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

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//===-- LoopIdiomRecognize.cpp - Loop idiom recognition -------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass implements an idiom recognizer that transforms simple loops into a
// non-loop form. In cases that this kicks in, it can be a significant
// performance win.
//
//===----------------------------------------------------------------------===//
//
// TODO List:
//
// Future loop memory idioms to recognize:
// memcmp, memmove, strlen, etc.
// Future floating point idioms to recognize in -ffast-math mode:
// fpowi
// Future integer operation idioms to recognize:
// ctpop, ctlz, cttz
//
// Beware that isel's default lowering for ctpop is highly inefficient for
// i64 and larger types when i64 is legal and the value has few bits set. It
// would be good to enhance isel to emit a loop for ctpop in this case.
//
// We should enhance the memset/memcpy recognition to handle multiple stores in
// the loop. This would handle things like:
// void foo(_Complex float *P)
// for (i) { __real__(*P) = 0; __imag__(*P) = 0; }
//
// This could recognize common matrix multiplies and dot product idioms and
// replace them with calls to BLAS (if linked in??).
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/BasicAliasAnalysis.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/ScalarEvolutionExpander.h"
#include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Module.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils/Local.h"
using namespace llvm;
#define DEBUG_TYPE "loop-idiom"
STATISTIC(NumMemSet, "Number of memset's formed from loop stores");
STATISTIC(NumMemCpy, "Number of memcpy's formed from loop load+stores");
namespace {
class LoopIdiomRecognize : public LoopPass {
Loop *CurLoop;
AliasAnalysis *AA;
DominatorTree *DT;
LoopInfo *LI;
ScalarEvolution *SE;
TargetLibraryInfo *TLI;
const TargetTransformInfo *TTI;
const DataLayout *DL;
public:
static char ID;
explicit LoopIdiomRecognize() : LoopPass(ID) {
initializeLoopIdiomRecognizePass(*PassRegistry::getPassRegistry());
}
bool runOnLoop(Loop *L, LPPassManager &LPM) override;
/// This transformation requires natural loop information & requires that
/// loop preheaders be inserted into the CFG.
///
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<LoopInfoWrapperPass>();
AU.addPreserved<LoopInfoWrapperPass>();
AU.addRequiredID(LoopSimplifyID);
AU.addPreservedID(LoopSimplifyID);
AU.addRequiredID(LCSSAID);
AU.addPreservedID(LCSSAID);
AU.addRequired<AAResultsWrapperPass>();
AU.addPreserved<AAResultsWrapperPass>();
AU.addRequired<ScalarEvolutionWrapperPass>();
AU.addPreserved<ScalarEvolutionWrapperPass>();
AU.addPreserved<SCEVAAWrapperPass>();
AU.addRequired<DominatorTreeWrapperPass>();
AU.addPreserved<DominatorTreeWrapperPass>();
AU.addRequired<TargetLibraryInfoWrapperPass>();
AU.addRequired<TargetTransformInfoWrapperPass>();
AU.addPreserved<BasicAAWrapperPass>();
AU.addPreserved<GlobalsAAWrapperPass>();
}
private:
/// \name Countable Loop Idiom Handling
/// @{
bool runOnCountableLoop();
bool runOnLoopBlock(BasicBlock *BB, const SCEV *BECount,
SmallVectorImpl<BasicBlock *> &ExitBlocks);
bool processLoopStore(StoreInst *SI, const SCEV *BECount);
bool processLoopMemSet(MemSetInst *MSI, const SCEV *BECount);
bool processLoopStridedStore(Value *DestPtr, unsigned StoreSize,
unsigned StoreAlignment, Value *SplatValue,
Instruction *TheStore, const SCEVAddRecExpr *Ev,
const SCEV *BECount, bool NegStride);
bool processLoopStoreOfLoopLoad(StoreInst *SI, unsigned StoreSize,
const SCEVAddRecExpr *StoreEv,
const SCEVAddRecExpr *LoadEv,
const SCEV *BECount);
/// @}
/// \name Noncountable Loop Idiom Handling
/// @{
bool runOnNoncountableLoop();
bool recognizePopcount();
void transformLoopToPopcount(BasicBlock *PreCondBB, Instruction *CntInst,
PHINode *CntPhi, Value *Var);
/// @}
};
} // End anonymous namespace.
char LoopIdiomRecognize::ID = 0;
INITIALIZE_PASS_BEGIN(LoopIdiomRecognize, "loop-idiom", "Recognize loop idioms",
false, false)
INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
INITIALIZE_PASS_DEPENDENCY(LCSSA)
INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(BasicAAWrapperPass)
INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
INITIALIZE_PASS_DEPENDENCY(GlobalsAAWrapperPass)
INITIALIZE_PASS_DEPENDENCY(SCEVAAWrapperPass)
INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
INITIALIZE_PASS_END(LoopIdiomRecognize, "loop-idiom", "Recognize loop idioms",
false, false)
Pass *llvm::createLoopIdiomPass() { return new LoopIdiomRecognize(); }
/// deleteDeadInstruction - Delete this instruction. Before we do, go through
/// and zero out all the operands of this instruction. If any of them become
/// dead, delete them and the computation tree that feeds them.
///
static void deleteDeadInstruction(Instruction *I,
const TargetLibraryInfo *TLI) {
SmallVector<Value *, 16> Operands(I->value_op_begin(), I->value_op_end());
I->replaceAllUsesWith(UndefValue::get(I->getType()));
I->eraseFromParent();
for (Value *Op : Operands)
RecursivelyDeleteTriviallyDeadInstructions(Op, TLI);
}
//===----------------------------------------------------------------------===//
//
// Implementation of LoopIdiomRecognize
//
//===----------------------------------------------------------------------===//
bool LoopIdiomRecognize::runOnLoop(Loop *L, LPPassManager &LPM) {
if (skipOptnoneFunction(L))
return false;
CurLoop = L;
// If the loop could not be converted to canonical form, it must have an
// indirectbr in it, just give up.
if (!L->getLoopPreheader())
return false;
// Disable loop idiom recognition if the function's name is a common idiom.
StringRef Name = L->getHeader()->getParent()->getName();
if (Name == "memset" || Name == "memcpy")
return false;
AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(
*CurLoop->getHeader()->getParent());
DL = &CurLoop->getHeader()->getModule()->getDataLayout();
if (SE->hasLoopInvariantBackedgeTakenCount(L))
return runOnCountableLoop();
return runOnNoncountableLoop();
}
bool LoopIdiomRecognize::runOnCountableLoop() {
const SCEV *BECount = SE->getBackedgeTakenCount(CurLoop);
assert(!isa<SCEVCouldNotCompute>(BECount) &&
"runOnCountableLoop() called on a loop without a predictable"
"backedge-taken count");
// If this loop executes exactly one time, then it should be peeled, not
// optimized by this pass.
if (const SCEVConstant *BECst = dyn_cast<SCEVConstant>(BECount))
if (BECst->getValue()->getValue() == 0)
return false;
SmallVector<BasicBlock *, 8> ExitBlocks;
CurLoop->getUniqueExitBlocks(ExitBlocks);
DEBUG(dbgs() << "loop-idiom Scanning: F["
<< CurLoop->getHeader()->getParent()->getName() << "] Loop %"
<< CurLoop->getHeader()->getName() << "\n");
bool MadeChange = false;
// Scan all the blocks in the loop that are not in subloops.
for (auto *BB : CurLoop->getBlocks()) {
// Ignore blocks in subloops.
if (LI->getLoopFor(BB) != CurLoop)
continue;
MadeChange |= runOnLoopBlock(BB, BECount, ExitBlocks);
}
return MadeChange;
}
/// runOnLoopBlock - Process the specified block, which lives in a counted loop
/// with the specified backedge count. This block is known to be in the current
/// loop and not in any subloops.
bool LoopIdiomRecognize::runOnLoopBlock(
BasicBlock *BB, const SCEV *BECount,
SmallVectorImpl<BasicBlock *> &ExitBlocks) {
// We can only promote stores in this block if they are unconditionally
// executed in the loop. For a block to be unconditionally executed, it has
// to dominate all the exit blocks of the loop. Verify this now.
for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
if (!DT->dominates(BB, ExitBlocks[i]))
return false;
bool MadeChange = false;
for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E;) {
Instruction *Inst = &*I++;
// Look for store instructions, which may be optimized to memset/memcpy.
if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
WeakVH InstPtr(&*I);
if (!processLoopStore(SI, BECount))
continue;
MadeChange = true;
// If processing the store invalidated our iterator, start over from the
// top of the block.
if (!InstPtr)
I = BB->begin();
continue;
}
// Look for memset instructions, which may be optimized to a larger memset.
if (MemSetInst *MSI = dyn_cast<MemSetInst>(Inst)) {
WeakVH InstPtr(&*I);
if (!processLoopMemSet(MSI, BECount))
continue;
MadeChange = true;
// If processing the memset invalidated our iterator, start over from the
// top of the block.
if (!InstPtr)
I = BB->begin();
continue;
}
}
return MadeChange;
}
/// processLoopStore - See if this store can be promoted to a memset or memcpy.
bool LoopIdiomRecognize::processLoopStore(StoreInst *SI, const SCEV *BECount) {
if (!SI->isSimple())
return false;
Value *StoredVal = SI->getValueOperand();
Value *StorePtr = SI->getPointerOperand();
// Reject stores that are so large that they overflow an unsigned.
uint64_t SizeInBits = DL->getTypeSizeInBits(StoredVal->getType());
if ((SizeInBits & 7) || (SizeInBits >> 32) != 0)
return false;
// See if the pointer expression is an AddRec like {base,+,1} on the current
// loop, which indicates a strided store. If we have something else, it's a
// random store we can't handle.
const SCEVAddRecExpr *StoreEv =
dyn_cast<SCEVAddRecExpr>(SE->getSCEV(StorePtr));
if (!StoreEv || StoreEv->getLoop() != CurLoop || !StoreEv->isAffine())
return false;
// Check to see if the stride matches the size of the store. If so, then we
// know that every byte is touched in the loop.
unsigned StoreSize = (unsigned)SizeInBits >> 3;
const SCEVConstant *ConstStride =
dyn_cast<SCEVConstant>(StoreEv->getOperand(1));
if (!ConstStride)
return false;
APInt Stride = ConstStride->getValue()->getValue();
if (StoreSize != Stride && StoreSize != -Stride)
return false;
bool NegStride = StoreSize == -Stride;
// See if we can optimize just this store in isolation.
if (processLoopStridedStore(StorePtr, StoreSize, SI->getAlignment(),
StoredVal, SI, StoreEv, BECount, NegStride))
return true;
// TODO: We don't handle negative stride memcpys.
if (NegStride)
return false;
// If the stored value is a strided load in the same loop with the same stride
// this may be transformable into a memcpy. This kicks in for stuff like
// for (i) A[i] = B[i];
if (LoadInst *LI = dyn_cast<LoadInst>(StoredVal)) {
const SCEVAddRecExpr *LoadEv =
dyn_cast<SCEVAddRecExpr>(SE->getSCEV(LI->getOperand(0)));
if (LoadEv && LoadEv->getLoop() == CurLoop && LoadEv->isAffine() &&
StoreEv->getOperand(1) == LoadEv->getOperand(1) && LI->isSimple())
if (processLoopStoreOfLoopLoad(SI, StoreSize, StoreEv, LoadEv, BECount))
return true;
}
// errs() << "UNHANDLED strided store: " << *StoreEv << " - " << *SI << "\n";
return false;
}
/// processLoopMemSet - See if this memset can be promoted to a large memset.
bool LoopIdiomRecognize::processLoopMemSet(MemSetInst *MSI,
const SCEV *BECount) {
// We can only handle non-volatile memsets with a constant size.
if (MSI->isVolatile() || !isa<ConstantInt>(MSI->getLength()))
return false;
// If we're not allowed to hack on memset, we fail.
if (!TLI->has(LibFunc::memset))
return false;
Value *Pointer = MSI->getDest();
// See if the pointer expression is an AddRec like {base,+,1} on the current
// loop, which indicates a strided store. If we have something else, it's a
// random store we can't handle.
const SCEVAddRecExpr *Ev = dyn_cast<SCEVAddRecExpr>(SE->getSCEV(Pointer));
if (!Ev || Ev->getLoop() != CurLoop || !Ev->isAffine())
return false;
// Reject memsets that are so large that they overflow an unsigned.
uint64_t SizeInBytes = cast<ConstantInt>(MSI->getLength())->getZExtValue();
if ((SizeInBytes >> 32) != 0)
return false;
// Check to see if the stride matches the size of the memset. If so, then we
// know that every byte is touched in the loop.
const SCEVConstant *Stride = dyn_cast<SCEVConstant>(Ev->getOperand(1));
// TODO: Could also handle negative stride here someday, that will require the
// validity check in mayLoopAccessLocation to be updated though.
if (!Stride || MSI->getLength() != Stride->getValue())
return false;
return processLoopStridedStore(Pointer, (unsigned)SizeInBytes,
MSI->getAlignment(), MSI->getValue(), MSI, Ev,
BECount, /*NegStride=*/false);
}
/// mayLoopAccessLocation - Return true if the specified loop might access the
/// specified pointer location, which is a loop-strided access. The 'Access'
/// argument specifies what the verboten forms of access are (read or write).
static bool mayLoopAccessLocation(Value *Ptr, ModRefInfo Access, Loop *L,
const SCEV *BECount, unsigned StoreSize,
AliasAnalysis &AA,
Instruction *IgnoredStore) {
// Get the location that may be stored across the loop. Since the access is
// strided positively through memory, we say that the modified location starts
// at the pointer and has infinite size.
uint64_t AccessSize = MemoryLocation::UnknownSize;
// If the loop iterates a fixed number of times, we can refine the access size
// to be exactly the size of the memset, which is (BECount+1)*StoreSize
if (const SCEVConstant *BECst = dyn_cast<SCEVConstant>(BECount))
AccessSize = (BECst->getValue()->getZExtValue() + 1) * StoreSize;
// TODO: For this to be really effective, we have to dive into the pointer
// operand in the store. Store to &A[i] of 100 will always return may alias
// with store of &A[100], we need to StoreLoc to be "A" with size of 100,
// which will then no-alias a store to &A[100].
MemoryLocation StoreLoc(Ptr, AccessSize);
for (Loop::block_iterator BI = L->block_begin(), E = L->block_end(); BI != E;
++BI)
for (BasicBlock::iterator I = (*BI)->begin(), E = (*BI)->end(); I != E; ++I)
if (&*I != IgnoredStore && (AA.getModRefInfo(&*I, StoreLoc) & Access))
return true;
return false;
}
/// getMemSetPatternValue - If a strided store of the specified value is safe to
/// turn into a memset_pattern16, return a ConstantArray of 16 bytes that should
/// be passed in. Otherwise, return null.
///
/// Note that we don't ever attempt to use memset_pattern8 or 4, because these
/// just replicate their input array and then pass on to memset_pattern16.
static Constant *getMemSetPatternValue(Value *V, const DataLayout *DL) {
// If the value isn't a constant, we can't promote it to being in a constant
// array. We could theoretically do a store to an alloca or something, but
// that doesn't seem worthwhile.
Constant *C = dyn_cast<Constant>(V);
if (!C)
return nullptr;
// Only handle simple values that are a power of two bytes in size.
uint64_t Size = DL->getTypeSizeInBits(V->getType());
if (Size == 0 || (Size & 7) || (Size & (Size - 1)))
return nullptr;
// Don't care enough about darwin/ppc to implement this.
if (DL->isBigEndian())
return nullptr;
// Convert to size in bytes.
Size /= 8;
// TODO: If CI is larger than 16-bytes, we can try slicing it in half to see
// if the top and bottom are the same (e.g. for vectors and large integers).
if (Size > 16)
return nullptr;
// If the constant is exactly 16 bytes, just use it.
if (Size == 16)
return C;
// Otherwise, we'll use an array of the constants.
unsigned ArraySize = 16 / Size;
ArrayType *AT = ArrayType::get(V->getType(), ArraySize);
return ConstantArray::get(AT, std::vector<Constant *>(ArraySize, C));
}
/// processLoopStridedStore - We see a strided store of some value. If we can
/// transform this into a memset or memset_pattern in the loop preheader, do so.
bool LoopIdiomRecognize::processLoopStridedStore(
Value *DestPtr, unsigned StoreSize, unsigned StoreAlignment,
Value *StoredVal, Instruction *TheStore, const SCEVAddRecExpr *Ev,
const SCEV *BECount, bool NegStride) {
// If the stored value is a byte-wise value (like i32 -1), then it may be
// turned into a memset of i8 -1, assuming that all the consecutive bytes
// are stored. A store of i32 0x01020304 can never be turned into a memset,
// but it can be turned into memset_pattern if the target supports it.
Value *SplatValue = isBytewiseValue(StoredVal);
Constant *PatternValue = nullptr;
unsigned DestAS = DestPtr->getType()->getPointerAddressSpace();
// If we're allowed to form a memset, and the stored value would be acceptable
// for memset, use it.
if (SplatValue && TLI->has(LibFunc::memset) &&
// Verify that the stored value is loop invariant. If not, we can't
// promote the memset.
CurLoop->isLoopInvariant(SplatValue)) {
// Keep and use SplatValue.
PatternValue = nullptr;
} else if (DestAS == 0 && TLI->has(LibFunc::memset_pattern16) &&
(PatternValue = getMemSetPatternValue(StoredVal, DL))) {
// Don't create memset_pattern16s with address spaces.
// It looks like we can use PatternValue!
SplatValue = nullptr;
} else {
// Otherwise, this isn't an idiom we can transform. For example, we can't
// do anything with a 3-byte store.
return false;
}
// The trip count of the loop and the base pointer of the addrec SCEV is
// guaranteed to be loop invariant, which means that it should dominate the
// header. This allows us to insert code for it in the preheader.
BasicBlock *Preheader = CurLoop->getLoopPreheader();
IRBuilder<> Builder(Preheader->getTerminator());
SCEVExpander Expander(*SE, *DL, "loop-idiom");
Type *DestInt8PtrTy = Builder.getInt8PtrTy(DestAS);
Type *IntPtr = Builder.getIntPtrTy(*DL, DestAS);
const SCEV *Start = Ev->getStart();
// If we have a negative stride, Start refers to the end of the memory
// location we're trying to memset. Therefore, we need to recompute the start
// point, which is just Start - BECount*Size.
if (NegStride) {
const SCEV *Index = SE->getTruncateOrZeroExtend(BECount, IntPtr);
if (StoreSize != 1)
Index = SE->getMulExpr(Index, SE->getConstant(IntPtr, StoreSize),
SCEV::FlagNUW);
Start = SE->getMinusSCEV(Ev->getStart(), Index);
}
// Okay, we have a strided store "p[i]" of a splattable value. We can turn
// this into a memset in the loop preheader now if we want. However, this
// would be unsafe to do if there is anything else in the loop that may read
// or write to the aliased location. Check for any overlap by generating the
// base pointer and checking the region.
Value *BasePtr =
Expander.expandCodeFor(Start, DestInt8PtrTy, Preheader->getTerminator());
if (mayLoopAccessLocation(BasePtr, MRI_ModRef, CurLoop, BECount, StoreSize,
*AA, TheStore)) {
Expander.clear();
// If we generated new code for the base pointer, clean up.
RecursivelyDeleteTriviallyDeadInstructions(BasePtr, TLI);
return false;
}
// Okay, everything looks good, insert the memset.
// The # stored bytes is (BECount+1)*Size. Expand the trip count out to
// pointer size if it isn't already.
BECount = SE->getTruncateOrZeroExtend(BECount, IntPtr);
const SCEV *NumBytesS =
SE->getAddExpr(BECount, SE->getOne(IntPtr), SCEV::FlagNUW);
if (StoreSize != 1) {
NumBytesS = SE->getMulExpr(NumBytesS, SE->getConstant(IntPtr, StoreSize),
SCEV::FlagNUW);
}
Value *NumBytes =
Expander.expandCodeFor(NumBytesS, IntPtr, Preheader->getTerminator());
CallInst *NewCall;
if (SplatValue) {
NewCall =
Builder.CreateMemSet(BasePtr, SplatValue, NumBytes, StoreAlignment);
} else {
// Everything is emitted in default address space
Type *Int8PtrTy = DestInt8PtrTy;
Module *M = TheStore->getParent()->getParent()->getParent();
Value *MSP =
M->getOrInsertFunction("memset_pattern16", Builder.getVoidTy(),
Int8PtrTy, Int8PtrTy, IntPtr, (void *)nullptr);
// Otherwise we should form a memset_pattern16. PatternValue is known to be
// an constant array of 16-bytes. Plop the value into a mergable global.
GlobalVariable *GV = new GlobalVariable(*M, PatternValue->getType(), true,
GlobalValue::PrivateLinkage,
PatternValue, ".memset_pattern");
GV->setUnnamedAddr(true); // Ok to merge these.
GV->setAlignment(16);
Value *PatternPtr = ConstantExpr::getBitCast(GV, Int8PtrTy);
NewCall = Builder.CreateCall(MSP, {BasePtr, PatternPtr, NumBytes});
}
DEBUG(dbgs() << " Formed memset: " << *NewCall << "\n"
<< " from store to: " << *Ev << " at: " << *TheStore << "\n");
NewCall->setDebugLoc(TheStore->getDebugLoc());
// Okay, the memset has been formed. Zap the original store and anything that
// feeds into it.
deleteDeadInstruction(TheStore, TLI);
++NumMemSet;
return true;
}
/// processLoopStoreOfLoopLoad - We see a strided store whose value is a
/// same-strided load.
bool LoopIdiomRecognize::processLoopStoreOfLoopLoad(
StoreInst *SI, unsigned StoreSize, const SCEVAddRecExpr *StoreEv,
const SCEVAddRecExpr *LoadEv, const SCEV *BECount) {
// If we're not allowed to form memcpy, we fail.
if (!TLI->has(LibFunc::memcpy))
return false;
LoadInst *LI = cast<LoadInst>(SI->getValueOperand());
// The trip count of the loop and the base pointer of the addrec SCEV is
// guaranteed to be loop invariant, which means that it should dominate the
// header. This allows us to insert code for it in the preheader.
BasicBlock *Preheader = CurLoop->getLoopPreheader();
IRBuilder<> Builder(Preheader->getTerminator());
SCEVExpander Expander(*SE, *DL, "loop-idiom");
// Okay, we have a strided store "p[i]" of a loaded value. We can turn
// this into a memcpy in the loop preheader now if we want. However, this
// would be unsafe to do if there is anything else in the loop that may read
// or write the memory region we're storing to. This includes the load that
// feeds the stores. Check for an alias by generating the base address and
// checking everything.
Value *StoreBasePtr = Expander.expandCodeFor(
StoreEv->getStart(), Builder.getInt8PtrTy(SI->getPointerAddressSpace()),
Preheader->getTerminator());
if (mayLoopAccessLocation(StoreBasePtr, MRI_ModRef, CurLoop, BECount,
StoreSize, *AA, SI)) {
Expander.clear();
// If we generated new code for the base pointer, clean up.
RecursivelyDeleteTriviallyDeadInstructions(StoreBasePtr, TLI);
return false;
}
// For a memcpy, we have to make sure that the input array is not being
// mutated by the loop.
Value *LoadBasePtr = Expander.expandCodeFor(
LoadEv->getStart(), Builder.getInt8PtrTy(LI->getPointerAddressSpace()),
Preheader->getTerminator());
if (mayLoopAccessLocation(LoadBasePtr, MRI_Mod, CurLoop, BECount, StoreSize,
*AA, SI)) {
Expander.clear();
// If we generated new code for the base pointer, clean up.
RecursivelyDeleteTriviallyDeadInstructions(LoadBasePtr, TLI);
RecursivelyDeleteTriviallyDeadInstructions(StoreBasePtr, TLI);
return false;
}
// Okay, everything is safe, we can transform this!
// The # stored bytes is (BECount+1)*Size. Expand the trip count out to
// pointer size if it isn't already.
Type *IntPtrTy = Builder.getIntPtrTy(*DL, SI->getPointerAddressSpace());
BECount = SE->getTruncateOrZeroExtend(BECount, IntPtrTy);
const SCEV *NumBytesS =
SE->getAddExpr(BECount, SE->getOne(IntPtrTy), SCEV::FlagNUW);
if (StoreSize != 1)
NumBytesS = SE->getMulExpr(NumBytesS, SE->getConstant(IntPtrTy, StoreSize),
SCEV::FlagNUW);
Value *NumBytes =
Expander.expandCodeFor(NumBytesS, IntPtrTy, Preheader->getTerminator());
CallInst *NewCall =
Builder.CreateMemCpy(StoreBasePtr, LoadBasePtr, NumBytes,
std::min(SI->getAlignment(), LI->getAlignment()));
NewCall->setDebugLoc(SI->getDebugLoc());
DEBUG(dbgs() << " Formed memcpy: " << *NewCall << "\n"
<< " from load ptr=" << *LoadEv << " at: " << *LI << "\n"
<< " from store ptr=" << *StoreEv << " at: " << *SI << "\n");
// Okay, the memcpy has been formed. Zap the original store and anything that
// feeds into it.
deleteDeadInstruction(SI, TLI);
++NumMemCpy;
return true;
}
bool LoopIdiomRecognize::runOnNoncountableLoop() {
return recognizePopcount();
}
/// Check if the given conditional branch is based on the comparison between
/// a variable and zero, and if the variable is non-zero, the control yields to
/// the loop entry. If the branch matches the behavior, the variable involved
/// in the comparion is returned. This function will be called to see if the
/// precondition and postcondition of the loop are in desirable form.
static Value *matchCondition(BranchInst *BI, BasicBlock *LoopEntry) {
if (!BI || !BI->isConditional())
return nullptr;
ICmpInst *Cond = dyn_cast<ICmpInst>(BI->getCondition());
if (!Cond)
return nullptr;
ConstantInt *CmpZero = dyn_cast<ConstantInt>(Cond->getOperand(1));
if (!CmpZero || !CmpZero->isZero())
return nullptr;
ICmpInst::Predicate Pred = Cond->getPredicate();
if ((Pred == ICmpInst::ICMP_NE && BI->getSuccessor(0) == LoopEntry) ||
(Pred == ICmpInst::ICMP_EQ && BI->getSuccessor(1) == LoopEntry))
return Cond->getOperand(0);
return nullptr;
}
/// Return true iff the idiom is detected in the loop.
///
/// Additionally:
/// 1) \p CntInst is set to the instruction counting the population bit.
/// 2) \p CntPhi is set to the corresponding phi node.
/// 3) \p Var is set to the value whose population bits are being counted.
///
/// The core idiom we are trying to detect is:
/// \code
/// if (x0 != 0)
/// goto loop-exit // the precondition of the loop
/// cnt0 = init-val;
/// do {
/// x1 = phi (x0, x2);
/// cnt1 = phi(cnt0, cnt2);
///
/// cnt2 = cnt1 + 1;
/// ...
/// x2 = x1 & (x1 - 1);
/// ...
/// } while(x != 0);
///
/// loop-exit:
/// \endcode
static bool detectPopcountIdiom(Loop *CurLoop, BasicBlock *PreCondBB,
Instruction *&CntInst, PHINode *&CntPhi,
Value *&Var) {
// step 1: Check to see if the look-back branch match this pattern:
// "if (a!=0) goto loop-entry".
BasicBlock *LoopEntry;
Instruction *DefX2, *CountInst;
Value *VarX1, *VarX0;
PHINode *PhiX, *CountPhi;
DefX2 = CountInst = nullptr;
VarX1 = VarX0 = nullptr;
PhiX = CountPhi = nullptr;
LoopEntry = *(CurLoop->block_begin());
// step 1: Check if the loop-back branch is in desirable form.
{
if (Value *T = matchCondition(
dyn_cast<BranchInst>(LoopEntry->getTerminator()), LoopEntry))
DefX2 = dyn_cast<Instruction>(T);
else
return false;
}
// step 2: detect instructions corresponding to "x2 = x1 & (x1 - 1)"
{
if (!DefX2 || DefX2->getOpcode() != Instruction::And)
return false;
BinaryOperator *SubOneOp;
if ((SubOneOp = dyn_cast<BinaryOperator>(DefX2->getOperand(0))))
VarX1 = DefX2->getOperand(1);
else {
VarX1 = DefX2->getOperand(0);
SubOneOp = dyn_cast<BinaryOperator>(DefX2->getOperand(1));
}
if (!SubOneOp)
return false;
Instruction *SubInst = cast<Instruction>(SubOneOp);
ConstantInt *Dec = dyn_cast<ConstantInt>(SubInst->getOperand(1));
if (!Dec ||
!((SubInst->getOpcode() == Instruction::Sub && Dec->isOne()) ||
(SubInst->getOpcode() == Instruction::Add &&
Dec->isAllOnesValue()))) {
return false;
}
}
// step 3: Check the recurrence of variable X
{
PhiX = dyn_cast<PHINode>(VarX1);
if (!PhiX ||
(PhiX->getOperand(0) != DefX2 && PhiX->getOperand(1) != DefX2)) {
return false;
}
}
// step 4: Find the instruction which count the population: cnt2 = cnt1 + 1
{
CountInst = nullptr;
for (BasicBlock::iterator Iter = LoopEntry->getFirstNonPHI()->getIterator(),
IterE = LoopEntry->end();
Iter != IterE; Iter++) {
Instruction *Inst = &*Iter;
if (Inst->getOpcode() != Instruction::Add)
continue;
ConstantInt *Inc = dyn_cast<ConstantInt>(Inst->getOperand(1));
if (!Inc || !Inc->isOne())
continue;
PHINode *Phi = dyn_cast<PHINode>(Inst->getOperand(0));
if (!Phi || Phi->getParent() != LoopEntry)
continue;
// Check if the result of the instruction is live of the loop.
bool LiveOutLoop = false;
for (User *U : Inst->users()) {
if ((cast<Instruction>(U))->getParent() != LoopEntry) {
LiveOutLoop = true;
break;
}
}
if (LiveOutLoop) {
CountInst = Inst;
CountPhi = Phi;
break;
}
}
if (!CountInst)
return false;
}
// step 5: check if the precondition is in this form:
// "if (x != 0) goto loop-head ; else goto somewhere-we-don't-care;"
{
auto *PreCondBr = dyn_cast<BranchInst>(PreCondBB->getTerminator());
Value *T = matchCondition(PreCondBr, CurLoop->getLoopPreheader());
if (T != PhiX->getOperand(0) && T != PhiX->getOperand(1))
return false;
CntInst = CountInst;
CntPhi = CountPhi;
Var = T;
}
return true;
}
/// Recognizes a population count idiom in a non-countable loop.
///
/// If detected, transforms the relevant code to issue the popcount intrinsic
/// function call, and returns true; otherwise, returns false.
bool LoopIdiomRecognize::recognizePopcount() {
if (TTI->getPopcntSupport(32) != TargetTransformInfo::PSK_FastHardware)
return false;
// Counting population are usually conducted by few arithmetic instructions.
// Such instructions can be easily "absorbed" by vacant slots in a
// non-compact loop. Therefore, recognizing popcount idiom only makes sense
// in a compact loop.
// Give up if the loop has multiple blocks or multiple backedges.
if (CurLoop->getNumBackEdges() != 1 || CurLoop->getNumBlocks() != 1)
return false;
BasicBlock *LoopBody = *(CurLoop->block_begin());
if (LoopBody->size() >= 20) {
// The loop is too big, bail out.
return false;
}
// It should have a preheader containing nothing but an unconditional branch.
BasicBlock *PH = CurLoop->getLoopPreheader();
if (!PH)
return false;
if (&PH->front() != PH->getTerminator())
return false;
auto *EntryBI = dyn_cast<BranchInst>(PH->getTerminator());
if (!EntryBI || EntryBI->isConditional())
return false;
// It should have a precondition block where the generated popcount instrinsic
// function can be inserted.
auto *PreCondBB = PH->getSinglePredecessor();
if (!PreCondBB)
return false;
auto *PreCondBI = dyn_cast<BranchInst>(PreCondBB->getTerminator());
if (!PreCondBI || PreCondBI->isUnconditional())
return false;
Instruction *CntInst;
PHINode *CntPhi;
Value *Val;
if (!detectPopcountIdiom(CurLoop, PreCondBB, CntInst, CntPhi, Val))
return false;
transformLoopToPopcount(PreCondBB, CntInst, CntPhi, Val);
return true;
}
static CallInst *createPopcntIntrinsic(IRBuilder<> &IRBuilder, Value *Val,
DebugLoc DL) {
Value *Ops[] = {Val};
Type *Tys[] = {Val->getType()};
Module *M = IRBuilder.GetInsertBlock()->getParent()->getParent();
Value *Func = Intrinsic::getDeclaration(M, Intrinsic::ctpop, Tys);
CallInst *CI = IRBuilder.CreateCall(Func, Ops);
CI->setDebugLoc(DL);
return CI;
}
void LoopIdiomRecognize::transformLoopToPopcount(BasicBlock *PreCondBB,
Instruction *CntInst,
PHINode *CntPhi, Value *Var) {
BasicBlock *PreHead = CurLoop->getLoopPreheader();
auto *PreCondBr = dyn_cast<BranchInst>(PreCondBB->getTerminator());
const DebugLoc DL = CntInst->getDebugLoc();
// Assuming before transformation, the loop is following:
// if (x) // the precondition
// do { cnt++; x &= x - 1; } while(x);
// Step 1: Insert the ctpop instruction at the end of the precondition block
IRBuilder<> Builder(PreCondBr);
Value *PopCnt, *PopCntZext, *NewCount, *TripCnt;
{
PopCnt = createPopcntIntrinsic(Builder, Var, DL);
NewCount = PopCntZext =
Builder.CreateZExtOrTrunc(PopCnt, cast<IntegerType>(CntPhi->getType()));
if (NewCount != PopCnt)
(cast<Instruction>(NewCount))->setDebugLoc(DL);
// TripCnt is exactly the number of iterations the loop has
TripCnt = NewCount;
// If the population counter's initial value is not zero, insert Add Inst.
Value *CntInitVal = CntPhi->getIncomingValueForBlock(PreHead);
ConstantInt *InitConst = dyn_cast<ConstantInt>(CntInitVal);
if (!InitConst || !InitConst->isZero()) {
NewCount = Builder.CreateAdd(NewCount, CntInitVal);
(cast<Instruction>(NewCount))->setDebugLoc(DL);
}
}
// Step 2: Replace the precondition from "if (x == 0) goto loop-exit" to
// "if (NewCount == 0) loop-exit". Without this change, the intrinsic
// function would be partial dead code, and downstream passes will drag
// it back from the precondition block to the preheader.
{
ICmpInst *PreCond = cast<ICmpInst>(PreCondBr->getCondition());
Value *Opnd0 = PopCntZext;
Value *Opnd1 = ConstantInt::get(PopCntZext->getType(), 0);
if (PreCond->getOperand(0) != Var)
std::swap(Opnd0, Opnd1);
ICmpInst *NewPreCond = cast<ICmpInst>(
Builder.CreateICmp(PreCond->getPredicate(), Opnd0, Opnd1));
PreCondBr->setCondition(NewPreCond);
RecursivelyDeleteTriviallyDeadInstructions(PreCond, TLI);
}
// Step 3: Note that the population count is exactly the trip count of the
// loop in question, which enable us to to convert the loop from noncountable
// loop into a countable one. The benefit is twofold:
//
// - If the loop only counts population, the entire loop becomes dead after
// the transformation. It is a lot easier to prove a countable loop dead
// than to prove a noncountable one. (In some C dialects, an infinite loop
// isn't dead even if it computes nothing useful. In general, DCE needs
// to prove a noncountable loop finite before safely delete it.)
//
// - If the loop also performs something else, it remains alive.
// Since it is transformed to countable form, it can be aggressively
// optimized by some optimizations which are in general not applicable
// to a noncountable loop.
//
// After this step, this loop (conceptually) would look like following:
// newcnt = __builtin_ctpop(x);
// t = newcnt;
// if (x)
// do { cnt++; x &= x-1; t--) } while (t > 0);
BasicBlock *Body = *(CurLoop->block_begin());
{
auto *LbBr = dyn_cast<BranchInst>(Body->getTerminator());
ICmpInst *LbCond = cast<ICmpInst>(LbBr->getCondition());
Type *Ty = TripCnt->getType();
PHINode *TcPhi = PHINode::Create(Ty, 2, "tcphi", &Body->front());
Builder.SetInsertPoint(LbCond);
Instruction *TcDec = cast<Instruction>(
Builder.CreateSub(TcPhi, ConstantInt::get(Ty, 1),
"tcdec", false, true));
TcPhi->addIncoming(TripCnt, PreHead);
TcPhi->addIncoming(TcDec, Body);
CmpInst::Predicate Pred =
(LbBr->getSuccessor(0) == Body) ? CmpInst::ICMP_UGT : CmpInst::ICMP_SLE;
LbCond->setPredicate(Pred);
LbCond->setOperand(0, TcDec);
LbCond->setOperand(1, ConstantInt::get(Ty, 0));
}
// Step 4: All the references to the original population counter outside
// the loop are replaced with the NewCount -- the value returned from
// __builtin_ctpop().
CntInst->replaceUsesOutsideBlock(NewCount, Body);
// step 5: Forget the "non-computable" trip-count SCEV associated with the
// loop. The loop would otherwise not be deleted even if it becomes empty.
SE->forgetLoop(CurLoop);
}
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