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//===-- LCSSA.cpp - Convert loops into loop-closed SSA form ---------------===//
// 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
// This pass transforms loops by placing phi nodes at the end of the loops for
// all values that are live across the loop boundary. For example, it turns
// the left into the right code:
// for (...) for (...)
// if (c) if (c)
// X1 = ... X1 = ...
// else else
// X2 = ... X2 = ...
// X3 = phi(X1, X2) X3 = phi(X1, X2)
// ... = X3 + 4 X4 = phi(X3)
// ... = X4 + 4
// This is still valid LLVM; the extra phi nodes are purely redundant, and will
// be trivially eliminated by InstCombine. The major benefit of this
// transformation is that it makes many other loop optimizations, such as
// LoopUnswitching, simpler.
#include "llvm/Transforms/Utils/LCSSA.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/BasicAliasAnalysis.h"
#include "llvm/Analysis/BranchProbabilityInfo.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/MemorySSA.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/PredIteratorCache.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Transforms/Utils.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
#include "llvm/Transforms/Utils/SSAUpdater.h"
using namespace llvm;
#define DEBUG_TYPE "lcssa"
STATISTIC(NumLCSSA, "Number of live out of a loop variables");
static bool VerifyLoopLCSSA = true;
static bool VerifyLoopLCSSA = false;
static cl::opt<bool, true>
VerifyLoopLCSSAFlag("verify-loop-lcssa", cl::location(VerifyLoopLCSSA),
cl::desc("Verify loop lcssa form (time consuming)"));
/// Return true if the specified block is in the list.
static bool isExitBlock(BasicBlock *BB,
const SmallVectorImpl<BasicBlock *> &ExitBlocks) {
return is_contained(ExitBlocks, BB);
/// For every instruction from the worklist, check to see if it has any uses
/// that are outside the current loop. If so, insert LCSSA PHI nodes and
/// rewrite the uses.
bool llvm::formLCSSAForInstructions(SmallVectorImpl<Instruction *> &Worklist,
const DominatorTree &DT, const LoopInfo &LI,
ScalarEvolution *SE, IRBuilderBase &Builder,
SmallVectorImpl<PHINode *> *PHIsToRemove) {
SmallVector<Use *, 16> UsesToRewrite;
SmallSetVector<PHINode *, 16> LocalPHIsToRemove;
PredIteratorCache PredCache;
bool Changed = false;
IRBuilderBase::InsertPointGuard InsertPtGuard(Builder);
// Cache the Loop ExitBlocks across this loop. We expect to get a lot of
// instructions within the same loops, computing the exit blocks is
// expensive, and we're not mutating the loop structure.
SmallDenseMap<Loop*, SmallVector<BasicBlock *,1>> LoopExitBlocks;
while (!Worklist.empty()) {
Instruction *I = Worklist.pop_back_val();
assert(!I->getType()->isTokenTy() && "Tokens shouldn't be in the worklist");
BasicBlock *InstBB = I->getParent();
Loop *L = LI.getLoopFor(InstBB);
assert(L && "Instruction belongs to a BB that's not part of a loop");
if (!LoopExitBlocks.count(L))
const SmallVectorImpl<BasicBlock *> &ExitBlocks = LoopExitBlocks[L];
if (ExitBlocks.empty())
for (Use &U : I->uses()) {
Instruction *User = cast<Instruction>(U.getUser());
BasicBlock *UserBB = User->getParent();
// For practical purposes, we consider that the use in a PHI
// occurs in the respective predecessor block. For more info,
// see the `phi` doc in LangRef and the LCSSA doc.
if (auto *PN = dyn_cast<PHINode>(User))
UserBB = PN->getIncomingBlock(U);
if (InstBB != UserBB && !L->contains(UserBB))
// If there are no uses outside the loop, exit with no change.
if (UsesToRewrite.empty())
++NumLCSSA; // We are applying the transformation
// Invoke instructions are special in that their result value is not
// available along their unwind edge. The code below tests to see whether
// DomBB dominates the value, so adjust DomBB to the normal destination
// block, which is effectively where the value is first usable.
BasicBlock *DomBB = InstBB;
if (auto *Inv = dyn_cast<InvokeInst>(I))
DomBB = Inv->getNormalDest();
const DomTreeNode *DomNode = DT.getNode(DomBB);
SmallVector<PHINode *, 16> AddedPHIs;
SmallVector<PHINode *, 8> PostProcessPHIs;
SmallVector<PHINode *, 4> InsertedPHIs;
SSAUpdater SSAUpdate(&InsertedPHIs);
SSAUpdate.Initialize(I->getType(), I->getName());
// Force re-computation of I, as some users now need to use the new PHI
// node.
if (SE)
// Insert the LCSSA phi's into all of the exit blocks dominated by the
// value, and add them to the Phi's map.
for (BasicBlock *ExitBB : ExitBlocks) {
if (!DT.dominates(DomNode, DT.getNode(ExitBB)))
// If we already inserted something for this BB, don't reprocess it.
if (SSAUpdate.HasValueForBlock(ExitBB))
PHINode *PN = Builder.CreatePHI(I->getType(), PredCache.size(ExitBB),
I->getName() + ".lcssa");
// Get the debug location from the original instruction.
// Add inputs from inside the loop for this PHI. This is valid
// because `I` dominates `ExitBB` (checked above). This implies
// that every incoming block/edge is dominated by `I` as well,
// i.e. we can add uses of `I` to those incoming edges/append to the incoming
// blocks without violating the SSA dominance property.
for (BasicBlock *Pred : PredCache.get(ExitBB)) {
PN->addIncoming(I, Pred);
// If the exit block has a predecessor not within the loop, arrange for
// the incoming value use corresponding to that predecessor to be
// rewritten in terms of a different LCSSA PHI.
if (!L->contains(Pred))
PN->getNumIncomingValues() - 1)));
// Remember that this phi makes the value alive in this block.
SSAUpdate.AddAvailableValue(ExitBB, PN);
// LoopSimplify might fail to simplify some loops (e.g. when indirect
// branches are involved). In such situations, it might happen that an
// exit for Loop L1 is the header of a disjoint Loop L2. Thus, when we
// create PHIs in such an exit block, we are also inserting PHIs into L2's
// header. This could break LCSSA form for L2 because these inserted PHIs
// can also have uses outside of L2. Remember all PHIs in such situation
// as to revisit than later on. FIXME: Remove this if indirectbr support
// into LoopSimplify gets improved.
if (auto *OtherLoop = LI.getLoopFor(ExitBB))
if (!L->contains(OtherLoop))
// Rewrite all uses outside the loop in terms of the new PHIs we just
// inserted.
for (Use *UseToRewrite : UsesToRewrite) {
Instruction *User = cast<Instruction>(UseToRewrite->getUser());
BasicBlock *UserBB = User->getParent();
// For practical purposes, we consider that the use in a PHI
// occurs in the respective predecessor block. For more info,
// see the `phi` doc in LangRef and the LCSSA doc.
if (auto *PN = dyn_cast<PHINode>(User))
UserBB = PN->getIncomingBlock(*UseToRewrite);
// If this use is in an exit block, rewrite to use the newly inserted PHI.
// This is required for correctness because SSAUpdate doesn't handle uses
// in the same block. It assumes the PHI we inserted is at the end of the
// block.
if (isa<PHINode>(UserBB->begin()) && isExitBlock(UserBB, ExitBlocks)) {
// If we added a single PHI, it must dominate all uses and we can directly
// rename it.
if (AddedPHIs.size() == 1) {
// Otherwise, do full PHI insertion.
SmallVector<DbgValueInst *, 4> DbgValues;
llvm::findDbgValues(DbgValues, I);
// Update pre-existing debug value uses that reside outside the loop.
for (auto DVI : DbgValues) {
BasicBlock *UserBB = DVI->getParent();
if (InstBB == UserBB || L->contains(UserBB))
// We currently only handle debug values residing in blocks that were
// traversed while rewriting the uses. If we inserted just a single PHI,
// we will handle all relevant debug values.
Value *V = AddedPHIs.size() == 1 ? AddedPHIs[0]
: SSAUpdate.FindValueForBlock(UserBB);
if (V)
DVI->replaceVariableLocationOp(I, V);
// SSAUpdater might have inserted phi-nodes inside other loops. We'll need
// to post-process them to keep LCSSA form.
for (PHINode *InsertedPN : InsertedPHIs) {
if (auto *OtherLoop = LI.getLoopFor(InsertedPN->getParent()))
if (!L->contains(OtherLoop))
// Post process PHI instructions that were inserted into another disjoint
// loop and update their exits properly.
for (auto *PostProcessPN : PostProcessPHIs)
if (!PostProcessPN->use_empty())
// Keep track of PHI nodes that we want to remove because they did not have
// any uses rewritten.
for (PHINode *PN : AddedPHIs)
if (PN->use_empty())
Changed = true;
// Remove PHI nodes that did not have any uses rewritten or add them to
// PHIsToRemove, so the caller can remove them after some additional cleanup.
// We need to redo the use_empty() check here, because even if the PHI node
// wasn't used when added to LocalPHIsToRemove, later added PHI nodes can be
// using it. This cleanup is not guaranteed to handle trees/cycles of PHI
// nodes that only are used by each other. Such situations has only been
// noticed when the input IR contains unreachable code, and leaving some extra
// redundant PHI nodes in such situations is considered a minor problem.
if (PHIsToRemove) {
PHIsToRemove->append(LocalPHIsToRemove.begin(), LocalPHIsToRemove.end());
} else {
for (PHINode *PN : LocalPHIsToRemove)
if (PN->use_empty())
return Changed;
// Compute the set of BasicBlocks in the loop `L` dominating at least one exit.
static void computeBlocksDominatingExits(
Loop &L, const DominatorTree &DT, SmallVector<BasicBlock *, 8> &ExitBlocks,
SmallSetVector<BasicBlock *, 8> &BlocksDominatingExits) {
// We start from the exit blocks, as every block trivially dominates itself
// (not strictly).
SmallVector<BasicBlock *, 8> BBWorklist(ExitBlocks);
while (!BBWorklist.empty()) {
BasicBlock *BB = BBWorklist.pop_back_val();
// Check if this is a loop header. If this is the case, we're done.
if (L.getHeader() == BB)
// Otherwise, add its immediate predecessor in the dominator tree to the
// worklist, unless we visited it already.
BasicBlock *IDomBB = DT.getNode(BB)->getIDom()->getBlock();
// Exit blocks can have an immediate dominator not beloinging to the
// loop. For an exit block to be immediately dominated by another block
// outside the loop, it implies not all paths from that dominator, to the
// exit block, go through the loop.
// Example:
// |---- A
// | |
// | B<--
// | | |
// |---> C --
// |
// D
// C is the exit block of the loop and it's immediately dominated by A,
// which doesn't belong to the loop.
if (!L.contains(IDomBB))
if (BlocksDominatingExits.insert(IDomBB))
bool llvm::formLCSSA(Loop &L, const DominatorTree &DT, const LoopInfo *LI,
ScalarEvolution *SE) {
bool Changed = false;
// Verify all sub-loops are in LCSSA form already.
for (Loop *SubLoop: L)
assert(SubLoop->isRecursivelyLCSSAForm(DT, *LI) && "Subloop not in LCSSA!");
SmallVector<BasicBlock *, 8> ExitBlocks;
if (ExitBlocks.empty())
return false;
SmallSetVector<BasicBlock *, 8> BlocksDominatingExits;
// We want to avoid use-scanning leveraging dominance informations.
// If a block doesn't dominate any of the loop exits, the none of the values
// defined in the loop can be used outside.
// We compute the set of blocks fullfilling the conditions in advance
// walking the dominator tree upwards until we hit a loop header.
computeBlocksDominatingExits(L, DT, ExitBlocks, BlocksDominatingExits);
SmallVector<Instruction *, 8> Worklist;
// Look at all the instructions in the loop, checking to see if they have uses
// outside the loop. If so, put them into the worklist to rewrite those uses.
for (BasicBlock *BB : BlocksDominatingExits) {
// Skip blocks that are part of any sub-loops, they must be in LCSSA
// already.
if (LI->getLoopFor(BB) != &L)
for (Instruction &I : *BB) {
// Reject two common cases fast: instructions with no uses (like stores)
// and instructions with one use that is in the same block as this.
if (I.use_empty() ||
(I.hasOneUse() && I.user_back()->getParent() == BB &&
// Tokens cannot be used in PHI nodes, so we skip over them.
// We can run into tokens which are live out of a loop with catchswitch
// instructions in Windows EH if the catchswitch has one catchpad which
// is inside the loop and another which is not.
if (I.getType()->isTokenTy())
IRBuilder<> Builder(L.getHeader()->getContext());
Changed = formLCSSAForInstructions(Worklist, DT, *LI, SE, Builder);
// If we modified the code, remove any caches about the loop from SCEV to
// avoid dangling entries.
// FIXME: This is a big hammer, can we clear the cache more selectively?
if (SE && Changed)
return Changed;
/// Process a loop nest depth first.
bool llvm::formLCSSARecursively(Loop &L, const DominatorTree &DT,
const LoopInfo *LI, ScalarEvolution *SE) {
bool Changed = false;
// Recurse depth-first through inner loops.
for (Loop *SubLoop : L.getSubLoops())
Changed |= formLCSSARecursively(*SubLoop, DT, LI, SE);
Changed |= formLCSSA(L, DT, LI, SE);
return Changed;
/// Process all loops in the function, inner-most out.
static bool formLCSSAOnAllLoops(const LoopInfo *LI, const DominatorTree &DT,
ScalarEvolution *SE) {
bool Changed = false;
for (auto &L : *LI)
Changed |= formLCSSARecursively(*L, DT, LI, SE);
return Changed;
namespace {
struct LCSSAWrapperPass : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
LCSSAWrapperPass() : FunctionPass(ID) {
// Cached analysis information for the current function.
DominatorTree *DT;
LoopInfo *LI;
ScalarEvolution *SE;
bool runOnFunction(Function &F) override;
void verifyAnalysis() const override {
// This check is very expensive. On the loop intensive compiles it may cause
// up to 10x slowdown. Currently it's disabled by default. LPPassManager
// always does limited form of the LCSSA verification. Similar reasoning
// was used for the LoopInfo verifier.
if (VerifyLoopLCSSA) {
[&](Loop *L) {
return L->isRecursivelyLCSSAForm(*DT, *LI);
}) &&
"LCSSA form is broken!");
/// This transformation requires natural loop information & requires that
/// loop preheaders be inserted into the CFG. It maintains both of these,
/// as well as the CFG. It also requires dominator information.
void getAnalysisUsage(AnalysisUsage &AU) const override {
// This is needed to perform LCSSA verification inside LPPassManager
char LCSSAWrapperPass::ID = 0;
INITIALIZE_PASS_BEGIN(LCSSAWrapperPass, "lcssa", "Loop-Closed SSA Form Pass",
false, false)
INITIALIZE_PASS_END(LCSSAWrapperPass, "lcssa", "Loop-Closed SSA Form Pass",
false, false)
Pass *llvm::createLCSSAPass() { return new LCSSAWrapperPass(); }
char &llvm::LCSSAID = LCSSAWrapperPass::ID;
/// Transform \p F into loop-closed SSA form.
bool LCSSAWrapperPass::runOnFunction(Function &F) {
LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>();
SE = SEWP ? &SEWP->getSE() : nullptr;
return formLCSSAOnAllLoops(LI, *DT, SE);
PreservedAnalyses LCSSAPass::run(Function &F, FunctionAnalysisManager &AM) {
auto &LI = AM.getResult<LoopAnalysis>(F);
auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
auto *SE = AM.getCachedResult<ScalarEvolutionAnalysis>(F);
if (!formLCSSAOnAllLoops(&LI, DT, SE))
return PreservedAnalyses::all();
PreservedAnalyses PA;
// BPI maps terminators to probabilities, since we don't modify the CFG, no
// updates are needed to preserve it.
return PA;