InstCombine: fsub nsz 0, X ==> fsub nsz -0.0, X
Some day the backend may handle instruction-level fast math flags and make
this transform unnecessary, but it's still better practice to use the canonical
representation of fneg when possible (use a -0.0).
This is a partial fix for PR20870 ( http://llvm.org/bugs/show_bug.cgi?id=20870 ).
See also http://reviews.llvm.org/D6723.
Differential Revision: http://reviews.llvm.org/D6731
llvm-svn: 225050
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index 9ea4bc5..7e6ead4 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1713,6 +1713,14 @@
TLI, DT, AT))
return ReplaceInstUsesWith(I, V);
+ // fsub nsz 0, X ==> fsub nsz -0.0, X
+ if (I.getFastMathFlags().noSignedZeros() && match(Op0, m_Zero())) {
+ // Subtraction from -0.0 is the canonical form of fneg.
+ Instruction *NewI = BinaryOperator::CreateFNeg(Op1);
+ NewI->copyFastMathFlags(&I);
+ return NewI;
+ }
+
if (isa<Constant>(Op0))
if (SelectInst *SI = dyn_cast<SelectInst>(Op1))
if (Instruction *NV = FoldOpIntoSelect(I, SI))