+
+bool LoopIdiomRecognize::runOnNoncountableLoop() {
+ 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() {
+ 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);
+}