Commit 42158f3e authored by David Majnemer's avatar David Majnemer
Browse files

InstCombine: Annotate sub with nuw when we prove it's safe 

We can prove that a 'sub' can be a 'sub nuw' if the left-hand side is
negative and the right-hand side is non-negative.

llvm-svn: 216045
parent 298f6380

llvm/lib/Transforms/InstCombine/InstCombine.h

+1 −0
Original line number Diff line number Diff line
@@ -251,6 +251,7 @@ private: 
  bool WillNotOverflowSignedAdd(Value *LHS, Value *RHS);

  bool WillNotOverflowUnsignedAdd(Value *LHS, Value *RHS);

  bool WillNotOverflowSignedSub(Value *LHS, Value *RHS);

  bool WillNotOverflowUnsignedSub(Value *LHS, Value *RHS);

  Value *EmitGEPOffset(User *GEP);

  Instruction *scalarizePHI(ExtractElementInst &EI, PHINode *PN);

  Value *EvaluateInDifferentElementOrder(Value *V, ArrayRef<int> Mask);

llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp

+18 −0
Original line number Diff line number Diff line
@@ -988,6 +988,20 @@ bool InstCombiner::WillNotOverflowSignedSub(Value *LHS, Value *RHS) { 
  return false;

}



/// \brief Return true if we can prove that:

///    (sub LHS, RHS)  === (sub nuw LHS, RHS)

bool InstCombiner::WillNotOverflowUnsignedSub(Value *LHS, Value *RHS) {

  // If the LHS is negative and the RHS is non-negative, no unsigned wrap.

  bool LHSKnownNonNegative, LHSKnownNegative;

  bool RHSKnownNonNegative, RHSKnownNegative;

  ComputeSignBit(LHS, LHSKnownNonNegative, LHSKnownNegative, DL, 0);

  ComputeSignBit(RHS, RHSKnownNonNegative, RHSKnownNegative, DL, 0);

  if (LHSKnownNegative && RHSKnownNonNegative)

    return true;



  return false;

}



// Checks if any operand is negative and we can convert add to sub.

// This function checks for following negative patterns

//   ADD(XOR(OR(Z, NOT(C)), C)), 1) == NEG(AND(Z, C))
@@ -1660,6 +1674,10 @@ Instruction *InstCombiner::visitSub(BinaryOperator &I) { 
    Changed = true;

    I.setHasNoSignedWrap(true);

  }

  if (!I.hasNoUnsignedWrap() && WillNotOverflowUnsignedSub(Op0, Op1)) {

    Changed = true;

    I.setHasNoUnsignedWrap(true);

  }



  return Changed ? &I : nullptr;

}

llvm/test/Transforms/InstCombine/sub.ll

+13 −1
Original line number Diff line number Diff line
@@ -506,6 +506,18 @@ define i4 @test42(i4 %x, i4 %y) { 
; CHECK-LABEL: @test42(

; CHECK-NEXT: [[AND:%.*]] = and i4 %y, 7

; CHECK-NEXT: [[AND1:%.*]] = and i4 %x, 7

; CHECK-NEXT: [[RET:%.*]] = sub nsw i4 %a, %b

; CHECK-NEXT: [[RET:%.*]] = sub nsw i4 [[AND]], [[AND1]]

; CHECK: ret i4 [[RET]]

}



define i4 @test43(i4 %x, i4 %y) {

  %a = or i4 %x, -8

  %b = and i4 %y, 7

  %c = sub i4 %a, %b

  ret i4 %c

; CHECK-LABEL: @test43(

; CHECK-NEXT: [[OR:%.*]] = or i4 %x, -8

; CHECK-NEXT: [[AND:%.*]] = and i4 %y, 7

; CHECK-NEXT: [[RET:%.*]] = sub nuw i4 [[OR]], [[AND]]

; CHECK: ret i4 [[RET]]

}