[CGP]: Optimize mul.overflow.

llvm/include/llvm/CodeGen/TargetLowering.h

@@ -3470,6 +3470,10 @@ class LLVM_ABI TargetLoweringBase {
     return MathUsed && (VT.isSimple() || !isOperationExpand(Opcode, VT));
   }
 
+  // Return true if the target wants to optimize the mul overflow intrinsic
+  // by detecting if there is no overflow.
+  virtual bool shouldOptimizeMulOverflowIntrinsic() const { return false; }
+
   // Return true if it is profitable to use a scalar input to a BUILD_VECTOR
   // even if the vector itself has multiple uses.
   virtual bool aggressivelyPreferBuildVectorSources(EVT VecVT) const {

llvm/lib/CodeGen/CodeGenPrepare.cpp

@@ -431,6 +431,8 @@ class CodeGenPrepare {
   bool optimizeMemoryInst(Instruction *MemoryInst, Value *Addr, Type *AccessTy,
                           unsigned AddrSpace);
   bool optimizeGatherScatterInst(Instruction *MemoryInst, Value *Ptr);
+  bool optimizeMulWithOverflow(Instruction *I, bool IsSigned,
+                               ModifyDT &ModifiedDT);
   bool optimizeInlineAsmInst(CallInst *CS);
   bool optimizeCallInst(CallInst *CI, ModifyDT &ModifiedDT);
   bool optimizeExt(Instruction *&I);
@@ -2778,6 +2780,10 @@ bool CodeGenPrepare::optimizeCallInst(CallInst *CI, ModifyDT &ModifiedDT) {
         }
       }
       return false;
+    case Intrinsic::umul_with_overflow:
+      return optimizeMulWithOverflow(II, /*IsSigned=*/false, ModifiedDT);
+    case Intrinsic::smul_with_overflow:
+      return optimizeMulWithOverflow(II, /*IsSigned=*/true, ModifiedDT);
     }
 
     SmallVector<Value *, 2> PtrOps;
@@ -6389,6 +6395,235 @@ bool CodeGenPrepare::optimizeGatherScatterInst(Instruction *MemoryInst,
   return true;
 }
 
+// Rewrite the mul_with_overflow intrinsic by checking if both of the
+// operands' value range is within the legal type. If so, we can optimize the
+// multiplication algorithm. This code is supposed to be written during the step
+// of type legalization, but given that we need to reconstruct the IR which is
+// not doable there, we do it here.
+bool CodeGenPrepare::optimizeMulWithOverflow(Instruction *I, bool IsSigned,
+                                             ModifyDT &ModifiedDT) {
+  if (!TLI->shouldOptimizeMulOverflowIntrinsic())
+    return false;
+
+  if (TLI->getTypeAction(
+          I->getContext(),
+          TLI->getValueType(*DL, I->getType()->getContainedType(0))) !=
+      TargetLowering::TypeExpandInteger)
+    return false;
+
+  Value *LHS = I->getOperand(0);
+  Value *RHS = I->getOperand(1);
+  Type *Ty = LHS->getType();
+  unsigned VTBitWidth = Ty->getScalarSizeInBits();
+  unsigned VTHalfBitWidth = VTBitWidth / 2;
+  IntegerType *LegalTy =
+      IntegerType::getIntNTy(I->getContext(), VTHalfBitWidth);
+
+  // Skip the optimization if the type with HalfBitWidth is not legal for the
+  // target.
+  if (TLI->getTypeAction(I->getContext(), TLI->getValueType(*DL, LegalTy)) !=
+      TargetLowering::TypeLegal)
+    return false;
+
+  // Make sure that the I->getType() is a struct type with two elements.
+  if (!I->getType()->isStructTy() || I->getType()->getStructNumElements() != 2)
+    return false;
+
+  // Keep track of the instruction to stop reoptimizing it again.
+  InsertedInsts.insert(I);
+  // ----------------------------
+
+  // For the simple case where IR just checks the overflow flag, new blocks
+  // should be:
+  //  entry:
+  //    if signed:
+  //      (lhs_lo ^ lhs_hi) || (rhs_lo ^ rhs_hi) ? overflow, overflow_no
+  //    else:
+  //      (lhs_hi != 0) || (rhs_hi != 0) ? overflow, overflow_no
+  //  overflow_no:
+  //  overflow:
+
+  // otherwise, new blocks should be:
+  //  entry:
+  //    if signed:
+  //      (lhs_lo ^ lhs_hi) || (rhs_lo ^ rhs_hi) ? overflow, overflow_no
+  //    else:
+  //      (lhs_hi != 0) || (rhs_hi != 0) ? overflow, overflow_no
+  //  overflow_no:
+  //  overflow:
+  //  overflow.res:
+
+  // New BBs:
+  std::string KeepBBName = I->getParent()->getName().str();
+  BasicBlock *OverflowEntryBB =
+      I->getParent()->splitBasicBlock(I, "overflow.entry", /*Before*/ true);
+  // Remove the 'br' instruction that is generated as a result of the split:
+  OverflowEntryBB->getTerminator()->eraseFromParent();
+  BasicBlock *NoOverflowBB =
+      BasicBlock::Create(I->getContext(), "overflow.no", I->getFunction());
+  NoOverflowBB->moveAfter(OverflowEntryBB);
+  BasicBlock *OverflowBB =
+      BasicBlock::Create(I->getContext(), "overflow", I->getFunction());
+  OverflowBB->moveAfter(NoOverflowBB);
+
+  // BB overflow.entry:
+  IRBuilder<> Builder(OverflowEntryBB);
+  // Get Lo and Hi of LHS & RHS:
+  Value *LoLHS = Builder.CreateTrunc(LHS, LegalTy, "lo.lhs");
+  Value *HiLHS = Builder.CreateLShr(LHS, VTHalfBitWidth, "lhs.lsr");
+  HiLHS = Builder.CreateTrunc(HiLHS, LegalTy, "hi.lhs");
+  Value *LoRHS = Builder.CreateTrunc(RHS, LegalTy, "lo.rhs");
+  Value *HiRHS = Builder.CreateLShr(RHS, VTHalfBitWidth, "rhs.lsr");
+  HiRHS = Builder.CreateTrunc(HiRHS, LegalTy, "hi.rhs");
+
+  Value *IsAnyBitTrue;
+  if (IsSigned) {
+    Value *SignLoLHS =
+        Builder.CreateAShr(LoLHS, VTHalfBitWidth - 1, "sign.lo.lhs");
+    Value *SignLoRHS =
+        Builder.CreateAShr(LoRHS, VTHalfBitWidth - 1, "sign.lo.rhs");
+    Value *XorLHS = Builder.CreateXor(HiLHS, SignLoLHS);
+    Value *XorRHS = Builder.CreateXor(HiRHS, SignLoRHS);
+    Value *Or = Builder.CreateOr(XorLHS, XorRHS, "or.lhs.rhs");
+    IsAnyBitTrue = Builder.CreateCmp(ICmpInst::ICMP_NE, Or,
+                                     ConstantInt::getNullValue(Or->getType()));
+  } else {
+    Value *CmpLHS = Builder.CreateCmp(ICmpInst::ICMP_NE, HiLHS,
+                                      ConstantInt::getNullValue(LegalTy));
+    Value *CmpRHS = Builder.CreateCmp(ICmpInst::ICMP_NE, HiRHS,
+                                      ConstantInt::getNullValue(LegalTy));
+    IsAnyBitTrue = Builder.CreateOr(CmpLHS, CmpRHS, "or.lhs.rhs");
+  }
+  Builder.CreateCondBr(IsAnyBitTrue, OverflowBB, NoOverflowBB);
+
+  // BB overflow.no:
+  Builder.SetInsertPoint(NoOverflowBB);
+  Value *ExtLoLHS, *ExtLoRHS;
+  if (IsSigned) {
+    ExtLoLHS = Builder.CreateSExt(LoLHS, Ty, "lo.lhs.ext");
+    ExtLoRHS = Builder.CreateSExt(LoRHS, Ty, "lo.rhs.ext");
+  } else {
+    ExtLoLHS = Builder.CreateZExt(LoLHS, Ty, "lo.lhs.ext");
+    ExtLoRHS = Builder.CreateZExt(LoRHS, Ty, "lo.rhs.ext");
+  }
+
+  Value *Mul = Builder.CreateMul(ExtLoLHS, ExtLoRHS, "mul.overflow.no");
+
+  // In overflow.no BB: we are sure that the overflow flag is false.
+  // So, if we found this pattern:
+  // br (extractvalue (%mul, 1)), label %if.then, label %if.end
+  // then we can jump directly to %if.end as we're sure that there is no
+  // overflow.
+  BasicBlock *DetectNoOverflowBrBB = nullptr;
+  StructType *STy = StructType::get(
+      I->getContext(), {Ty, IntegerType::getInt1Ty(I->getContext())});
+  // Look for the pattern in the users of I, and make sure that all the users
+  // are either part of the pattern or NOT in the same BB as I.
+  for (User *U : I->users()) {
+    if (auto *Instr = dyn_cast<Instruction>(U);
+        Instr && Instr->getParent() != I->getParent())
+      continue;
+
+    if (auto *ExtUser = dyn_cast<ExtractValueInst>(U)) {
+      if (ExtUser->hasOneUse() && ExtUser->getNumIndices() == 1 &&
+          ExtUser->getIndices()[0] == 1) {
+        if (auto *Br = dyn_cast<BranchInst>(*ExtUser->user_begin())) {
+          DetectNoOverflowBrBB = Br->getSuccessor(1) /*if.end*/;
+          continue;
+        }
+      }
+    }
+    // If we come here, it means that either the pattern doesn't exist or
+    // there are multiple users in the same BB
+    DetectNoOverflowBrBB = nullptr;
+    break;
+  }
+  if (DetectNoOverflowBrBB) {
+    // BB overflow.no: jump directly to if.end BB
+    Builder.CreateBr(DetectNoOverflowBrBB);
+
+    // BB if.end:
+    Builder.SetInsertPoint(DetectNoOverflowBrBB,
+                           DetectNoOverflowBrBB->getFirstInsertionPt());
+    // Create PHI node to get the results of multiplication from 'overflow.no'
+    // and 'overflow' BBs
+    PHINode *NoOverflowPHI = Builder.CreatePHI(Ty, 2);
+    NoOverflowPHI->addIncoming(Mul, NoOverflowBB);
+    // Create struct value to replace all uses of I
+    Value *StructValNoOverflow = PoisonValue::get(STy);
+    StructValNoOverflow =
+        Builder.CreateInsertValue(StructValNoOverflow, NoOverflowPHI, {0});
+    // Overflow flag is always false as we are sure it's not overflow.
+    StructValNoOverflow = Builder.CreateInsertValue(
+        StructValNoOverflow, ConstantInt::getFalse(I->getContext()), {1});
+    // Replace all uses of I, only uses dominated by the if.end BB
+    I->replaceUsesOutsideBlock(StructValNoOverflow, I->getParent());
+
+    // Remove the original BB as it's divided into 'overflow.entry' and
+    // 'overflow' BBs.
+    BasicBlock *ToBeRemoveBB = I->getParent();
+    // BB overflow:
+    OverflowBB->splice(OverflowBB->end(), ToBeRemoveBB);
+    // Extract the multiplication result to add it to the PHI node in the if.end
+    // BB
+    Builder.SetInsertPoint(OverflowBB, OverflowBB->end());
+    Value *IntrinsicMulRes = Builder.CreateExtractValue(I, {0}, "mul.extract");
+    cast<Instruction>(IntrinsicMulRes)->moveAfter(I);
+    NoOverflowPHI->addIncoming(IntrinsicMulRes, OverflowBB);
+
+    ToBeRemoveBB->eraseFromParent();
+    // Restore the original name of the overflow.entry BB:
+    OverflowEntryBB->setName(KeepBBName);
+    ModifiedDT = ModifyDT::ModifyBBDT;
+    return true;
+  }
+
+  // Otherwise, we need to create the 'overflow.res' BB to merge the results of
+  // the two paths:
+  I->getParent()->setName("overflow.res");
+  BasicBlock *OverflowResBB = I->getParent();
+
+  // BB overflow.no: jump to overflow.res BB
+  Builder.CreateBr(OverflowResBB);
+
+  // BB overflow.res:
+  Builder.SetInsertPoint(OverflowResBB, OverflowResBB->getFirstInsertionPt());
+  PHINode *OverflowResPHI = Builder.CreatePHI(Ty, 2),
+          *OverflowFlagPHI =
+              Builder.CreatePHI(IntegerType::getInt1Ty(I->getContext()), 2);
+
+  Value *StructValOverflowRes = PoisonValue::get(STy);
+  StructValOverflowRes =
+      Builder.CreateInsertValue(StructValOverflowRes, OverflowResPHI, {0});
+  StructValOverflowRes =
+      Builder.CreateInsertValue(StructValOverflowRes, OverflowFlagPHI, {1});
+  OverflowResPHI->addIncoming(Mul, NoOverflowBB);
+  OverflowFlagPHI->addIncoming(ConstantInt::getFalse(I->getContext()),
+                               NoOverflowBB);
+
+  // Before moving the mul.overflow intrinsic to the overflowBB, replace all its
+  // uses by StructValOverflowRes.
+  I->replaceAllUsesWith(StructValOverflowRes);
+  I->removeFromParent();
+
+  // BB overflow:
+  I->insertInto(OverflowBB, OverflowBB->end());
+  Builder.SetInsertPoint(OverflowBB, OverflowBB->end());
+  Value *MulOverflow = Builder.CreateExtractValue(I, {0}, "mul.overflow");
+  Value *OverflowFlag = Builder.CreateExtractValue(I, {1}, "overflow.flag");
+  Builder.CreateBr(OverflowResBB);
+
+  // Add The Extracted values to the PHINodes in the overflow.res block.
+  OverflowResPHI->addIncoming(MulOverflow, OverflowBB);
+  OverflowFlagPHI->addIncoming(OverflowFlag, OverflowBB);
+
+  // Restore the original name of the overflow.entry BB:
+  OverflowEntryBB->setName(KeepBBName);
+
+  ModifiedDT = ModifyDT::ModifyBBDT;
+  return true;
+}
+
 /// If there are any memory operands, use OptimizeMemoryInst to sink their
 /// address computing into the block when possible / profitable.
 bool CodeGenPrepare::optimizeInlineAsmInst(CallInst *CS) {

llvm/lib/Target/AArch64/AArch64ISelLowering.h

@@ -321,6 +321,10 @@ class AArch64TargetLowering : public TargetLowering {
     return TargetLowering::shouldFormOverflowOp(Opcode, VT, true);
   }
 
+  // Return true if the target wants to optimize the mul overflow intrinsic
+  // by detecting if there is no overflow.
+  bool shouldOptimizeMulOverflowIntrinsic() const override { return true; }
+
   Value *emitLoadLinked(IRBuilderBase &Builder, Type *ValueTy, Value *Addr,
                         AtomicOrdering Ord) const override;
   Value *emitStoreConditional(IRBuilderBase &Builder, Value *Val, Value *Addr,