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llvm-project/llvm/lib/Transforms/Scalar/LICM.cpp

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//===-- LICM.cpp - Loop Invariant Code Motion Pass ------------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass performs loop invariant code motion, attempting to remove as much
// code from the body of a loop as possible. It does this by either hoisting
// code into the preheader block, or by sinking code to the exit blocks if it is
// safe. This pass also promotes must-aliased memory locations in the loop to
// live in registers, thus hoisting and sinking "invariant" loads and stores.
//
// This pass uses alias analysis for two purposes:
//
// 1. Moving loop invariant loads and calls out of loops. If we can determine
// that a load or call inside of a loop never aliases anything stored to,
// we can hoist it or sink it like any other instruction.
// 2. Scalar Promotion of Memory - If there is a store instruction inside of
// the loop, we try to move the store to happen AFTER the loop instead of
// inside of the loop. This can only happen if a few conditions are true:
// A. The pointer stored through is loop invariant
// B. There are no stores or loads in the loop which _may_ alias the
// pointer. There are no calls in the loop which mod/ref the pointer.
// If these conditions are true, we can promote the loads and stores in the
// loop of the pointer to use a temporary alloca'd variable. We then use
// the SSAUpdater to construct the appropriate SSA form for the value.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar/LICM.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/AliasSetTracker.h"
#include "llvm/Analysis/BasicAliasAnalysis.h"
#include "llvm/Analysis/CaptureTracking.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/Loads.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/Analysis/MemorySSA.h"
#include "llvm/Analysis/OptimizationRemarkEmitter.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Metadata.h"
#include "llvm/IR/PredIteratorCache.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Scalar/LoopPassManager.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
#include "llvm/Transforms/Utils/SSAUpdater.h"
#include <algorithm>
#include <utility>
using namespace llvm;
#define DEBUG_TYPE "licm"
STATISTIC(NumSunk, "Number of instructions sunk out of loop");
STATISTIC(NumHoisted, "Number of instructions hoisted out of loop");
STATISTIC(NumMovedLoads, "Number of load insts hoisted or sunk");
STATISTIC(NumMovedCalls, "Number of call insts hoisted or sunk");
STATISTIC(NumPromoted, "Number of memory locations promoted to registers");
/// Memory promotion is enabled by default.
static cl::opt<bool>
DisablePromotion("disable-licm-promotion", cl::Hidden, cl::init(false),
cl::desc("Disable memory promotion in LICM pass"));
static cl::opt<uint32_t> MaxNumUsesTraversed(
"licm-max-num-uses-traversed", cl::Hidden, cl::init(8),
cl::desc("Max num uses visited for identifying load "
"invariance in loop using invariant start (default = 8)"));
static bool inSubLoop(BasicBlock *BB, Loop *CurLoop, LoopInfo *LI);
static bool isNotUsedOrFreeInLoop(const Instruction &I, const Loop *CurLoop,
const LoopSafetyInfo *SafetyInfo,
TargetTransformInfo *TTI, bool &FreeInLoop);
static bool hoist(Instruction &I, const DominatorTree *DT, const Loop *CurLoop,
const LoopSafetyInfo *SafetyInfo,
OptimizationRemarkEmitter *ORE);
static bool sink(Instruction &I, LoopInfo *LI, DominatorTree *DT,
const Loop *CurLoop, LoopSafetyInfo *SafetyInfo,
OptimizationRemarkEmitter *ORE, bool FreeInLoop);
static bool isSafeToExecuteUnconditionally(Instruction &Inst,
const DominatorTree *DT,
const Loop *CurLoop,
const LoopSafetyInfo *SafetyInfo,
OptimizationRemarkEmitter *ORE,
const Instruction *CtxI = nullptr);
static bool pointerInvalidatedByLoop(Value *V, uint64_t Size,
const AAMDNodes &AAInfo,
AliasSetTracker *CurAST);
static Instruction *
CloneInstructionInExitBlock(Instruction &I, BasicBlock &ExitBlock, PHINode &PN,
const LoopInfo *LI,
const LoopSafetyInfo *SafetyInfo);
namespace {
struct LoopInvariantCodeMotion {
bool runOnLoop(Loop *L, AliasAnalysis *AA, LoopInfo *LI, DominatorTree *DT,
TargetLibraryInfo *TLI, TargetTransformInfo *TTI,
ScalarEvolution *SE, MemorySSA *MSSA,
OptimizationRemarkEmitter *ORE, bool DeleteAST);
DenseMap<Loop *, AliasSetTracker *> &getLoopToAliasSetMap() {
return LoopToAliasSetMap;
}
private:
DenseMap<Loop *, AliasSetTracker *> LoopToAliasSetMap;
AliasSetTracker *collectAliasInfoForLoop(Loop *L, LoopInfo *LI,
AliasAnalysis *AA);
};
struct LegacyLICMPass : public LoopPass {
static char ID; // Pass identification, replacement for typeid
LegacyLICMPass() : LoopPass(ID) {
initializeLegacyLICMPassPass(*PassRegistry::getPassRegistry());
}
bool runOnLoop(Loop *L, LPPassManager &LPM) override {
if (skipLoop(L)) {
// If we have run LICM on a previous loop but now we are skipping
// (because we've hit the opt-bisect limit), we need to clear the
// loop alias information.
for (auto &LTAS : LICM.getLoopToAliasSetMap())
delete LTAS.second;
LICM.getLoopToAliasSetMap().clear();
return false;
}
auto *SE = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>();
MemorySSA *MSSA = EnableMSSALoopDependency
? (&getAnalysis<MemorySSAWrapperPass>().getMSSA())
: nullptr;
// For the old PM, we can't use OptimizationRemarkEmitter as an analysis
// pass. Function analyses need to be preserved across loop transformations
// but ORE cannot be preserved (see comment before the pass definition).
OptimizationRemarkEmitter ORE(L->getHeader()->getParent());
return LICM.runOnLoop(L,
&getAnalysis<AAResultsWrapperPass>().getAAResults(),
&getAnalysis<LoopInfoWrapperPass>().getLoopInfo(),
&getAnalysis<DominatorTreeWrapperPass>().getDomTree(),
&getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(),
&getAnalysis<TargetTransformInfoWrapperPass>().getTTI(
*L->getHeader()->getParent()),
SE ? &SE->getSE() : nullptr, MSSA, &ORE, false);
}
/// This transformation requires natural loop information & requires that
/// loop preheaders be inserted into the CFG...
///
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addPreserved<DominatorTreeWrapperPass>();
AU.addPreserved<LoopInfoWrapperPass>();
AU.addRequired<TargetLibraryInfoWrapperPass>();
if (EnableMSSALoopDependency)
AU.addRequired<MemorySSAWrapperPass>();
AU.addRequired<TargetTransformInfoWrapperPass>();
getLoopAnalysisUsage(AU);
}
using llvm::Pass::doFinalization;
bool doFinalization() override {
assert(LICM.getLoopToAliasSetMap().empty() &&
"Didn't free loop alias sets");
return false;
}
private:
LoopInvariantCodeMotion LICM;
/// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info.
void cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To,
Loop *L) override;
/// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias
/// set.
void deleteAnalysisValue(Value *V, Loop *L) override;
/// Simple Analysis hook. Delete loop L from alias set map.
void deleteAnalysisLoop(Loop *L) override;
};
} // namespace
PreservedAnalyses LICMPass::run(Loop &L, LoopAnalysisManager &AM,
LoopStandardAnalysisResults &AR, LPMUpdater &) {
const auto &FAM =
AM.getResult<FunctionAnalysisManagerLoopProxy>(L, AR).getManager();
Function *F = L.getHeader()->getParent();
auto *ORE = FAM.getCachedResult<OptimizationRemarkEmitterAnalysis>(*F);
// FIXME: This should probably be optional rather than required.
if (!ORE)
report_fatal_error("LICM: OptimizationRemarkEmitterAnalysis not "
"cached at a higher level");
LoopInvariantCodeMotion LICM;
if (!LICM.runOnLoop(&L, &AR.AA, &AR.LI, &AR.DT, &AR.TLI, &AR.TTI, &AR.SE,
AR.MSSA, ORE, true))
return PreservedAnalyses::all();
auto PA = getLoopPassPreservedAnalyses();
PA.preserve<DominatorTreeAnalysis>();
PA.preserve<LoopAnalysis>();
return PA;
}
char LegacyLICMPass::ID = 0;
INITIALIZE_PASS_BEGIN(LegacyLICMPass, "licm", "Loop Invariant Code Motion",
false, false)
INITIALIZE_PASS_DEPENDENCY(LoopPass)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(MemorySSAWrapperPass)
INITIALIZE_PASS_END(LegacyLICMPass, "licm", "Loop Invariant Code Motion", false,
false)
Pass *llvm::createLICMPass() { return new LegacyLICMPass(); }
/// Hoist expressions out of the specified loop. Note, alias info for inner
/// loop is not preserved so it is not a good idea to run LICM multiple
/// times on one loop.
/// We should delete AST for inner loops in the new pass manager to avoid
/// memory leak.
///
bool LoopInvariantCodeMotion::runOnLoop(
Loop *L, AliasAnalysis *AA, LoopInfo *LI, DominatorTree *DT,
TargetLibraryInfo *TLI, TargetTransformInfo *TTI, ScalarEvolution *SE,
MemorySSA *MSSA, OptimizationRemarkEmitter *ORE, bool DeleteAST) {
bool Changed = false;
assert(L->isLCSSAForm(*DT) && "Loop is not in LCSSA form.");
AliasSetTracker *CurAST = collectAliasInfoForLoop(L, LI, AA);
// Get the preheader block to move instructions into...
BasicBlock *Preheader = L->getLoopPreheader();
// Compute loop safety information.
LoopSafetyInfo SafetyInfo;
computeLoopSafetyInfo(&SafetyInfo, L);
// We want to visit all of the instructions in this loop... that are not parts
// of our subloops (they have already had their invariants hoisted out of
// their loop, into this loop, so there is no need to process the BODIES of
// the subloops).
//
// Traverse the body of the loop in depth first order on the dominator tree so
// that we are guaranteed to see definitions before we see uses. This allows
// us to sink instructions in one pass, without iteration. After sinking
// instructions, we perform another pass to hoist them out of the loop.
//
if (L->hasDedicatedExits())
Changed |= sinkRegion(DT->getNode(L->getHeader()), AA, LI, DT, TLI, TTI, L,
CurAST, &SafetyInfo, ORE);
if (Preheader)
Changed |= hoistRegion(DT->getNode(L->getHeader()), AA, LI, DT, TLI, L,
CurAST, &SafetyInfo, ORE);
// Now that all loop invariants have been removed from the loop, promote any
// memory references to scalars that we can.
// Don't sink stores from loops without dedicated block exits. Exits
// containing indirect branches are not transformed by loop simplify,
// make sure we catch that. An additional load may be generated in the
// preheader for SSA updater, so also avoid sinking when no preheader
// is available.
if (!DisablePromotion && Preheader && L->hasDedicatedExits()) {
// Figure out the loop exits and their insertion points
SmallVector<BasicBlock *, 8> ExitBlocks;
L->getUniqueExitBlocks(ExitBlocks);
// We can't insert into a catchswitch.
bool HasCatchSwitch = llvm::any_of(ExitBlocks, [](BasicBlock *Exit) {
return isa<CatchSwitchInst>(Exit->getTerminator());
});
if (!HasCatchSwitch) {
SmallVector<Instruction *, 8> InsertPts;
InsertPts.reserve(ExitBlocks.size());
for (BasicBlock *ExitBlock : ExitBlocks)
InsertPts.push_back(&*ExitBlock->getFirstInsertionPt());
PredIteratorCache PIC;
bool Promoted = false;
// Loop over all of the alias sets in the tracker object.
for (AliasSet &AS : *CurAST) {
// We can promote this alias set if it has a store, if it is a "Must"
// alias set, if the pointer is loop invariant, and if we are not
// eliminating any volatile loads or stores.
if (AS.isForwardingAliasSet() || !AS.isMod() || !AS.isMustAlias() ||
AS.isVolatile() || !L->isLoopInvariant(AS.begin()->getValue()))
continue;
assert(
!AS.empty() &&
"Must alias set should have at least one pointer element in it!");
SmallSetVector<Value *, 8> PointerMustAliases;
for (const auto &ASI : AS)
PointerMustAliases.insert(ASI.getValue());
Promoted |= promoteLoopAccessesToScalars(PointerMustAliases, ExitBlocks,
InsertPts, PIC, LI, DT, TLI, L,
CurAST, &SafetyInfo, ORE);
}
// Once we have promoted values across the loop body we have to
// recursively reform LCSSA as any nested loop may now have values defined
// within the loop used in the outer loop.
// FIXME: This is really heavy handed. It would be a bit better to use an
// SSAUpdater strategy during promotion that was LCSSA aware and reformed
// it as it went.
if (Promoted)
formLCSSARecursively(*L, *DT, LI, SE);
Changed |= Promoted;
}
}
// Check that neither this loop nor its parent have had LCSSA broken. LICM is
// specifically moving instructions across the loop boundary and so it is
// especially in need of sanity checking here.
assert(L->isLCSSAForm(*DT) && "Loop not left in LCSSA form after LICM!");
assert((!L->getParentLoop() || L->getParentLoop()->isLCSSAForm(*DT)) &&
"Parent loop not left in LCSSA form after LICM!");
// If this loop is nested inside of another one, save the alias information
// for when we process the outer loop.
if (L->getParentLoop() && !DeleteAST)
LoopToAliasSetMap[L] = CurAST;
else
delete CurAST;
if (Changed && SE)
SE->forgetLoopDispositions(L);
return Changed;
}
/// Walk the specified region of the CFG (defined by all blocks dominated by
/// the specified block, and that are in the current loop) in reverse depth
/// first order w.r.t the DominatorTree. This allows us to visit uses before
/// definitions, allowing us to sink a loop body in one pass without iteration.
///
bool llvm::sinkRegion(DomTreeNode *N, AliasAnalysis *AA, LoopInfo *LI,
DominatorTree *DT, TargetLibraryInfo *TLI,
TargetTransformInfo *TTI, Loop *CurLoop,
AliasSetTracker *CurAST, LoopSafetyInfo *SafetyInfo,
OptimizationRemarkEmitter *ORE) {
// Verify inputs.
assert(N != nullptr && AA != nullptr && LI != nullptr && DT != nullptr &&
CurLoop != nullptr && CurAST != nullptr && SafetyInfo != nullptr &&
"Unexpected input to sinkRegion");
// We want to visit children before parents. We will enque all the parents
// before their children in the worklist and process the worklist in reverse
// order.
SmallVector<DomTreeNode *, 16> Worklist = collectChildrenInLoop(N, CurLoop);
bool Changed = false;
for (DomTreeNode *DTN : reverse(Worklist)) {
BasicBlock *BB = DTN->getBlock();
// Only need to process the contents of this block if it is not part of a
// subloop (which would already have been processed).
if (inSubLoop(BB, CurLoop, LI))
continue;
for (BasicBlock::iterator II = BB->end(); II != BB->begin();) {
Instruction &I = *--II;
// If the instruction is dead, we would try to sink it because it isn't
// used in the loop, instead, just delete it.
if (isInstructionTriviallyDead(&I, TLI)) {
LLVM_DEBUG(dbgs() << "LICM deleting dead inst: " << I << '\n');
salvageDebugInfo(I);
++II;
CurAST->deleteValue(&I);
I.eraseFromParent();
Changed = true;
continue;
}
// Check to see if we can sink this instruction to the exit blocks
// of the loop. We can do this if the all users of the instruction are
// outside of the loop. In this case, it doesn't even matter if the
// operands of the instruction are loop invariant.
//
bool FreeInLoop = false;
if (isNotUsedOrFreeInLoop(I, CurLoop, SafetyInfo, TTI, FreeInLoop) &&
canSinkOrHoistInst(I, AA, DT, CurLoop, CurAST, SafetyInfo, ORE)) {
if (sink(I, LI, DT, CurLoop, SafetyInfo, ORE, FreeInLoop)) {
if (!FreeInLoop) {
++II;
CurAST->deleteValue(&I);
I.eraseFromParent();
}
Changed = true;
}
}
}
}
return Changed;
}
/// Walk the specified region of the CFG (defined by all blocks dominated by
/// the specified block, and that are in the current loop) in depth first
/// order w.r.t the DominatorTree. This allows us to visit definitions before
/// uses, allowing us to hoist a loop body in one pass without iteration.
///
bool llvm::hoistRegion(DomTreeNode *N, AliasAnalysis *AA, LoopInfo *LI,
DominatorTree *DT, TargetLibraryInfo *TLI, Loop *CurLoop,
AliasSetTracker *CurAST, LoopSafetyInfo *SafetyInfo,
OptimizationRemarkEmitter *ORE) {
// Verify inputs.
assert(N != nullptr && AA != nullptr && LI != nullptr && DT != nullptr &&
CurLoop != nullptr && CurAST != nullptr && SafetyInfo != nullptr &&
"Unexpected input to hoistRegion");
// We want to visit parents before children. We will enque all the parents
// before their children in the worklist and process the worklist in order.
SmallVector<DomTreeNode *, 16> Worklist = collectChildrenInLoop(N, CurLoop);
bool Changed = false;
for (DomTreeNode *DTN : Worklist) {
BasicBlock *BB = DTN->getBlock();
// Only need to process the contents of this block if it is not part of a
// subloop (which would already have been processed).
if (inSubLoop(BB, CurLoop, LI))
continue;
// Keep track of whether the prefix of instructions visited so far are such
// that the next instruction visited is guaranteed to execute if the loop
// is entered.
bool IsMustExecute = CurLoop->getHeader() == BB;
for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;) {
Instruction &I = *II++;
// Try constant folding this instruction. If all the operands are
// constants, it is technically hoistable, but it would be better to
// just fold it.
if (Constant *C = ConstantFoldInstruction(
&I, I.getModule()->getDataLayout(), TLI)) {
LLVM_DEBUG(dbgs() << "LICM folding inst: " << I << " --> " << *C
<< '\n');
CurAST->copyValue(&I, C);
I.replaceAllUsesWith(C);
if (isInstructionTriviallyDead(&I, TLI)) {
CurAST->deleteValue(&I);
I.eraseFromParent();
}
Changed = true;
continue;
}
// Try hoisting the instruction out to the preheader. We can only do
// this if all of the operands of the instruction are loop invariant and
// if it is safe to hoist the instruction.
//
if (CurLoop->hasLoopInvariantOperands(&I) &&
canSinkOrHoistInst(I, AA, DT, CurLoop, CurAST, SafetyInfo, ORE) &&
(IsMustExecute ||
isSafeToExecuteUnconditionally(
I, DT, CurLoop, SafetyInfo, ORE,
CurLoop->getLoopPreheader()->getTerminator()))) {
Changed |= hoist(I, DT, CurLoop, SafetyInfo, ORE);
continue;
}
// Attempt to remove floating point division out of the loop by
// converting it to a reciprocal multiplication.
if (I.getOpcode() == Instruction::FDiv &&
CurLoop->isLoopInvariant(I.getOperand(1)) &&
I.hasAllowReciprocal()) {
auto Divisor = I.getOperand(1);
auto One = llvm::ConstantFP::get(Divisor->getType(), 1.0);
auto ReciprocalDivisor = BinaryOperator::CreateFDiv(One, Divisor);
ReciprocalDivisor->setFastMathFlags(I.getFastMathFlags());
ReciprocalDivisor->insertBefore(&I);
auto Product =
BinaryOperator::CreateFMul(I.getOperand(0), ReciprocalDivisor);
Product->setFastMathFlags(I.getFastMathFlags());
Product->insertAfter(&I);
I.replaceAllUsesWith(Product);
I.eraseFromParent();
hoist(*ReciprocalDivisor, DT, CurLoop, SafetyInfo, ORE);
Changed = true;
continue;
}
if (IsMustExecute)
IsMustExecute = isGuaranteedToTransferExecutionToSuccessor(&I);
}
}
return Changed;
}
// Return true if LI is invariant within scope of the loop. LI is invariant if
// CurLoop is dominated by an invariant.start representing the same memory
// location and size as the memory location LI loads from, and also the
// invariant.start has no uses.
static bool isLoadInvariantInLoop(LoadInst *LI, DominatorTree *DT,
Loop *CurLoop) {
Value *Addr = LI->getOperand(0);
const DataLayout &DL = LI->getModule()->getDataLayout();
const uint32_t LocSizeInBits = DL.getTypeSizeInBits(
cast<PointerType>(Addr->getType())->getElementType());
// if the type is i8 addrspace(x)*, we know this is the type of
// llvm.invariant.start operand
auto *PtrInt8Ty = PointerType::get(Type::getInt8Ty(LI->getContext()),
LI->getPointerAddressSpace());
unsigned BitcastsVisited = 0;
// Look through bitcasts until we reach the i8* type (this is invariant.start
// operand type).
while (Addr->getType() != PtrInt8Ty) {
auto *BC = dyn_cast<BitCastInst>(Addr);
// Avoid traversing high number of bitcast uses.
if (++BitcastsVisited > MaxNumUsesTraversed || !BC)
return false;
Addr = BC->getOperand(0);
}
unsigned UsesVisited = 0;
// Traverse all uses of the load operand value, to see if invariant.start is
// one of the uses, and whether it dominates the load instruction.
for (auto *U : Addr->users()) {
// Avoid traversing for Load operand with high number of users.
if (++UsesVisited > MaxNumUsesTraversed)
return false;
IntrinsicInst *II = dyn_cast<IntrinsicInst>(U);
// If there are escaping uses of invariant.start instruction, the load maybe
// non-invariant.
if (!II || II->getIntrinsicID() != Intrinsic::invariant_start ||
!II->use_empty())
continue;
unsigned InvariantSizeInBits =
cast<ConstantInt>(II->getArgOperand(0))->getSExtValue() * 8;
// Confirm the invariant.start location size contains the load operand size
// in bits. Also, the invariant.start should dominate the load, and we
// should not hoist the load out of a loop that contains this dominating
// invariant.start.
if (LocSizeInBits <= InvariantSizeInBits &&
DT->properlyDominates(II->getParent(), CurLoop->getHeader()))
return true;
}
return false;
}
bool llvm::canSinkOrHoistInst(Instruction &I, AAResults *AA, DominatorTree *DT,
Loop *CurLoop, AliasSetTracker *CurAST,
LoopSafetyInfo *SafetyInfo,
OptimizationRemarkEmitter *ORE) {
// SafetyInfo is nullptr if we are checking for sinking from preheader to
// loop body.
const bool SinkingToLoopBody = !SafetyInfo;
// Loads have extra constraints we have to verify before we can hoist them.
if (LoadInst *LI = dyn_cast<LoadInst>(&I)) {
if (!LI->isUnordered())
return false; // Don't sink/hoist volatile or ordered atomic loads!
// Loads from constant memory are always safe to move, even if they end up
// in the same alias set as something that ends up being modified.
if (AA->pointsToConstantMemory(LI->getOperand(0)))
return true;
if (LI->getMetadata(LLVMContext::MD_invariant_load))
return true;
if (LI->isAtomic() && SinkingToLoopBody)
return false; // Don't sink unordered atomic loads to loop body.
// This checks for an invariant.start dominating the load.
if (isLoadInvariantInLoop(LI, DT, CurLoop))
return true;
// Don't hoist loads which have may-aliased stores in loop.
uint64_t Size = 0;
if (LI->getType()->isSized())
Size = I.getModule()->getDataLayout().getTypeStoreSize(LI->getType());
AAMDNodes AAInfo;
LI->getAAMetadata(AAInfo);
bool Invalidated =
pointerInvalidatedByLoop(LI->getOperand(0), Size, AAInfo, CurAST);
// Check loop-invariant address because this may also be a sinkable load
// whose address is not necessarily loop-invariant.
if (ORE && Invalidated && CurLoop->isLoopInvariant(LI->getPointerOperand()))
ORE->emit([&]() {
return OptimizationRemarkMissed(
DEBUG_TYPE, "LoadWithLoopInvariantAddressInvalidated", LI)
<< "failed to move load with loop-invariant address "
"because the loop may invalidate its value";
});
return !Invalidated;
} else if (CallInst *CI = dyn_cast<CallInst>(&I)) {
// Don't sink or hoist dbg info; it's legal, but not useful.
if (isa<DbgInfoIntrinsic>(I))
return false;
// Don't sink calls which can throw.
if (CI->mayThrow())
return false;
// Handle simple cases by querying alias analysis.
FunctionModRefBehavior Behavior = AA->getModRefBehavior(CI);
if (Behavior == FMRB_DoesNotAccessMemory)
return true;
if (AliasAnalysis::onlyReadsMemory(Behavior)) {
// A readonly argmemonly function only reads from memory pointed to by
// it's arguments with arbitrary offsets. If we can prove there are no
// writes to this memory in the loop, we can hoist or sink.
if (AliasAnalysis::onlyAccessesArgPointees(Behavior)) {
for (Value *Op : CI->arg_operands())
if (Op->getType()->isPointerTy() &&
pointerInvalidatedByLoop(Op, MemoryLocation::UnknownSize,
AAMDNodes(), CurAST))
return false;
return true;
}
// If this call only reads from memory and there are no writes to memory
// in the loop, we can hoist or sink the call as appropriate.
bool FoundMod = false;
for (AliasSet &AS : *CurAST) {
if (!AS.isForwardingAliasSet() && AS.isMod()) {
FoundMod = true;
break;
}
}
if (!FoundMod)
return true;
}
// FIXME: This should use mod/ref information to see if we can hoist or
// sink the call.
return false;
}
// Only these instructions are hoistable/sinkable.
if (!isa<BinaryOperator>(I) && !isa<CastInst>(I) && !isa<SelectInst>(I) &&
!isa<GetElementPtrInst>(I) && !isa<CmpInst>(I) &&
!isa<InsertElementInst>(I) && !isa<ExtractElementInst>(I) &&
!isa<ShuffleVectorInst>(I) && !isa<ExtractValueInst>(I) &&
!isa<InsertValueInst>(I))
return false;
// If we are checking for sinking from preheader to loop body it will be
// always safe as there is no speculative execution.
if (SinkingToLoopBody)
return true;
// TODO: Plumb the context instruction through to make hoisting and sinking
// more powerful. Hoisting of loads already works due to the special casing
// above.
return isSafeToExecuteUnconditionally(I, DT, CurLoop, SafetyInfo, nullptr);
}
/// Returns true if a PHINode is a trivially replaceable with an
/// Instruction.
/// This is true when all incoming values are that instruction.
/// This pattern occurs most often with LCSSA PHI nodes.
///
static bool isTriviallyReplaceablePHI(const PHINode &PN, const Instruction &I) {
for (const Value *IncValue : PN.incoming_values())
if (IncValue != &I)
return false;
return true;
}
/// Return true if the instruction is free in the loop.
static bool isFreeInLoop(const Instruction &I, const Loop *CurLoop,
const TargetTransformInfo *TTI) {
if (const GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(&I)) {
if (TTI->getUserCost(GEP) != TargetTransformInfo::TCC_Free)
return false;
// For a GEP, we cannot simply use getUserCost because currently it
// optimistically assume that a GEP will fold into addressing mode
// regardless of its users.
const BasicBlock *BB = GEP->getParent();
for (const User *U : GEP->users()) {
const Instruction *UI = cast<Instruction>(U);
if (CurLoop->contains(UI) &&
(BB != UI->getParent() ||
(!isa<StoreInst>(UI) && !isa<LoadInst>(UI))))
return false;
}
return true;
} else
return TTI->getUserCost(&I) == TargetTransformInfo::TCC_Free;
}
/// Return true if the only users of this instruction are outside of
/// the loop. If this is true, we can sink the instruction to the exit
/// blocks of the loop.
///
/// We also return true if the instruction could be folded away in lowering.
/// (e.g., a GEP can be folded into a load as an addressing mode in the loop).
static bool isNotUsedOrFreeInLoop(const Instruction &I, const Loop *CurLoop,
const LoopSafetyInfo *SafetyInfo,
TargetTransformInfo *TTI, bool &FreeInLoop) {
const auto &BlockColors = SafetyInfo->BlockColors;
bool IsFree = isFreeInLoop(I, CurLoop, TTI);
for (const User *U : I.users()) {
const Instruction *UI = cast<Instruction>(U);
if (const PHINode *PN = dyn_cast<PHINode>(UI)) {
const BasicBlock *BB = PN->getParent();
// We cannot sink uses in catchswitches.
if (isa<CatchSwitchInst>(BB->getTerminator()))
return false;
// We need to sink a callsite to a unique funclet. Avoid sinking if the
// phi use is too muddled.
if (isa<CallInst>(I))
if (!BlockColors.empty() &&
BlockColors.find(const_cast<BasicBlock *>(BB))->second.size() != 1)
return false;
}
if (CurLoop->contains(UI)) {
if (IsFree) {
FreeInLoop = true;
continue;
}
return false;
}
}
return true;
}
static Instruction *
CloneInstructionInExitBlock(Instruction &I, BasicBlock &ExitBlock, PHINode &PN,
const LoopInfo *LI,
const LoopSafetyInfo *SafetyInfo) {
Instruction *New;
if (auto *CI = dyn_cast<CallInst>(&I)) {
const auto &BlockColors = SafetyInfo->BlockColors;
// Sinking call-sites need to be handled differently from other
// instructions. The cloned call-site needs a funclet bundle operand
// appropriate for it's location in the CFG.
SmallVector<OperandBundleDef, 1> OpBundles;
for (unsigned BundleIdx = 0, BundleEnd = CI->getNumOperandBundles();
BundleIdx != BundleEnd; ++BundleIdx) {
OperandBundleUse Bundle = CI->getOperandBundleAt(BundleIdx);
if (Bundle.getTagID() == LLVMContext::OB_funclet)
continue;
OpBundles.emplace_back(Bundle);
}
if (!BlockColors.empty()) {
const ColorVector &CV = BlockColors.find(&ExitBlock)->second;
assert(CV.size() == 1 && "non-unique color for exit block!");
BasicBlock *BBColor = CV.front();
Instruction *EHPad = BBColor->getFirstNonPHI();
if (EHPad->isEHPad())
OpBundles.emplace_back("funclet", EHPad);
}
New = CallInst::Create(CI, OpBundles);
} else {
New = I.clone();
}
ExitBlock.getInstList().insert(ExitBlock.getFirstInsertionPt(), New);
if (!I.getName().empty())
New->setName(I.getName() + ".le");
// Build LCSSA PHI nodes for any in-loop operands. Note that this is
// particularly cheap because we can rip off the PHI node that we're
// replacing for the number and blocks of the predecessors.
// OPT: If this shows up in a profile, we can instead finish sinking all
// invariant instructions, and then walk their operands to re-establish
// LCSSA. That will eliminate creating PHI nodes just to nuke them when
// sinking bottom-up.
for (User::op_iterator OI = New->op_begin(), OE = New->op_end(); OI != OE;
++OI)
if (Instruction *OInst = dyn_cast<Instruction>(*OI))
if (Loop *OLoop = LI->getLoopFor(OInst->getParent()))
if (!OLoop->contains(&PN)) {
PHINode *OpPN =
PHINode::Create(OInst->getType(), PN.getNumIncomingValues(),
OInst->getName() + ".lcssa", &ExitBlock.front());
for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i)
OpPN->addIncoming(OInst, PN.getIncomingBlock(i));
*OI = OpPN;
}
return New;
}
static Instruction *sinkThroughTriviallyReplaceablePHI(
PHINode *TPN, Instruction *I, LoopInfo *LI,
SmallDenseMap<BasicBlock *, Instruction *, 32> &SunkCopies,
const LoopSafetyInfo *SafetyInfo, const Loop *CurLoop) {
assert(isTriviallyReplaceablePHI(*TPN, *I) &&
"Expect only trivially replaceable PHI");
BasicBlock *ExitBlock = TPN->getParent();
Instruction *New;
auto It = SunkCopies.find(ExitBlock);
if (It != SunkCopies.end())
New = It->second;
else
New = SunkCopies[ExitBlock] =
CloneInstructionInExitBlock(*I, *ExitBlock, *TPN, LI, SafetyInfo);
return New;
}
static bool canSplitPredecessors(PHINode *PN, LoopSafetyInfo *SafetyInfo) {
BasicBlock *BB = PN->getParent();
if (!BB->canSplitPredecessors())
return false;
// It's not impossible to split EHPad blocks, but if BlockColors already exist
// it require updating BlockColors for all offspring blocks accordingly. By
// skipping such corner case, we can make updating BlockColors after splitting
// predecessor fairly simple.
if (!SafetyInfo->BlockColors.empty() && BB->getFirstNonPHI()->isEHPad())
return false;
for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
BasicBlock *BBPred = *PI;
if (isa<IndirectBrInst>(BBPred->getTerminator()))
return false;
}
return true;
}
static void splitPredecessorsOfLoopExit(PHINode *PN, DominatorTree *DT,
LoopInfo *LI, const Loop *CurLoop,
LoopSafetyInfo *SafetyInfo) {
#ifndef NDEBUG
SmallVector<BasicBlock *, 32> ExitBlocks;
CurLoop->getUniqueExitBlocks(ExitBlocks);
SmallPtrSet<BasicBlock *, 32> ExitBlockSet(ExitBlocks.begin(),
ExitBlocks.end());
#endif
BasicBlock *ExitBB = PN->getParent();
assert(ExitBlockSet.count(ExitBB) && "Expect the PHI is in an exit block.");
// Split predecessors of the loop exit to make instructions in the loop are
// exposed to exit blocks through trivially replaceable PHIs while keeping the
// loop in the canonical form where each predecessor of each exit block should
// be contained within the loop. For example, this will convert the loop below
// from
//
// LB1:
// %v1 =
// br %LE, %LB2
// LB2:
// %v2 =
// br %LE, %LB1
// LE:
// %p = phi [%v1, %LB1], [%v2, %LB2] <-- non-trivially replaceable
//
// to
//
// LB1:
// %v1 =
// br %LE.split, %LB2
// LB2:
// %v2 =
// br %LE.split2, %LB1
// LE.split:
// %p1 = phi [%v1, %LB1] <-- trivially replaceable
// br %LE
// LE.split2:
// %p2 = phi [%v2, %LB2] <-- trivially replaceable
// br %LE
// LE:
// %p = phi [%p1, %LE.split], [%p2, %LE.split2]
//
auto &BlockColors = SafetyInfo->BlockColors;
SmallSetVector<BasicBlock *, 8> PredBBs(pred_begin(ExitBB), pred_end(ExitBB));
while (!PredBBs.empty()) {
BasicBlock *PredBB = *PredBBs.begin();
assert(CurLoop->contains(PredBB) &&
"Expect all predecessors are in the loop");
if (PN->getBasicBlockIndex(PredBB) >= 0) {
BasicBlock *NewPred = SplitBlockPredecessors(
ExitBB, PredBB, ".split.loop.exit", DT, LI, true);
// Since we do not allow splitting EH-block with BlockColors in
// canSplitPredecessors(), we can simply assign predecessor's color to
// the new block.
if (!BlockColors.empty()) {
// Grab a reference to the ColorVector to be inserted before getting the
// reference to the vector we are copying because inserting the new
// element in BlockColors might cause the map to be reallocated.
ColorVector &ColorsForNewBlock = BlockColors[NewPred];
ColorVector &ColorsForOldBlock = BlockColors[PredBB];
ColorsForNewBlock = ColorsForOldBlock;
}
}
PredBBs.remove(PredBB);
}
}
/// When an instruction is found to only be used outside of the loop, this
/// function moves it to the exit blocks and patches up SSA form as needed.
/// This method is guaranteed to remove the original instruction from its
/// position, and may either delete it or move it to outside of the loop.
///
static bool sink(Instruction &I, LoopInfo *LI, DominatorTree *DT,
const Loop *CurLoop, LoopSafetyInfo *SafetyInfo,
OptimizationRemarkEmitter *ORE, bool FreeInLoop) {
LLVM_DEBUG(dbgs() << "LICM sinking instruction: " << I << "\n");
ORE->emit([&]() {
return OptimizationRemark(DEBUG_TYPE, "InstSunk", &I)
<< "sinking " << ore::NV("Inst", &I);
});
bool Changed = false;
if (isa<LoadInst>(I))
++NumMovedLoads;
else if (isa<CallInst>(I))
++NumMovedCalls;
++NumSunk;
// Iterate over users to be ready for actual sinking. Replace users via
// unrechable blocks with undef and make all user PHIs trivially replcable.
SmallPtrSet<Instruction *, 8> VisitedUsers;
for (Value::user_iterator UI = I.user_begin(), UE = I.user_end(); UI != UE;) {
auto *User = cast<Instruction>(*UI);
Use &U = UI.getUse();
++UI;
if (VisitedUsers.count(User) || CurLoop->contains(User))
continue;
if (!DT->isReachableFromEntry(User->getParent())) {
U = UndefValue::get(I.getType());
Changed = true;
continue;
}
// The user must be a PHI node.
PHINode *PN = cast<PHINode>(User);
// Surprisingly, instructions can be used outside of loops without any
// exits. This can only happen in PHI nodes if the incoming block is
// unreachable.
BasicBlock *BB = PN->getIncomingBlock(U);
if (!DT->isReachableFromEntry(BB)) {
U = UndefValue::get(I.getType());
Changed = true;
continue;
}
VisitedUsers.insert(PN);
if (isTriviallyReplaceablePHI(*PN, I))
continue;
if (!canSplitPredecessors(PN, SafetyInfo))
return Changed;
// Split predecessors of the PHI so that we can make users trivially
// replaceable.
splitPredecessorsOfLoopExit(PN, DT, LI, CurLoop, SafetyInfo);
// Should rebuild the iterators, as they may be invalidated by
// splitPredecessorsOfLoopExit().
UI = I.user_begin();
UE = I.user_end();
}
if (VisitedUsers.empty())
return Changed;
#ifndef NDEBUG
SmallVector<BasicBlock *, 32> ExitBlocks;
CurLoop->getUniqueExitBlocks(ExitBlocks);
SmallPtrSet<BasicBlock *, 32> ExitBlockSet(ExitBlocks.begin(),
ExitBlocks.end());
#endif
// Clones of this instruction. Don't create more than one per exit block!
SmallDenseMap<BasicBlock *, Instruction *, 32> SunkCopies;
// If this instruction is only used outside of the loop, then all users are
// PHI nodes in exit blocks due to LCSSA form. Just RAUW them with clones of
// the instruction.
SmallSetVector<User*, 8> Users(I.user_begin(), I.user_end());
for (auto *UI : Users) {
auto *User = cast<Instruction>(UI);
if (CurLoop->contains(User))
continue;
PHINode *PN = cast<PHINode>(User);
assert(ExitBlockSet.count(PN->getParent()) &&
"The LCSSA PHI is not in an exit block!");
// The PHI must be trivially replaceable.
Instruction *New = sinkThroughTriviallyReplaceablePHI(PN, &I, LI, SunkCopies,
SafetyInfo, CurLoop);
PN->replaceAllUsesWith(New);
PN->eraseFromParent();
Changed = true;
}
return Changed;
}
/// When an instruction is found to only use loop invariant operands that
/// is safe to hoist, this instruction is called to do the dirty work.
///
static bool hoist(Instruction &I, const DominatorTree *DT, const Loop *CurLoop,
const LoopSafetyInfo *SafetyInfo,
OptimizationRemarkEmitter *ORE) {
auto *Preheader = CurLoop->getLoopPreheader();
LLVM_DEBUG(dbgs() << "LICM hoisting to " << Preheader->getName() << ": " << I
<< "\n");
ORE->emit([&]() {
return OptimizationRemark(DEBUG_TYPE, "Hoisted", &I) << "hoisting "
<< ore::NV("Inst", &I);
});
// Metadata can be dependent on conditions we are hoisting above.
// Conservatively strip all metadata on the instruction unless we were
// guaranteed to execute I if we entered the loop, in which case the metadata
// is valid in the loop preheader.
if (I.hasMetadataOtherThanDebugLoc() &&
// The check on hasMetadataOtherThanDebugLoc is to prevent us from burning
// time in isGuaranteedToExecute if we don't actually have anything to
// drop. It is a compile time optimization, not required for correctness.
!isGuaranteedToExecute(I, DT, CurLoop, SafetyInfo))
I.dropUnknownNonDebugMetadata();
// Move the new node to the Preheader, before its terminator.
I.moveBefore(Preheader->getTerminator());
// Do not retain debug locations when we are moving instructions to different
// basic blocks, because we want to avoid jumpy line tables. Calls, however,
// need to retain their debug locs because they may be inlined.
// FIXME: How do we retain source locations without causing poor debugging
// behavior?
if (!isa<CallInst>(I))
I.setDebugLoc(DebugLoc());
if (isa<LoadInst>(I))
++NumMovedLoads;
else if (isa<CallInst>(I))
++NumMovedCalls;
++NumHoisted;
return true;
}
/// Only sink or hoist an instruction if it is not a trapping instruction,
/// or if the instruction is known not to trap when moved to the preheader.
/// or if it is a trapping instruction and is guaranteed to execute.
static bool isSafeToExecuteUnconditionally(Instruction &Inst,
const DominatorTree *DT,
const Loop *CurLoop,
const LoopSafetyInfo *SafetyInfo,
OptimizationRemarkEmitter *ORE,
const Instruction *CtxI) {
if (isSafeToSpeculativelyExecute(&Inst, CtxI, DT))
return true;
bool GuaranteedToExecute =
isGuaranteedToExecute(Inst, DT, CurLoop, SafetyInfo);
if (!GuaranteedToExecute) {
auto *LI = dyn_cast<LoadInst>(&Inst);
if (LI && CurLoop->isLoopInvariant(LI->getPointerOperand()))
ORE->emit([&]() {
return OptimizationRemarkMissed(
DEBUG_TYPE, "LoadWithLoopInvariantAddressCondExecuted", LI)
<< "failed to hoist load with loop-invariant address "
"because load is conditionally executed";
});
}
return GuaranteedToExecute;
}
namespace {
class LoopPromoter : public LoadAndStorePromoter {
Value *SomePtr; // Designated pointer to store to.
const SmallSetVector<Value *, 8> &PointerMustAliases;
SmallVectorImpl<BasicBlock *> &LoopExitBlocks;
SmallVectorImpl<Instruction *> &LoopInsertPts;
PredIteratorCache &PredCache;
AliasSetTracker &AST;
LoopInfo &LI;
DebugLoc DL;
int Alignment;
bool UnorderedAtomic;
AAMDNodes AATags;
Value *maybeInsertLCSSAPHI(Value *V, BasicBlock *BB) const {
if (Instruction *I = dyn_cast<Instruction>(V))
if (Loop *L = LI.getLoopFor(I->getParent()))
if (!L->contains(BB)) {
// We need to create an LCSSA PHI node for the incoming value and
// store that.
PHINode *PN = PHINode::Create(I->getType(), PredCache.size(BB),
I->getName() + ".lcssa", &BB->front());
for (BasicBlock *Pred : PredCache.get(BB))
PN->addIncoming(I, Pred);
return PN;
}
return V;
}
public:
LoopPromoter(Value *SP, ArrayRef<const Instruction *> Insts, SSAUpdater &S,
const SmallSetVector<Value *, 8> &PMA,
SmallVectorImpl<BasicBlock *> &LEB,
SmallVectorImpl<Instruction *> &LIP, PredIteratorCache &PIC,
AliasSetTracker &ast, LoopInfo &li, DebugLoc dl, int alignment,
bool UnorderedAtomic, const AAMDNodes &AATags)
: LoadAndStorePromoter(Insts, S), SomePtr(SP), PointerMustAliases(PMA),
LoopExitBlocks(LEB), LoopInsertPts(LIP), PredCache(PIC), AST(ast),
LI(li), DL(std::move(dl)), Alignment(alignment),
UnorderedAtomic(UnorderedAtomic), AATags(AATags) {}
bool isInstInList(Instruction *I,
const SmallVectorImpl<Instruction *> &) const override {
Value *Ptr;
if (LoadInst *LI = dyn_cast<LoadInst>(I))
Ptr = LI->getOperand(0);
else
Ptr = cast<StoreInst>(I)->getPointerOperand();
return PointerMustAliases.count(Ptr);
}
void doExtraRewritesBeforeFinalDeletion() const override {
// Insert stores after in the loop exit blocks. Each exit block gets a
// store of the live-out values that feed them. Since we've already told
// the SSA updater about the defs in the loop and the preheader
// definition, it is all set and we can start using it.
for (unsigned i = 0, e = LoopExitBlocks.size(); i != e; ++i) {
BasicBlock *ExitBlock = LoopExitBlocks[i];
Value *LiveInValue = SSA.GetValueInMiddleOfBlock(ExitBlock);
LiveInValue = maybeInsertLCSSAPHI(LiveInValue, ExitBlock);
Value *Ptr = maybeInsertLCSSAPHI(SomePtr, ExitBlock);
Instruction *InsertPos = LoopInsertPts[i];
StoreInst *NewSI = new StoreInst(LiveInValue, Ptr, InsertPos);
if (UnorderedAtomic)
NewSI->setOrdering(AtomicOrdering::Unordered);
NewSI->setAlignment(Alignment);
NewSI->setDebugLoc(DL);
if (AATags)
NewSI->setAAMetadata(AATags);
}
}
void replaceLoadWithValue(LoadInst *LI, Value *V) const override {
// Update alias analysis.
AST.copyValue(LI, V);
}
void instructionDeleted(Instruction *I) const override { AST.deleteValue(I); }
};
/// Return true iff we can prove that a caller of this function can not inspect
/// the contents of the provided object in a well defined program.
bool isKnownNonEscaping(Value *Object, const TargetLibraryInfo *TLI) {
if (isa<AllocaInst>(Object))
// Since the alloca goes out of scope, we know the caller can't retain a
// reference to it and be well defined. Thus, we don't need to check for
// capture.
return true;
// For all other objects we need to know that the caller can't possibly
// have gotten a reference to the object. There are two components of
// that:
// 1) Object can't be escaped by this function. This is what
// PointerMayBeCaptured checks.
// 2) Object can't have been captured at definition site. For this, we
// need to know the return value is noalias. At the moment, we use a
// weaker condition and handle only AllocLikeFunctions (which are
// known to be noalias). TODO
return isAllocLikeFn(Object, TLI) &&
!PointerMayBeCaptured(Object, true, true);
}
} // namespace
/// Try to promote memory values to scalars by sinking stores out of the
/// loop and moving loads to before the loop. We do this by looping over
/// the stores in the loop, looking for stores to Must pointers which are
/// loop invariant.
///
bool llvm::promoteLoopAccessesToScalars(
const SmallSetVector<Value *, 8> &PointerMustAliases,
SmallVectorImpl<BasicBlock *> &ExitBlocks,
SmallVectorImpl<Instruction *> &InsertPts, PredIteratorCache &PIC,
LoopInfo *LI, DominatorTree *DT, const TargetLibraryInfo *TLI,
Loop *CurLoop, AliasSetTracker *CurAST, LoopSafetyInfo *SafetyInfo,
OptimizationRemarkEmitter *ORE) {
// Verify inputs.
assert(LI != nullptr && DT != nullptr && CurLoop != nullptr &&
CurAST != nullptr && SafetyInfo != nullptr &&
"Unexpected Input to promoteLoopAccessesToScalars");
Value *SomePtr = *PointerMustAliases.begin();
BasicBlock *Preheader = CurLoop->getLoopPreheader();
// It is not safe to promote a load/store from the loop if the load/store is
// conditional. For example, turning:
//
// for () { if (c) *P += 1; }
//
// into:
//
// tmp = *P; for () { if (c) tmp +=1; } *P = tmp;
//
// is not safe, because *P may only be valid to access if 'c' is true.
//
// The safety property divides into two parts:
// p1) The memory may not be dereferenceable on entry to the loop. In this
// case, we can't insert the required load in the preheader.
// p2) The memory model does not allow us to insert a store along any dynamic
// path which did not originally have one.
//
// If at least one store is guaranteed to execute, both properties are
// satisfied, and promotion is legal.
//
// This, however, is not a necessary condition. Even if no store/load is
// guaranteed to execute, we can still establish these properties.
// We can establish (p1) by proving that hoisting the load into the preheader
// is safe (i.e. proving dereferenceability on all paths through the loop). We
// can use any access within the alias set to prove dereferenceability,
// since they're all must alias.
//
// There are two ways establish (p2):
// a) Prove the location is thread-local. In this case the memory model
// requirement does not apply, and stores are safe to insert.
// b) Prove a store dominates every exit block. In this case, if an exit
// blocks is reached, the original dynamic path would have taken us through
// the store, so inserting a store into the exit block is safe. Note that this
// is different from the store being guaranteed to execute. For instance,
// if an exception is thrown on the first iteration of the loop, the original
// store is never executed, but the exit blocks are not executed either.
bool DereferenceableInPH = false;
bool SafeToInsertStore = false;
SmallVector<Instruction *, 64> LoopUses;
// We start with an alignment of one and try to find instructions that allow
// us to prove better alignment.
unsigned Alignment = 1;
// Keep track of which types of access we see
bool SawUnorderedAtomic = false;
bool SawNotAtomic = false;
AAMDNodes AATags;
const DataLayout &MDL = Preheader->getModule()->getDataLayout();
bool IsKnownThreadLocalObject = false;
if (SafetyInfo->MayThrow) {
// If a loop can throw, we have to insert a store along each unwind edge.
// That said, we can't actually make the unwind edge explicit. Therefore,
// we have to prove that the store is dead along the unwind edge. We do
// this by proving that the caller can't have a reference to the object
// after return and thus can't possibly load from the object.
Value *Object = GetUnderlyingObject(SomePtr, MDL);
if (!isKnownNonEscaping(Object, TLI))
return false;
// Subtlety: Alloca's aren't visible to callers, but *are* potentially
// visible to other threads if captured and used during their lifetimes.
IsKnownThreadLocalObject = !isa<AllocaInst>(Object);
}
// Check that all of the pointers in the alias set have the same type. We
// cannot (yet) promote a memory location that is loaded and stored in
// different sizes. While we are at it, collect alignment and AA info.
for (Value *ASIV : PointerMustAliases) {
// Check that all of the pointers in the alias set have the same type. We
// cannot (yet) promote a memory location that is loaded and stored in
// different sizes.
if (SomePtr->getType() != ASIV->getType())
return false;
for (User *U : ASIV->users()) {
// Ignore instructions that are outside the loop.
Instruction *UI = dyn_cast<Instruction>(U);
if (!UI || !CurLoop->contains(UI))
continue;
// If there is an non-load/store instruction in the loop, we can't promote
// it.
if (LoadInst *Load = dyn_cast<LoadInst>(UI)) {
assert(!Load->isVolatile() && "AST broken");
if (!Load->isUnordered())
return false;
SawUnorderedAtomic |= Load->isAtomic();
SawNotAtomic |= !Load->isAtomic();
if (!DereferenceableInPH)
DereferenceableInPH = isSafeToExecuteUnconditionally(
*Load, DT, CurLoop, SafetyInfo, ORE, Preheader->getTerminator());
} else if (const StoreInst *Store = dyn_cast<StoreInst>(UI)) {
// Stores *of* the pointer are not interesting, only stores *to* the
// pointer.
if (UI->getOperand(1) != ASIV)
continue;
assert(!Store->isVolatile() && "AST broken");
if (!Store->isUnordered())
return false;
SawUnorderedAtomic |= Store->isAtomic();
SawNotAtomic |= !Store->isAtomic();
// If the store is guaranteed to execute, both properties are satisfied.
// We may want to check if a store is guaranteed to execute even if we
// already know that promotion is safe, since it may have higher
// alignment than any other guaranteed stores, in which case we can
// raise the alignment on the promoted store.
unsigned InstAlignment = Store->getAlignment();
if (!InstAlignment)
InstAlignment =
MDL.getABITypeAlignment(Store->getValueOperand()->getType());
if (!DereferenceableInPH || !SafeToInsertStore ||
(InstAlignment > Alignment)) {
if (isGuaranteedToExecute(*UI, DT, CurLoop, SafetyInfo)) {
DereferenceableInPH = true;
SafeToInsertStore = true;
Alignment = std::max(Alignment, InstAlignment);
}
}
// If a store dominates all exit blocks, it is safe to sink.
// As explained above, if an exit block was executed, a dominating
// store must have been executed at least once, so we are not
// introducing stores on paths that did not have them.
// Note that this only looks at explicit exit blocks. If we ever
// start sinking stores into unwind edges (see above), this will break.
if (!SafeToInsertStore)
SafeToInsertStore = llvm::all_of(ExitBlocks, [&](BasicBlock *Exit) {
return DT->dominates(Store->getParent(), Exit);
});
// If the store is not guaranteed to execute, we may still get
// deref info through it.
if (!DereferenceableInPH) {
DereferenceableInPH = isDereferenceableAndAlignedPointer(
Store->getPointerOperand(), Store->getAlignment(), MDL,
Preheader->getTerminator(), DT);
}
} else
return false; // Not a load or store.
// Merge the AA tags.
if (LoopUses.empty()) {
// On the first load/store, just take its AA tags.
UI->getAAMetadata(AATags);
} else if (AATags) {
UI->getAAMetadata(AATags, /* Merge = */ true);
}
LoopUses.push_back(UI);
}
}
// If we found both an unordered atomic instruction and a non-atomic memory
// access, bail. We can't blindly promote non-atomic to atomic since we
// might not be able to lower the result. We can't downgrade since that
// would violate memory model. Also, align 0 is an error for atomics.
if (SawUnorderedAtomic && SawNotAtomic)
return false;
// If we couldn't prove we can hoist the load, bail.
if (!DereferenceableInPH)
return false;
// We know we can hoist the load, but don't have a guaranteed store.
// Check whether the location is thread-local. If it is, then we can insert
// stores along paths which originally didn't have them without violating the
// memory model.
if (!SafeToInsertStore) {
if (IsKnownThreadLocalObject)
SafeToInsertStore = true;
else {
Value *Object = GetUnderlyingObject(SomePtr, MDL);
SafeToInsertStore =
(isAllocLikeFn(Object, TLI) || isa<AllocaInst>(Object)) &&
!PointerMayBeCaptured(Object, true, true);
}
}
// If we've still failed to prove we can sink the store, give up.
if (!SafeToInsertStore)
return false;
// Otherwise, this is safe to promote, lets do it!
LLVM_DEBUG(dbgs() << "LICM: Promoting value stored to in loop: " << *SomePtr
<< '\n');
ORE->emit([&]() {
return OptimizationRemark(DEBUG_TYPE, "PromoteLoopAccessesToScalar",
LoopUses[0])
<< "Moving accesses to memory location out of the loop";
});
++NumPromoted;
// Grab a debug location for the inserted loads/stores; given that the
// inserted loads/stores have little relation to the original loads/stores,
// this code just arbitrarily picks a location from one, since any debug
// location is better than none.
DebugLoc DL = LoopUses[0]->getDebugLoc();
// We use the SSAUpdater interface to insert phi nodes as required.
SmallVector<PHINode *, 16> NewPHIs;
SSAUpdater SSA(&NewPHIs);
LoopPromoter Promoter(SomePtr, LoopUses, SSA, PointerMustAliases, ExitBlocks,
InsertPts, PIC, *CurAST, *LI, DL, Alignment,
SawUnorderedAtomic, AATags);
// Set up the preheader to have a definition of the value. It is the live-out
// value from the preheader that uses in the loop will use.
LoadInst *PreheaderLoad = new LoadInst(
SomePtr, SomePtr->getName() + ".promoted", Preheader->getTerminator());
if (SawUnorderedAtomic)
PreheaderLoad->setOrdering(AtomicOrdering::Unordered);
PreheaderLoad->setAlignment(Alignment);
PreheaderLoad->setDebugLoc(DL);
if (AATags)
PreheaderLoad->setAAMetadata(AATags);
SSA.AddAvailableValue(Preheader, PreheaderLoad);
// Rewrite all the loads in the loop and remember all the definitions from
// stores in the loop.
Promoter.run(LoopUses);
// If the SSAUpdater didn't use the load in the preheader, just zap it now.
if (PreheaderLoad->use_empty())
PreheaderLoad->eraseFromParent();
return true;
}
/// Returns an owning pointer to an alias set which incorporates aliasing info
/// from L and all subloops of L.
/// FIXME: In new pass manager, there is no helper function to handle loop
/// analysis such as cloneBasicBlockAnalysis, so the AST needs to be recomputed
/// from scratch for every loop. Hook up with the helper functions when
/// available in the new pass manager to avoid redundant computation.
AliasSetTracker *
LoopInvariantCodeMotion::collectAliasInfoForLoop(Loop *L, LoopInfo *LI,
AliasAnalysis *AA) {
AliasSetTracker *CurAST = nullptr;
SmallVector<Loop *, 4> RecomputeLoops;
for (Loop *InnerL : L->getSubLoops()) {
auto MapI = LoopToAliasSetMap.find(InnerL);
// If the AST for this inner loop is missing it may have been merged into
// some other loop's AST and then that loop unrolled, and so we need to
// recompute it.
if (MapI == LoopToAliasSetMap.end()) {
RecomputeLoops.push_back(InnerL);
continue;
}
AliasSetTracker *InnerAST = MapI->second;
if (CurAST != nullptr) {
// What if InnerLoop was modified by other passes ?
CurAST->add(*InnerAST);
// Once we've incorporated the inner loop's AST into ours, we don't need
// the subloop's anymore.
delete InnerAST;
} else {
CurAST = InnerAST;
}
LoopToAliasSetMap.erase(MapI);
}
if (CurAST == nullptr)
CurAST = new AliasSetTracker(*AA);
auto mergeLoop = [&](Loop *L) {
// Loop over the body of this loop, looking for calls, invokes, and stores.
for (BasicBlock *BB : L->blocks())
CurAST->add(*BB); // Incorporate the specified basic block
};
// Add everything from the sub loops that are no longer directly available.
for (Loop *InnerL : RecomputeLoops)
mergeLoop(InnerL);
// And merge in this loop.
mergeLoop(L);
return CurAST;
}
/// Simple analysis hook. Clone alias set info.
///
void LegacyLICMPass::cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To,
Loop *L) {
AliasSetTracker *AST = LICM.getLoopToAliasSetMap().lookup(L);
if (!AST)
return;
AST->copyValue(From, To);
}
/// Simple Analysis hook. Delete value V from alias set
///
void LegacyLICMPass::deleteAnalysisValue(Value *V, Loop *L) {
AliasSetTracker *AST = LICM.getLoopToAliasSetMap().lookup(L);
if (!AST)
return;
AST->deleteValue(V);
}
/// Simple Analysis hook. Delete value L from alias set map.
///
void LegacyLICMPass::deleteAnalysisLoop(Loop *L) {
AliasSetTracker *AST = LICM.getLoopToAliasSetMap().lookup(L);
if (!AST)
return;
delete AST;
LICM.getLoopToAliasSetMap().erase(L);
}
/// Return true if the body of this loop may store into the memory
/// location pointed to by V.
///
static bool pointerInvalidatedByLoop(Value *V, uint64_t Size,
const AAMDNodes &AAInfo,
AliasSetTracker *CurAST) {
// Check to see if any of the basic blocks in CurLoop invalidate *V.
return CurAST->getAliasSetForPointer(V, Size, AAInfo).isMod();
}
/// Little predicate that returns true if the specified basic block is in
/// a subloop of the current one, not the current one itself.
///
static bool inSubLoop(BasicBlock *BB, Loop *CurLoop, LoopInfo *LI) {
assert(CurLoop->contains(BB) && "Only valid if BB is IN the loop");
return LI->getLoopFor(BB) != CurLoop;
}
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