[PATCH] [x86] add a reassociation optimization to increase ILP via the MachineCombiner pass

[Sanjay Patel]

In http://reviews.llvm.org/D10321#185353, @Gerolf wrote:

> I'm also curious about performance numbers.


Hi Gerolf -

Thanks for the review! I'll work on the inline comments soon.

For perf numbers, I see no difference running the benchmarking subset of test-suite on x86 with AVX and -ffast-math; this is as expected because this pass isn't firing more than a handful of times on any test.

Do you have suggestions for other benchmarks? My motivating examples for this patch are:
https://llvm.org/bugs/show_bug.cgi?id=17305
https://llvm.org/bugs/show_bug.cgi?id=21768
https://llvm.org/bugs/show_bug.cgi?id=23116
http://reviews.llvm.org/D8941

I can create a toy test case and measure that the perf matches the IACA output in:
https://llvm.org/bugs/show_bug.cgi?id=17305#c0


http://reviews.llvm.org/D10321

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