[InstCombine] fold copysign with constant sign argument to (fneg+)fabs
If the sign of the sign argument is known (this could be extended to use ValueTracking),
then we can use fneg+fabs to clear/set the sign bit of the magnitude argument.
http://llvm.org/docs/LangRef.html#llvm-copysign-intrinsic
This transform is already done in DAGCombiner, but we can do it sooner in IR as
suggested in PR44153:
https://bugs.llvm.org/show_bug.cgi?id=44153
We have effectively no analysis for copysign in IR, so we are taking the unusual step
of increasing the number of IR instructions for the negative constant case.
Differential Revision: https://reviews.llvm.org/D70792
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index 23ca03f..536e84b 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -2279,6 +2279,21 @@
break;
}
+ case Intrinsic::copysign: {
+ const APFloat *C;
+ if (match(II->getArgOperand(1), m_APFloat(C))) {
+ // If we know the sign bit of the sign argument, reduce to FABS/FNABS:
+ // copysign X, PosC --> fabs X
+ // copysign X, NegC --> fneg (fabs X)
+ Value *Fabs = Builder.CreateUnaryIntrinsic(Intrinsic::fabs,
+ II->getArgOperand(0), II);
+ if (C->isNegative())
+ Fabs = Builder.CreateFNegFMF(Fabs, II);
+
+ return replaceInstUsesWith(*II, Fabs);
+ }
+ break;
+ }
case Intrinsic::fabs: {
Value *Cond;
Constant *LHS, *RHS;