[llvm] r216045 - InstCombine: Annotate sub with nuw when we prove it's safe
David Majnemer
david.majnemer at gmail.com
Wed Aug 20 00:17:31 PDT 2014
Author: majnemer
Date: Wed Aug 20 02:17:31 2014
New Revision: 216045
URL: http://llvm.org/viewvc/llvm-project?rev=216045&view=rev
Log:
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.
Modified:
llvm/trunk/lib/Transforms/InstCombine/InstCombine.h
llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp
llvm/trunk/test/Transforms/InstCombine/sub.ll
Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombine.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombine.h?rev=216045&r1=216044&r2=216045&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombine.h (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombine.h Wed Aug 20 02:17:31 2014
@@ -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);
Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp?rev=216045&r1=216044&r2=216045&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp Wed Aug 20 02:17:31 2014
@@ -988,6 +988,20 @@ bool InstCombiner::WillNotOverflowSigned
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(Bina
Changed = true;
I.setHasNoSignedWrap(true);
}
+ if (!I.hasNoUnsignedWrap() && WillNotOverflowUnsignedSub(Op0, Op1)) {
+ Changed = true;
+ I.setHasNoUnsignedWrap(true);
+ }
return Changed ? &I : nullptr;
}
Modified: llvm/trunk/test/Transforms/InstCombine/sub.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/sub.ll?rev=216045&r1=216044&r2=216045&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/sub.ll (original)
+++ llvm/trunk/test/Transforms/InstCombine/sub.ll Wed Aug 20 02:17:31 2014
@@ -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]]
}
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