[PATCH] D16122: [LibCallSimplifier] use instruction-level fast-math-flags to transform pow(x, [small integer]) calls

[Sanjay Patel via llvm-commits]

spatel created this revision.
spatel added reviewers: scanon, davide, hfinkel, mgrang.
spatel added a subscriber: llvm-commits.

This is a continuation of adding FMF to call instructions:
http://reviews.llvm.org/rL255555

As with D15937, the intent of the patch is to preserve the current behavior of the transform except that we use the pow call's 'fast' attribute as a trigger rather than a function-level attribute.

But this raises a possible improvement noted by the TODO comment:
Should all of the new instructions be marked 'fast' since they were derived from a fast pow()? I think a user of -ffast-math would expect that despite the potentially increased risk of numerical disaster. In particular, the possible 'fdiv' would benefit from being turned into a reciprocal estimate on a target that supports that. There's probably less risk/opportunity to do anything with the chain of dependent multiplies, but if we're going 'fast', we might as well flag them all?

http://reviews.llvm.org/D16122

Files:
  lib/Transforms/Utils/SimplifyLibCalls.cpp
  test/Transforms/InstCombine/pow-4.ll

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