diff options
Diffstat (limited to 'llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp')
| -rw-r--r-- | llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp | 735 |
1 files changed, 343 insertions, 392 deletions
diff --git a/llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp b/llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp index 95aadb19075..8ccbf9dcb1a 100644 --- a/llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp +++ b/llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp @@ -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); + /// @} }; @@ -236,352 +196,6 @@ static void deleteDeadInstruction(Instruction *I, //===----------------------------------------------------------------------===// // -// 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); +} |
