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//===- LoopSimplify.cpp - Loop Canonicalization Pass ----------------------===//
// 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 performs several transformations to transform natural loops into a
// simpler form, which makes subsequent analyses and transformations simpler and
// more effective.
// Loop pre-header insertion guarantees that there is a single, non-critical
// entry edge from outside of the loop to the loop header. This simplifies a
// number of analyses and transformations, such as LICM.
// Loop exit-block insertion guarantees that all exit blocks from the loop
// (blocks which are outside of the loop that have predecessors inside of the
// loop) only have predecessors from inside of the loop (and are thus dominated
// by the loop header). This simplifies transformations such as store-sinking
// that are built into LICM.
// This pass also guarantees that loops will have exactly one backedge.
// Indirectbr instructions introduce several complications. If the loop
// contains or is entered by an indirectbr instruction, it may not be possible
// to transform the loop and make these guarantees. Client code should check
// that these conditions are true before relying on them.
// Similar complications arise from callbr instructions, particularly in
// asm-goto where blockaddress expressions are used.
// Note that the simplifycfg pass will clean up blocks which are split out but
// end up being unnecessary, so usage of this pass should not pessimize
// generated code.
// This pass obviously modifies the CFG, but updates loop information and
// dominator information.
#include "llvm/Transforms/Utils/LoopSimplify.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/SetOperations.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/BasicAliasAnalysis.h"
#include "llvm/Analysis/BranchProbabilityInfo.h"
#include "llvm/Analysis/DependenceAnalysis.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/MemorySSA.h"
#include "llvm/Analysis/MemorySSAUpdater.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Type.h"
#include "llvm/InitializePasses.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
using namespace llvm;
#define DEBUG_TYPE "loop-simplify"
STATISTIC(NumNested , "Number of nested loops split out");
// If the block isn't already, move the new block to right after some 'outside
// block' block. This prevents the preheader from being placed inside the loop
// body, e.g. when the loop hasn't been rotated.
static void placeSplitBlockCarefully(BasicBlock *NewBB,
SmallVectorImpl<BasicBlock *> &SplitPreds,
Loop *L) {
// Check to see if NewBB is already well placed.
Function::iterator BBI = --NewBB->getIterator();
for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
if (&*BBI == SplitPreds[i])
// If it isn't already after an outside block, move it after one. This is
// always good as it makes the uncond branch from the outside block into a
// fall-through.
// Figure out *which* outside block to put this after. Prefer an outside
// block that neighbors a BB actually in the loop.
BasicBlock *FoundBB = nullptr;
for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
Function::iterator BBI = SplitPreds[i]->getIterator();
if (++BBI != NewBB->getParent()->end() && L->contains(&*BBI)) {
FoundBB = SplitPreds[i];
// If our heuristic for a *good* bb to place this after doesn't find
// anything, just pick something. It's likely better than leaving it within
// the loop.
if (!FoundBB)
FoundBB = SplitPreds[0];
/// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
/// preheader, this method is called to insert one. This method has two phases:
/// preheader insertion and analysis updating.
BasicBlock *llvm::InsertPreheaderForLoop(Loop *L, DominatorTree *DT,
LoopInfo *LI, MemorySSAUpdater *MSSAU,
bool PreserveLCSSA) {
BasicBlock *Header = L->getHeader();
// Compute the set of predecessors of the loop that are not in the loop.
SmallVector<BasicBlock*, 8> OutsideBlocks;
for (BasicBlock *P : predecessors(Header)) {
if (!L->contains(P)) { // Coming in from outside the loop?
// If the loop is branched to from an indirect terminator, we won't
// be able to fully transform the loop, because it prohibits
// edge splitting.
if (P->getTerminator()->isIndirectTerminator())
return nullptr;
// Keep track of it.
// Split out the loop pre-header.
BasicBlock *PreheaderBB;
PreheaderBB = SplitBlockPredecessors(Header, OutsideBlocks, ".preheader", DT,
LI, MSSAU, PreserveLCSSA);
if (!PreheaderBB)
return nullptr;
LLVM_DEBUG(dbgs() << "LoopSimplify: Creating pre-header "
<< PreheaderBB->getName() << "\n");
// Make sure that NewBB is put someplace intelligent, which doesn't mess up
// code layout too horribly.
placeSplitBlockCarefully(PreheaderBB, OutsideBlocks, L);
return PreheaderBB;
/// Add the specified block, and all of its predecessors, to the specified set,
/// if it's not already in there. Stop predecessor traversal when we reach
/// StopBlock.
static void addBlockAndPredsToSet(BasicBlock *InputBB, BasicBlock *StopBlock,
SmallPtrSetImpl<BasicBlock *> &Blocks) {
SmallVector<BasicBlock *, 8> Worklist;
do {
BasicBlock *BB = Worklist.pop_back_val();
if (Blocks.insert(BB).second && BB != StopBlock)
// If BB is not already processed and it is not a stop block then
// insert its predecessor in the work list
append_range(Worklist, predecessors(BB));
} while (!Worklist.empty());
/// The first part of loop-nestification is to find a PHI node that tells
/// us how to partition the loops.
static PHINode *findPHIToPartitionLoops(Loop *L, DominatorTree *DT,
AssumptionCache *AC) {
const DataLayout &DL = L->getHeader()->getModule()->getDataLayout();
for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) {
PHINode *PN = cast<PHINode>(I);
if (Value *V = SimplifyInstruction(PN, {DL, nullptr, DT, AC})) {
// This is a degenerate PHI already, don't modify it!
// Scan this PHI node looking for a use of the PHI node by itself.
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
if (PN->getIncomingValue(i) == PN &&
// We found something tasty to remove.
return PN;
return nullptr;
/// If this loop has multiple backedges, try to pull one of them out into
/// a nested loop.
/// This is important for code that looks like
/// this:
/// Loop:
/// ...
/// br cond, Loop, Next
/// ...
/// br cond2, Loop, Out
/// To identify this common case, we look at the PHI nodes in the header of the
/// loop. PHI nodes with unchanging values on one backedge correspond to values
/// that change in the "outer" loop, but not in the "inner" loop.
/// If we are able to separate out a loop, return the new outer loop that was
/// created.
static Loop *separateNestedLoop(Loop *L, BasicBlock *Preheader,
DominatorTree *DT, LoopInfo *LI,
ScalarEvolution *SE, bool PreserveLCSSA,
AssumptionCache *AC, MemorySSAUpdater *MSSAU) {
// Don't try to separate loops without a preheader.
if (!Preheader)
return nullptr;
// Treat the presence of convergent functions conservatively. The
// transformation is invalid if calls to certain convergent
// functions (like an AMDGPU barrier) get included in the resulting
// inner loop. But blocks meant for the inner loop will be
// identified later at a point where it's too late to abort the
// transformation. Also, the convergent attribute is not really
// sufficient to express the semantics of functions that are
// affected by this transformation. So we choose to back off if such
// a function call is present until a better alternative becomes
// available. This is similar to the conservative treatment of
// convergent function calls in GVNHoist and JumpThreading.
for (auto BB : L->blocks()) {
for (auto &II : *BB) {
if (auto CI = dyn_cast<CallBase>(&II)) {
if (CI->isConvergent()) {
return nullptr;
// The header is not a landing pad; preheader insertion should ensure this.
BasicBlock *Header = L->getHeader();
assert(!Header->isEHPad() && "Can't insert backedge to EH pad");
PHINode *PN = findPHIToPartitionLoops(L, DT, AC);
if (!PN) return nullptr; // No known way to partition.
// Pull out all predecessors that have varying values in the loop. This
// handles the case when a PHI node has multiple instances of itself as
// arguments.
SmallVector<BasicBlock*, 8> OuterLoopPreds;
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
if (PN->getIncomingValue(i) != PN ||
!L->contains(PN->getIncomingBlock(i))) {
// We can't split indirect control flow edges.
if (PN->getIncomingBlock(i)->getTerminator()->isIndirectTerminator())
return nullptr;
LLVM_DEBUG(dbgs() << "LoopSimplify: Splitting out a new outer loop\n");
// If ScalarEvolution is around and knows anything about values in
// this loop, tell it to forget them, because we're about to
// substantially change it.
if (SE)
BasicBlock *NewBB = SplitBlockPredecessors(Header, OuterLoopPreds, ".outer",
DT, LI, MSSAU, PreserveLCSSA);
// Make sure that NewBB is put someplace intelligent, which doesn't mess up
// code layout too horribly.
placeSplitBlockCarefully(NewBB, OuterLoopPreds, L);
// Create the new outer loop.
Loop *NewOuter = LI->AllocateLoop();
// Change the parent loop to use the outer loop as its child now.
if (Loop *Parent = L->getParentLoop())
Parent->replaceChildLoopWith(L, NewOuter);
LI->changeTopLevelLoop(L, NewOuter);
// L is now a subloop of our outer loop.
for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
I != E; ++I)
// Now reset the header in L, which had been moved by
// SplitBlockPredecessors for the outer loop.
// Determine which blocks should stay in L and which should be moved out to
// the Outer loop now.
SmallPtrSet<BasicBlock *, 4> BlocksInL;
for (BasicBlock *P : predecessors(Header)) {
if (DT->dominates(Header, P))
addBlockAndPredsToSet(P, Header, BlocksInL);
// Scan all of the loop children of L, moving them to OuterLoop if they are
// not part of the inner loop.
const std::vector<Loop*> &SubLoops = L->getSubLoops();
for (size_t I = 0; I != SubLoops.size(); )
if (BlocksInL.count(SubLoops[I]->getHeader()))
++I; // Loop remains in L
NewOuter->addChildLoop(L->removeChildLoop(SubLoops.begin() + I));
SmallVector<BasicBlock *, 8> OuterLoopBlocks;
// Now that we know which blocks are in L and which need to be moved to
// OuterLoop, move any blocks that need it.
for (unsigned i = 0; i != L->getBlocks().size(); ++i) {
BasicBlock *BB = L->getBlocks()[i];
if (!BlocksInL.count(BB)) {
// Move this block to the parent, updating the exit blocks sets
if ((*LI)[BB] == L) {
LI->changeLoopFor(BB, NewOuter);
// Split edges to exit blocks from the inner loop, if they emerged in the
// process of separating the outer one.
formDedicatedExitBlocks(L, DT, LI, MSSAU, PreserveLCSSA);
if (PreserveLCSSA) {
// Fix LCSSA form for L. Some values, which previously were only used inside
// L, can now be used in NewOuter loop. We need to insert phi-nodes for them
// in corresponding exit blocks.
// We don't need to form LCSSA recursively, because there cannot be uses
// inside a newly created loop of defs from inner loops as those would
// already be a use of an LCSSA phi node.
formLCSSA(*L, *DT, LI, SE);
assert(NewOuter->isRecursivelyLCSSAForm(*DT, *LI) &&
"LCSSA is broken after separating nested loops!");
return NewOuter;
/// This method is called when the specified loop has more than one
/// backedge in it.
/// If this occurs, revector all of these backedges to target a new basic block
/// and have that block branch to the loop header. This ensures that loops
/// have exactly one backedge.
static BasicBlock *insertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader,
DominatorTree *DT, LoopInfo *LI,
MemorySSAUpdater *MSSAU) {
assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!");
// Get information about the loop
BasicBlock *Header = L->getHeader();
Function *F = Header->getParent();
// Unique backedge insertion currently depends on having a preheader.
if (!Preheader)
return nullptr;
// The header is not an EH pad; preheader insertion should ensure this.
assert(!Header->isEHPad() && "Can't insert backedge to EH pad");
// Figure out which basic blocks contain back-edges to the loop header.
std::vector<BasicBlock*> BackedgeBlocks;
for (BasicBlock *P : predecessors(Header)) {
// Indirect edges cannot be split, so we must fail if we find one.
if (P->getTerminator()->isIndirectTerminator())
return nullptr;
if (P != Preheader) BackedgeBlocks.push_back(P);
// Create and insert the new backedge block...
BasicBlock *BEBlock = BasicBlock::Create(Header->getContext(),
Header->getName() + ".backedge", F);
BranchInst *BETerminator = BranchInst::Create(Header, BEBlock);
LLVM_DEBUG(dbgs() << "LoopSimplify: Inserting unique backedge block "
<< BEBlock->getName() << "\n");
// Move the new backedge block to right after the last backedge block.
Function::iterator InsertPos = ++BackedgeBlocks.back()->getIterator();
F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock);
// Now that the block has been inserted into the function, create PHI nodes in
// the backedge block which correspond to any PHI nodes in the header block.
for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
PHINode *PN = cast<PHINode>(I);
PHINode *NewPN = PHINode::Create(PN->getType(), BackedgeBlocks.size(),
PN->getName()+".be", BETerminator);
// Loop over the PHI node, moving all entries except the one for the
// preheader over to the new PHI node.
unsigned PreheaderIdx = ~0U;
bool HasUniqueIncomingValue = true;
Value *UniqueValue = nullptr;
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
BasicBlock *IBB = PN->getIncomingBlock(i);
Value *IV = PN->getIncomingValue(i);
if (IBB == Preheader) {
PreheaderIdx = i;
} else {
NewPN->addIncoming(IV, IBB);
if (HasUniqueIncomingValue) {
if (!UniqueValue)
UniqueValue = IV;
else if (UniqueValue != IV)
HasUniqueIncomingValue = false;
// Delete all of the incoming values from the old PN except the preheader's
assert(PreheaderIdx != ~0U && "PHI has no preheader entry??");
if (PreheaderIdx != 0) {
PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx));
PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx));
// Nuke all entries except the zero'th.
for (unsigned i = 0, e = PN->getNumIncomingValues()-1; i != e; ++i)
PN->removeIncomingValue(e-i, false);
// Finally, add the newly constructed PHI node as the entry for the BEBlock.
PN->addIncoming(NewPN, BEBlock);
// As an optimization, if all incoming values in the new PhiNode (which is a
// subset of the incoming values of the old PHI node) have the same value,
// eliminate the PHI Node.
if (HasUniqueIncomingValue) {
// Now that all of the PHI nodes have been inserted and adjusted, modify the
// backedge blocks to jump to the BEBlock instead of the header.
// If one of the backedges has llvm.loop metadata attached, we remove
// it from the backedge and add it to BEBlock.
unsigned LoopMDKind = BEBlock->getContext().getMDKindID("llvm.loop");
MDNode *LoopMD = nullptr;
for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) {
Instruction *TI = BackedgeBlocks[i]->getTerminator();
if (!LoopMD)
LoopMD = TI->getMetadata(LoopMDKind);
TI->setMetadata(LoopMDKind, nullptr);
TI->replaceSuccessorWith(Header, BEBlock);
BEBlock->getTerminator()->setMetadata(LoopMDKind, LoopMD);
//===--- Update all analyses which we must preserve now -----------------===//
// Update Loop Information - we know that this block is now in the current
// loop and all parent loops.
L->addBasicBlockToLoop(BEBlock, *LI);
// Update dominator information
if (MSSAU)
MSSAU->updatePhisWhenInsertingUniqueBackedgeBlock(Header, Preheader,
return BEBlock;
/// Simplify one loop and queue further loops for simplification.
static bool simplifyOneLoop(Loop *L, SmallVectorImpl<Loop *> &Worklist,
DominatorTree *DT, LoopInfo *LI,
ScalarEvolution *SE, AssumptionCache *AC,
MemorySSAUpdater *MSSAU, bool PreserveLCSSA) {
bool Changed = false;
if (MSSAU && VerifyMemorySSA)
// Check to see that no blocks (other than the header) in this loop have
// predecessors that are not in the loop. This is not valid for natural
// loops, but can occur if the blocks are unreachable. Since they are
// unreachable we can just shamelessly delete those CFG edges!
for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
BB != E; ++BB) {
if (*BB == L->getHeader()) continue;
SmallPtrSet<BasicBlock*, 4> BadPreds;
for (BasicBlock *P : predecessors(*BB))
if (!L->contains(P))
// Delete each unique out-of-loop (and thus dead) predecessor.
for (BasicBlock *P : BadPreds) {
LLVM_DEBUG(dbgs() << "LoopSimplify: Deleting edge from dead predecessor "
<< P->getName() << "\n");
// Zap the dead pred's terminator and replace it with unreachable.
Instruction *TI = P->getTerminator();
changeToUnreachable(TI, /*UseLLVMTrap=*/false, PreserveLCSSA,
/*DTU=*/nullptr, MSSAU);
Changed = true;
if (MSSAU && VerifyMemorySSA)
// If there are exiting blocks with branches on undef, resolve the undef in
// the direction which will exit the loop. This will help simplify loop
// trip count computations.
SmallVector<BasicBlock*, 8> ExitingBlocks;
for (BasicBlock *ExitingBlock : ExitingBlocks)
if (BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator()))
if (BI->isConditional()) {
if (UndefValue *Cond = dyn_cast<UndefValue>(BI->getCondition())) {
<< "LoopSimplify: Resolving \"br i1 undef\" to exit in "
<< ExitingBlock->getName() << "\n");
Changed = true;
// Does the loop already have a preheader? If so, don't insert one.
BasicBlock *Preheader = L->getLoopPreheader();
if (!Preheader) {
Preheader = InsertPreheaderForLoop(L, DT, LI, MSSAU, PreserveLCSSA);
if (Preheader)
Changed = true;
// Next, check to make sure that all exit nodes of the loop only have
// predecessors that are inside of the loop. This check guarantees that the
// loop preheader/header will dominate the exit blocks. If the exit block has
// predecessors from outside of the loop, split the edge now.
if (formDedicatedExitBlocks(L, DT, LI, MSSAU, PreserveLCSSA))
Changed = true;
if (MSSAU && VerifyMemorySSA)
// If the header has more than two predecessors at this point (from the
// preheader and from multiple backedges), we must adjust the loop.
BasicBlock *LoopLatch = L->getLoopLatch();
if (!LoopLatch) {
// If this is really a nested loop, rip it out into a child loop. Don't do
// this for loops with a giant number of backedges, just factor them into a
// common backedge instead.
if (L->getNumBackEdges() < 8) {
if (Loop *OuterL = separateNestedLoop(L, Preheader, DT, LI, SE,
PreserveLCSSA, AC, MSSAU)) {
// Enqueue the outer loop as it should be processed next in our
// depth-first nest walk.
// This is a big restructuring change, reprocess the whole loop.
Changed = true;
// GCC doesn't tail recursion eliminate this.
// FIXME: It isn't clear we can't rely on LLVM to TRE this.
goto ReprocessLoop;
// If we either couldn't, or didn't want to, identify nesting of the loops,
// insert a new block that all backedges target, then make it jump to the
// loop header.
LoopLatch = insertUniqueBackedgeBlock(L, Preheader, DT, LI, MSSAU);
if (LoopLatch)
Changed = true;
if (MSSAU && VerifyMemorySSA)
const DataLayout &DL = L->getHeader()->getModule()->getDataLayout();
// Scan over the PHI nodes in the loop header. Since they now have only two
// incoming values (the loop is canonicalized), we may have simplified the PHI
// down to 'X = phi [X, Y]', which should be replaced with 'Y'.
PHINode *PN;
for (BasicBlock::iterator I = L->getHeader()->begin();
(PN = dyn_cast<PHINode>(I++)); )
if (Value *V = SimplifyInstruction(PN, {DL, nullptr, DT, AC})) {
if (SE) SE->forgetValue(PN);
if (!PreserveLCSSA || LI->replacementPreservesLCSSAForm(PN, V)) {
Changed = true;
// If this loop has multiple exits and the exits all go to the same
// block, attempt to merge the exits. This helps several passes, such
// as LoopRotation, which do not support loops with multiple exits.
// SimplifyCFG also does this (and this code uses the same utility
// function), however this code is loop-aware, where SimplifyCFG is
// not. That gives it the advantage of being able to hoist
// loop-invariant instructions out of the way to open up more
// opportunities, and the disadvantage of having the responsibility
// to preserve dominator information.
auto HasUniqueExitBlock = [&]() {
BasicBlock *UniqueExit = nullptr;
for (auto *ExitingBB : ExitingBlocks)
for (auto *SuccBB : successors(ExitingBB)) {
if (L->contains(SuccBB))
if (!UniqueExit)
UniqueExit = SuccBB;
else if (UniqueExit != SuccBB)
return false;
return true;
if (HasUniqueExitBlock()) {
for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
BasicBlock *ExitingBlock = ExitingBlocks[i];
if (!ExitingBlock->getSinglePredecessor()) continue;
BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator());
if (!BI || !BI->isConditional()) continue;
CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition());
if (!CI || CI->getParent() != ExitingBlock) continue;
// Attempt to hoist out all instructions except for the
// comparison and the branch.
bool AllInvariant = true;
bool AnyInvariant = false;
for (auto I = ExitingBlock->instructionsWithoutDebug().begin(); &*I != BI; ) {
Instruction *Inst = &*I++;
if (Inst == CI)
if (!L->makeLoopInvariant(
Inst, AnyInvariant,
Preheader ? Preheader->getTerminator() : nullptr, MSSAU)) {
AllInvariant = false;
if (AnyInvariant) {
Changed = true;
// The loop disposition of all SCEV expressions that depend on any
// hoisted values have also changed.
if (SE)
if (!AllInvariant) continue;
// The block has now been cleared of all instructions except for
// a comparison and a conditional branch. SimplifyCFG may be able
// to fold it now.
if (!FoldBranchToCommonDest(BI, /*DTU=*/nullptr, MSSAU))
// Success. The block is now dead, so remove it from the loop,
// update the dominator tree and delete it.
LLVM_DEBUG(dbgs() << "LoopSimplify: Eliminating exiting block "
<< ExitingBlock->getName() << "\n");
Changed = true;
DomTreeNode *Node = DT->getNode(ExitingBlock);
while (!Node->isLeaf()) {
DomTreeNode *Child = Node->back();
DT->changeImmediateDominator(Child, Node->getIDom());
if (MSSAU) {
SmallSetVector<BasicBlock *, 8> ExitBlockSet;
ExitingBlock, /* KeepOneInputPHIs */ PreserveLCSSA);
ExitingBlock, /* KeepOneInputPHIs */ PreserveLCSSA);
// Changing exit conditions for blocks may affect exit counts of this loop and
// any of its paretns, so we must invalidate the entire subtree if we've made
// any changes.
if (Changed && SE)
if (MSSAU && VerifyMemorySSA)
return Changed;
bool llvm::simplifyLoop(Loop *L, DominatorTree *DT, LoopInfo *LI,
ScalarEvolution *SE, AssumptionCache *AC,
MemorySSAUpdater *MSSAU, bool PreserveLCSSA) {
bool Changed = false;
#ifndef NDEBUG
// If we're asked to preserve LCSSA, the loop nest needs to start in LCSSA
// form.
if (PreserveLCSSA) {
assert(DT && "DT not available.");
assert(LI && "LI not available.");
assert(L->isRecursivelyLCSSAForm(*DT, *LI) &&
"Requested to preserve LCSSA, but it's already broken.");
// Worklist maintains our depth-first queue of loops in this nest to process.
SmallVector<Loop *, 4> Worklist;
// Walk the worklist from front to back, pushing newly found sub loops onto
// the back. This will let us process loops from back to front in depth-first
// order. We can use this simple process because loops form a tree.
for (unsigned Idx = 0; Idx != Worklist.size(); ++Idx) {
Loop *L2 = Worklist[Idx];
Worklist.append(L2->begin(), L2->end());
while (!Worklist.empty())
Changed |= simplifyOneLoop(Worklist.pop_back_val(), Worklist, DT, LI, SE,
AC, MSSAU, PreserveLCSSA);
return Changed;
namespace {
struct LoopSimplify : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
LoopSimplify() : FunctionPass(ID) {
bool runOnFunction(Function &F) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
// We need loop information to identify the loops...
AU.addPreservedID(BreakCriticalEdgesID); // No critical edges added.
if (EnableMSSALoopDependency)
/// verifyAnalysis() - Verify LoopSimplifyForm's guarantees.
void verifyAnalysis() const override;
char LoopSimplify::ID = 0;
INITIALIZE_PASS_BEGIN(LoopSimplify, "loop-simplify",
"Canonicalize natural loops", false, false)
INITIALIZE_PASS_END(LoopSimplify, "loop-simplify",
"Canonicalize natural loops", false, false)
// Publicly exposed interface to pass...
char &llvm::LoopSimplifyID = LoopSimplify::ID;
Pass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); }
/// runOnFunction - Run down all loops in the CFG (recursively, but we could do
/// it in any convenient order) inserting preheaders...
bool LoopSimplify::runOnFunction(Function &F) {
bool Changed = false;
LoopInfo *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
DominatorTree *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>();
ScalarEvolution *SE = SEWP ? &SEWP->getSE() : nullptr;
AssumptionCache *AC =
MemorySSA *MSSA = nullptr;
std::unique_ptr<MemorySSAUpdater> MSSAU;
if (EnableMSSALoopDependency) {
auto *MSSAAnalysis = getAnalysisIfAvailable<MemorySSAWrapperPass>();
if (MSSAAnalysis) {
MSSA = &MSSAAnalysis->getMSSA();
MSSAU = std::make_unique<MemorySSAUpdater>(MSSA);
bool PreserveLCSSA = mustPreserveAnalysisID(LCSSAID);
// Simplify each loop nest in the function.
for (auto *L : *LI)
Changed |= simplifyLoop(L, DT, LI, SE, AC, MSSAU.get(), PreserveLCSSA);
#ifndef NDEBUG
if (PreserveLCSSA) {
bool InLCSSA = all_of(
*LI, [&](Loop *L) { return L->isRecursivelyLCSSAForm(*DT, *LI); });
assert(InLCSSA && "LCSSA is broken after loop-simplify.");
return Changed;
PreservedAnalyses LoopSimplifyPass::run(Function &F,
FunctionAnalysisManager &AM) {
bool Changed = false;
LoopInfo *LI = &AM.getResult<LoopAnalysis>(F);
DominatorTree *DT = &AM.getResult<DominatorTreeAnalysis>(F);
ScalarEvolution *SE = AM.getCachedResult<ScalarEvolutionAnalysis>(F);
AssumptionCache *AC = &AM.getResult<AssumptionAnalysis>(F);
auto *MSSAAnalysis = AM.getCachedResult<MemorySSAAnalysis>(F);
std::unique_ptr<MemorySSAUpdater> MSSAU;
if (MSSAAnalysis) {
auto *MSSA = &MSSAAnalysis->getMSSA();
MSSAU = std::make_unique<MemorySSAUpdater>(MSSA);
// Note that we don't preserve LCSSA in the new PM, if you need it run LCSSA
// after simplifying the loops. MemorySSA is preserved if it exists.
for (auto *L : *LI)
Changed |=
simplifyLoop(L, DT, LI, SE, AC, MSSAU.get(), /*PreserveLCSSA*/ false);
if (!Changed)
return PreservedAnalyses::all();
PreservedAnalyses PA;
if (MSSAAnalysis)
// BPI maps conditional terminators to probabilities, LoopSimplify can insert
// blocks, but it does so only by splitting existing blocks and edges. This
// results in the interesting property that all new terminators inserted are
// unconditional branches which do not appear in BPI. All deletions are
// handled via ValueHandle callbacks w/in BPI.
return PA;
// FIXME: Restore this code when we re-enable verification in verifyAnalysis
// below.
#if 0
static void verifyLoop(Loop *L) {
// Verify subloops.
for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
// It used to be possible to just assert L->isLoopSimplifyForm(), however
// with the introduction of indirectbr, there are now cases where it's
// not possible to transform a loop as necessary. We can at least check
// that there is an indirectbr near any time there's trouble.
// Indirectbr can interfere with preheader and unique backedge insertion.
if (!L->getLoopPreheader() || !L->getLoopLatch()) {
bool HasIndBrPred = false;
for (BasicBlock *Pred : predecessors(L->getHeader()))
if (isa<IndirectBrInst>(Pred->getTerminator())) {
HasIndBrPred = true;
assert(HasIndBrPred &&
"LoopSimplify has no excuse for missing loop header info!");
// Indirectbr can interfere with exit block canonicalization.
if (!L->hasDedicatedExits()) {
bool HasIndBrExiting = false;
SmallVector<BasicBlock*, 8> ExitingBlocks;
for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
if (isa<IndirectBrInst>((ExitingBlocks[i])->getTerminator())) {
HasIndBrExiting = true;
assert(HasIndBrExiting &&
"LoopSimplify has no excuse for missing exit block info!");
void LoopSimplify::verifyAnalysis() const {
// FIXME: This routine is being called mid-way through the loop pass manager
// as loop passes destroy this analysis. That's actually fine, but we have no
// way of expressing that here. Once all of the passes that destroy this are
// hoisted out of the loop pass manager we can add back verification here.
#if 0
for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)