[SimplifyLibCalls] require fast-math-flags for pow(X, -0.5) transforms

As discussed in PR44330:
https://bugs.llvm.org/show_bug.cgi?id=44330
...the transform from pow(X, -0.5) libcall/intrinsic to
reciprocal square root can result in small deviations from
the expected result due to differences in the pow()
implementation and/or the extra rounding step from the division.

This patch proposes to allow that difference with either the
'approximate functions' or 'reassociate' FMF:
http://llvm.org/docs/LangRef.html#fast-math-flags

In practice, this likely means that the code is compiled with
all of 'fast' (-ffast-math), but I have preserved the existing
specializations for -0.0/-INF that enable generating safe code
if those special values are allowed simultaneously with
allowing approximation/reassociation.

The question about whether a similar restriction is needed for
the non-reciprocal case -- pow(X, 0.5) -- is deferred. That
transform is allowed without FMF currently, and this patch does
not change that behavior.

Differential Revision: https://reviews.llvm.org/D71706

1 files changed, 5 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp b/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
index 6c09600..fa3a9d2 100644
--- a/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -1644,6 +1644,11 @@ Value *LibCallSimplifier::replacePowWithSqrt(CallInst *Pow, IRBuilder<> &B) {
       (!ExpoF->isExactlyValue(0.5) && !ExpoF->isExactlyValue(-0.5)))
     return nullptr;
 
+  // Converting pow(X, -0.5) to 1/sqrt(X) may introduce an extra rounding step,
+  // so that requires fast-math-flags (afn or reassoc).
+  if (ExpoF->isNegative() && (!Pow->hasApproxFunc() && !Pow->hasAllowReassoc()))
+    return nullptr;
+
   Sqrt = getSqrtCall(Base, Attrs, Pow->doesNotAccessMemory(), Mod, B, TLI);
   if (!Sqrt)
     return nullptr;