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[LIR] Remove the dedicated class for popcount recognition and sink the

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code into methods on LoopIdiomRecognize.

This simplifies the code somewhat and also makes it much easier to move
the analyses around. Ultimately, the separate class wasn't providing
significant value over methods -- it contained the precondition basic
block and the current loop. The current loop is already available and
the precondition block wasn't needed everywhere and is easy to pass
around.

In several cases I just moved things to be static functions because they
already accepted most of their inputs as arguments.

This doesn't fix the way we manage analyses yet, that will be the next
patch, but it already makes the code over 50 lines shorter.

No functionality changed.

llvm-svn: 244851
This commit is contained in:
Chandler Carruth 2015-08-13 00:44:29 +00:00
parent 7937d8a6c3
commit 3d8b7669f2
1 changed files with 343 additions and 392 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

735
lib/Transforms/Scalar/LoopIdiomRecognize.cpp
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@ -67,50 +67,6 @@ STATISTIC(NumMemCpy, "Number of memcpy's formed from loop load+stores");
namespace {
class LoopIdiomRecognize;
/// This class is to recoginize idioms of population-count conducted in
/// a noncountable loop. Currently it only recognizes this pattern:
/// \code
/// while(x) {cnt++; ...; x &= x - 1; ...}
/// \endcode
class NclPopcountRecognize {
LoopIdiomRecognize &LIR;
Loop *CurLoop;
BasicBlock *PreCondBB;
typedef IRBuilder<> IRBuilderTy;
public:
explicit NclPopcountRecognize(LoopIdiomRecognize &TheLIR);
bool recognize();
private:
/// Take a glimpse of the loop to see if we need to go ahead recoginizing
/// the idiom.
bool preliminaryScreen();
/// 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.
Value *matchCondition(BranchInst *Br, BasicBlock *NonZeroTarget) const;
/// Return true iff the idiom is detected in the loop. and 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.
bool detectIdiom(Instruction *&CntInst, PHINode *&CntPhi, Value *&Var) const;
/// Insert ctpop intrinsic function and some obviously dead instructions.
void transform(Instruction *CntInst, PHINode *CntPhi, Value *Var);
/// Create llvm.ctpop.* intrinsic function.
CallInst *createPopcntIntrinsic(IRBuilderTy &IRB, Value *Val, DebugLoc DL);
};
class LoopIdiomRecognize : public LoopPass {
Loop *CurLoop;
DominatorTree *DT;
@ -200,6 +156,10 @@ private:
bool runOnNoncountableLoop();
bool recognizePopcount();
void transformLoopToPopcount(BasicBlock *PreCondBB, Instruction *CntInst,
PHINode *CntPhi, Value *Var);
/// @}
};
@ -234,352 +194,6 @@ static void deleteDeadInstruction(Instruction *I,
RecursivelyDeleteTriviallyDeadInstructions(Op, TLI);
}
//===----------------------------------------------------------------------===//
//
// Implementation of NclPopcountRecognize
//
//===----------------------------------------------------------------------===//
NclPopcountRecognize::NclPopcountRecognize(LoopIdiomRecognize &TheLIR)
: LIR(TheLIR), CurLoop(TheLIR.getLoop()), PreCondBB(nullptr) {}
bool NclPopcountRecognize::preliminaryScreen() {
const TargetTransformInfo *TTI = LIR.getTargetTransformInfo();
if (TTI->getPopcntSupport(32) != TargetTransformInfo::PSK_FastHardware)
return false;
// Counting population are usually conducted by few arithmetic instructions.
// Such instructions can be easilly "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.
PreCondBB = PH->getSinglePredecessor();
if (!PreCondBB)
return false;
auto *PreCondBI = dyn_cast<BranchInst>(PreCondBB->getTerminator());
if (!PreCondBI || PreCondBI->isUnconditional())
return false;
return true;
}
Value *NclPopcountRecognize::matchCondition(BranchInst *Br,
BasicBlock *LoopEntry) const {
if (!Br || !Br->isConditional())
return nullptr;
ICmpInst *Cond = dyn_cast<ICmpInst>(Br->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 && Br->getSuccessor(0) == LoopEntry) ||
(Pred == ICmpInst::ICMP_EQ && Br->getSuccessor(1) == LoopEntry))
return Cond->getOperand(0);
return nullptr;
}
bool NclPopcountRecognize::detectIdiom(Instruction *&CntInst, PHINode *&CntPhi,
Value *&Var) const {
// Following code tries to detect this idiom:
//
// 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:
//
// 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(),
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;
}
void NclPopcountRecognize::transform(Instruction *CntInst, PHINode *CntPhi,
Value *Var) {
ScalarEvolution *SE = LIR.getScalarEvolution();
TargetLibraryInfo *TLI = LIR.getTargetLibraryInfo();
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
IRBuilderTy 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". Withtout 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 enble 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 become dead after
// the transformation. It is lots easier to prove a countable loop dead
// than to prove a noncountable one. (In some C dialects, a infite 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->begin());
Builder.SetInsertPoint(LbCond);
Value *Opnd1 = cast<Value>(TcPhi);
Value *Opnd2 = cast<Value>(ConstantInt::get(Ty, 1));
Instruction *TcDec = cast<Instruction>(
Builder.CreateSub(Opnd1, Opnd2, "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, cast<Value>(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);
}
CallInst *NclPopcountRecognize::createPopcntIntrinsic(IRBuilderTy &IRBuilder,
Value *Val, DebugLoc DL) {
Value *Ops[] = {Val};
Type *Tys[] = {Val->getType()};
Module *M = (*(CurLoop->block_begin()))->getParent()->getParent();
Value *Func = Intrinsic::getDeclaration(M, Intrinsic::ctpop, Tys);
CallInst *CI = IRBuilder.CreateCall(Func, Ops);
CI->setDebugLoc(DL);
return CI;
}
/// recognize - detect population count idiom in a non-countable loop. If
/// detected, transform the relevant code to popcount intrinsic function
/// call, and return true; otherwise, return false.
bool NclPopcountRecognize::recognize() {
if (!LIR.getTargetTransformInfo())
return false;
LIR.getScalarEvolution();
if (!preliminaryScreen())
return false;
Instruction *CntInst;
PHINode *CntPhi;
Value *Val;
if (!detectIdiom(CntInst, CntPhi, Val))
return false;
transform(CntInst, CntPhi, Val);
return true;
}
//===----------------------------------------------------------------------===//
//
// Implementation of LoopIdiomRecognize
@ -1065,9 +679,346 @@ bool LoopIdiomRecognize::processLoopStoreOfLoopLoad(
}
bool LoopIdiomRecognize::runOnNoncountableLoop() {
NclPopcountRecognize Popcount(*this);
if (Popcount.recognize())
if (recognizePopcount())
return true;
return false;
}
/// 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(),
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() {
(void)getScalarEvolution();
(void)getTargetLibraryInfo();
(void)getTargetTransformInfo();
if (TTI->getPopcntSupport(32) != TargetTransformInfo::PSK_FastHardware)
return false;
// Counting population are usually conducted by few arithmetic instructions.
// Such instructions can be easilly "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". Withtout 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 enble 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 become dead after
// the transformation. It is lots easier to prove a countable loop dead
// than to prove a noncountable one. (In some C dialects, a infite 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->begin());
Builder.SetInsertPoint(LbCond);
Value *Opnd1 = cast<Value>(TcPhi);
Value *Opnd2 = cast<Value>(ConstantInt::get(Ty, 1));
Instruction *TcDec = cast<Instruction>(
Builder.CreateSub(Opnd1, Opnd2, "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, cast<Value>(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);
}