[PATCH] propagate IR-level fast-math-flags to DAG nodes

[Sanjay Patel]

re: the -freciprocal-math flag

"see existing gcc behavior for a starting point"

I should've taken my own advice more seriously. :)

It turns out that -freciprocal-math is only used for a reassociation optimization on divisions. Specifically, this one:
https://llvm.org/bugs/show_bug.cgi?id=16218

With gcc, all of the machine-level hackery that we've put into generating hardware estimate instructions is controlled by a flag that doesn't even exist in clang yet: -mrecip
https://llvm.org/bugs/show_bug.cgi?id=20912

So the end result is that this patch's recip change is wrong. Update to at least that part of the patch coming soon.

I'm assuming that we should not diverge from gcc's user-visible behavior without good reason, so we'll still make -freciprocal-math cause 'arcp' generation in IR ( http://reviews.llvm.org/rL234493 ), but 'arcp' has nothing to do with HW estimate codegen. For that, we need -mrecip with its multitude of options:
https://gcc.gnu.org/onlinedocs/gcc-4.9.2/gcc/i386-and-x86-64-Options.html#index-mrecip_003dopt-1627


http://reviews.llvm.org/D8900

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