[CGP]: Optimize mul.overflow. (#148343)

- Detect cases where LHS & RHS values will not cause overflow
(when the Hi halfs are zero).

Author: hassnaaHamdi

llvm/include/llvm/CodeGen/TargetLowering.h

@@ -3492,6 +3492,13 @@ class LLVM_ABI TargetLoweringBase {
     return MathUsed && (VT.isSimple() || !isOperationExpand(Opcode, VT));
   }
 
+  // Return true if the target wants to optimize the mul overflow intrinsic
+  // for the given \p VT.
+  virtual bool shouldOptimizeMulOverflowWithZeroHighBits(LLVMContext &Context,
+                                                         EVT VT) 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);
@@ -2797,6 +2799,10 @@ 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;
@@ -6391,6 +6397,182 @@ bool CodeGenPrepare::optimizeGatherScatterInst(Instruction *MemoryInst,
   return true;
 }
 
+// This is a helper for CodeGenPrepare::optimizeMulWithOverflow.
+// Check the pattern we are interested in where there are maximum 2 uses
+// of the intrinsic which are the extract instructions.
+static bool matchOverflowPattern(Instruction *&I, ExtractValueInst *&MulExtract,
+                                 ExtractValueInst *&OverflowExtract) {
+  // Bail out if it's more than 2 users:
+  if (I->hasNUsesOrMore(3))
+    return false;
+
+  for (User *U : I->users()) {
+    auto *Extract = dyn_cast<ExtractValueInst>(U);
+    if (!Extract || Extract->getNumIndices() != 1)
+      return false;
+
+    unsigned Index = Extract->getIndices()[0];
+    if (Index == 0)
+      MulExtract = Extract;
+    else if (Index == 1)
+      OverflowExtract = Extract;
+    else
+      return false;
+  }
+  return true;
+}
+
+// Rewrite the mul_with_overflow intrinsic by checking if both of the
+// operands' value ranges are 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.
+// The IR after the optimization will look like:
+// entry:
+//   if signed:
+//     ( (lhs_lo>>BW-1) ^ lhs_hi) || ( (rhs_lo>>BW-1) ^ rhs_hi) ? overflow,
+//     overflow_no
+//   else:
+//     (lhs_hi != 0) || (rhs_hi != 0) ? overflow, overflow_no
+// overflow_no:
+// overflow:
+// overflow.res:
+// \returns true if optimization was applied
+// TODO: This optimization can be further improved to optimize branching on
+// overflow where the 'overflow_no' BB can branch directly to the false
+// successor of overflow, but that would add additional complexity so we leave
+// it for future work.
+bool CodeGenPrepare::optimizeMulWithOverflow(Instruction *I, bool IsSigned,
+                                             ModifyDT &ModifiedDT) {
+  // Check if target supports this optimization.
+  if (!TLI->shouldOptimizeMulOverflowWithZeroHighBits(
+          I->getContext(),
+          TLI->getValueType(*DL, I->getType()->getContainedType(0))))
+    return false;
+
+  ExtractValueInst *MulExtract = nullptr, *OverflowExtract = nullptr;
+  if (!matchOverflowPattern(I, MulExtract, OverflowExtract))
+    return false;
+
+  // Keep track of the instruction to stop reoptimizing it again.
+  InsertedInsts.insert(I);
+
+  Value *LHS = I->getOperand(0);
+  Value *RHS = I->getOperand(1);
+  Type *Ty = LHS->getType();
+  unsigned VTHalfBitWidth = Ty->getScalarSizeInBits() / 2;
+  Type *LegalTy = Ty->getWithNewBitWidth(VTHalfBitWidth);
+
+  // New BBs:
+  BasicBlock *OverflowEntryBB =
+      I->getParent()->splitBasicBlock(I, "", /*Before*/ true);
+  OverflowEntryBB->takeName(I->getParent());
+  // Keep the 'br' instruction that is generated as a result of the split to be
+  // erased/replaced later.
+  Instruction *OldTerminator = OverflowEntryBB->getTerminator();
+  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);
+  // Extract low and high halves of LHS:
+  Value *LoLHS = Builder.CreateTrunc(LHS, LegalTy, "lo.lhs");
+  Value *HiLHS = Builder.CreateLShr(LHS, VTHalfBitWidth, "lhs.lsr");
+  HiLHS = Builder.CreateTrunc(HiLHS, LegalTy, "hi.lhs");
+
+  // Extract low and high halves of RHS:
+  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");
+
+  // Create the 'overflow.res' BB to merge the results of
+  // the two paths:
+  BasicBlock *OverflowResBB = I->getParent();
+  OverflowResBB->setName("overflow.res");
+
+  // BB overflow.no: jump to overflow.res BB
+  Builder.CreateBr(OverflowResBB);
+  // No we don't need the old terminator in overflow.entry BB, erase it:
+  OldTerminator->eraseFromParent();
+
+  // BB overflow.res:
+  Builder.SetInsertPoint(OverflowResBB, OverflowResBB->getFirstInsertionPt());
+  // Create PHI nodes to merge results from no.overflow BB and overflow BB to
+  // replace the extract instructions.
+  PHINode *OverflowResPHI = Builder.CreatePHI(Ty, 2),
+          *OverflowFlagPHI =
+              Builder.CreatePHI(IntegerType::getInt1Ty(I->getContext()), 2);
+
+  // Add the incoming values from no.overflow BB and later from overflow BB.
+  OverflowResPHI->addIncoming(Mul, NoOverflowBB);
+  OverflowFlagPHI->addIncoming(ConstantInt::getFalse(I->getContext()),
+                               NoOverflowBB);
+
+  // Replace all users of MulExtract and OverflowExtract to use the PHI nodes.
+  if (MulExtract) {
+    MulExtract->replaceAllUsesWith(OverflowResPHI);
+    MulExtract->eraseFromParent();
+  }
+  if (OverflowExtract) {
+    OverflowExtract->replaceAllUsesWith(OverflowFlagPHI);
+    OverflowExtract->eraseFromParent();
+  }
+
+  // Remove the intrinsic from parent (overflow.res BB) as it will be part of
+  // overflow BB
+  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 BB.
+  OverflowResPHI->addIncoming(MulOverflow, OverflowBB);
+  OverflowFlagPHI->addIncoming(OverflowFlag, OverflowBB);
+
+  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.cpp

@@ -18851,6 +18851,15 @@ bool AArch64TargetLowering::isExtractSubvectorCheap(EVT ResVT, EVT SrcVT,
   return (Index == 0 || Index == ResVT.getVectorMinNumElements());
 }
 
+bool AArch64TargetLowering::shouldOptimizeMulOverflowWithZeroHighBits(
+    LLVMContext &Context, EVT VT) const {
+  if (getTypeAction(Context, VT) != TypeExpandInteger)
+    return false;
+
+  EVT LegalTy = EVT::getIntegerVT(Context, VT.getSizeInBits() / 2);
+  return getTypeAction(Context, LegalTy) == TargetLowering::TypeLegal;
+}
+
 /// Turn vector tests of the signbit in the form of:
 ///   xor (sra X, elt_size(X)-1), -1
 /// into:

llvm/lib/Target/AArch64/AArch64ISelLowering.h

@@ -333,6 +333,11 @@ class AArch64TargetLowering : public TargetLowering {
     return TargetLowering::shouldFormOverflowOp(Opcode, VT, true);
   }
 
+  // Return true if the target wants to optimize the mul overflow intrinsic
+  // for the given \p VT.
+  bool shouldOptimizeMulOverflowWithZeroHighBits(LLVMContext &Context,
+                                                 EVT VT) const override;
+
   Value *emitLoadLinked(IRBuilderBase &Builder, Type *ValueTy, Value *Addr,
                         AtomicOrdering Ord) const override;
   Value *emitStoreConditional(IRBuilderBase &Builder, Value *Val, Value *Addr,