the outer loop scope.
for (Loop *UpdatedL :
llvm::concat<Loop *>(NonChildClonedLoops, HoistedLoops)) {
if (UpdatedL->isOutermost())
OuterExitL = nullptr;
if (IsStillLoop) {
if (L.isOutermost())
OuterExitL = nullptr;
// If the original loop had exit blocks, walk up through the outer most loop
// of those exit blocks to update LCSSA and form updated dedicated exits.
if (OuterExitL != &L)
for (Loop *OuterL = ParentL; OuterL != OuterExitL;
OuterL = OuterL->getParentLoop())
#ifndef NDEBUG
// Verify the entire loop structure to catch any incorrect updates before we
// progress in the pass pipeline.
// Now that we've unswitched something, make callbacks to report the changes.
// For that we need to merge together the updated loops and the cloned loops
// and check whether the original loop survived.
SmallVector<Loop *, 4> SibLoops;
for (Loop *UpdatedL : llvm::concat<Loop *>(NonChildClonedLoops, HoistedLoops))
if (UpdatedL->getParentLoop() == ParentL)
UnswitchCB(IsStillLoop, SibLoops);
if (MSSAU && VerifyMemorySSA)
if (BI)
/// Recursively compute the cost of a dominator subtree based on the per-block
/// cost map provided.
/// The recursive computation is memozied into the provided DT-indexed cost map
/// to allow querying it for most nodes in the domtree without it becoming
/// quadratic.
static InstructionCost computeDomSubtreeCost(
DomTreeNode &N,
const SmallDenseMap<BasicBlock *, InstructionCost, 4> &BBCostMap,
SmallDenseMap<DomTreeNode *, InstructionCost, 4> &DTCostMap) {
// Don't accumulate cost (or recurse through) blocks not in our block cost
// map and thus not part of the duplication cost being considered.
auto BBCostIt = BBCostMap.find(N.getBlock());
if (BBCostIt == BBCostMap.end())
return 0;
// Lookup this node to see if we already computed its cost.
auto DTCostIt = DTCostMap.find(&N);
if (DTCostIt != DTCostMap.end())
return DTCostIt->second;
// If not, we have to compute it. We can't use insert above and update
// because computing the cost may insert more things into the map.
InstructionCost Cost = std::accumulate(
N.begin(), N.end(), BBCostIt->second,
[&](InstructionCost Sum, DomTreeNode *ChildN) -> InstructionCost {
return Sum + computeDomSubtreeCost(*ChildN, BBCostMap, DTCostMap);
bool Inserted = DTCostMap.insert({&N, Cost}).second;
assert(Inserted && "Should not insert a node while visiting children!");
return Cost;
/// Turns a llvm.experimental.guard intrinsic into implicit control flow branch,
/// making the following replacement:
/// --code before guard--
/// call void (i1, ...) @llvm.experimental.guard(i1 %cond) [ "deopt"() ]
/// --code after guard--
/// into
/// --code before guard--
/// br i1 %cond, label %guarded, label %deopt
/// guarded:
/// --code after guard--
/// deopt:
/// call void (i1, ...) @llvm.experimental.guard(i1 false) [ "deopt"() ]
/// unreachable
/// It also makes all relevant DT and LI updates, so that all structures are in
/// valid state after this transform.
static BranchInst *
turnGuardIntoBranch(IntrinsicInst *GI, Loop &L,
SmallVectorImpl<BasicBlock *> &ExitBlocks,
DominatorTree &DT, LoopInfo &LI, MemorySSAUpdater *MSSAU) {
SmallVector<DominatorTree::UpdateType, 4> DTUpdates;
LLVM_DEBUG(dbgs() << "Turning " << *GI << " into a branch.\n");
BasicBlock *CheckBB = GI->getParent();
if (MSSAU && VerifyMemorySSA)
// Remove all CheckBB's successors from DomTree. A block can be seen among
// successors more than once, but for DomTree it should be added only once.
SmallPtrSet<BasicBlock *, 4> Successors;
for (auto *Succ : successors(CheckBB))
if (Successors.insert(Succ).second)
DTUpdates.push_back({DominatorTree::Delete, CheckBB, Succ});
Instruction *DeoptBlockTerm =
SplitBlockAndInsertIfThen(GI->getArgOperand(0), GI, true);
BranchInst *CheckBI = cast<BranchInst>(CheckBB->getTerminator());
// SplitBlockAndInsertIfThen inserts control flow that branches to
// DeoptBlockTerm if the condition is true. We want the opposite.
BasicBlock *GuardedBlock = CheckBI->getSuccessor(0);
BasicBlock *DeoptBlock = CheckBI->getSuccessor(1);
// We now have a new exit block.
if (MSSAU)
MSSAU->moveAllAfterSpliceBlocks(CheckBB, GuardedBlock, GI);
GI->setArgOperand(0, ConstantInt::getFalse(GI->getContext()));
// Add new successors of CheckBB into DomTree.
for (auto *Succ : successors(CheckBB))
DTUpdates.push_back({DominatorTree::Insert, CheckBB, Succ});
// Now the blocks that used to be CheckBB's successors are GuardedBlock's
// successors.
for (auto *Succ : Successors)
DTUpdates.push_back({DominatorTree::Insert, GuardedBlock, Succ});
// Make proper changes to DT.
// Inform LI of a new loop block.
L.addBasicBlockToLoop(GuardedBlock, LI);
if (MSSAU) {
MemoryDef *MD = cast<MemoryDef>(MSSAU->getMemorySSA()->getMemoryAccess(GI));
MSSAU->moveToPlace(MD, DeoptBlock, MemorySSA::BeforeTerminator);
if (VerifyMemorySSA)
return CheckBI;
/// Cost multiplier is a way to limit potentially exponential behavior
/// of loop-unswitch. Cost is multipied in proportion of 2^number of unswitch
/// candidates available. Also accounting for the number of "sibling" loops with
/// the idea to account for previous unswitches that already happened on this
/// cluster of loops. There was an attempt to keep this formula simple,
/// just enough to limit the worst case behavior. Even if it is not that simple
/// now it is still not an attempt to provide a detailed heuristic size
/// prediction.
/// TODO: Make a proper accounting of "explosion" effect for all kinds of
/// unswitch candidates, making adequate predictions instead of wild guesses.
/// That requires knowing not just the number of "remaining" candidates but
/// also costs of unswitching for each of these candidates.