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[TM] Restore default TargetOptions in TargetMachine::resetTargetOptions.

Authored by jlebar on Jan 9 2017, 7:30 PM.



Previously if you had

  • a function with the fast-math-enabled attr, followed by
  • a function without the fast-math attr,

the second function would inherit the first function's fast-math-ness.

This means that mixing fast-math and non-fast-math functions in a module
was completely broken unless you explicitly annotated every
non-fast-math function with "unsafe-fp-math"="false". This appears to
have been broken since at r176986 (March 2013), when the
resetTargetOptions function was introduced.

This patch tests the correct behavior as best we can. I don't think I
can test FPDenormalMode and NoTrappingFPMath, because they aren't used
in any backends during function lowering. Surprisingly, I also can't
find any uses at all of LessPreciseFPMAD affecting generated code.

The NVPTX/fast-math.ll test changes are an expected result of fixing
this bug. When FMA is disabled, we emit add as "add.rn.f32", which
prevents fma combining. Before this patch, fast-math was enabled in all
functions following the one which explicitly enabled it on itself, so we
were emitting plain "add.f32" where we should have generated

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jlebar updated this revision to Diff 83768.Jan 9 2017, 7:30 PM
jlebar retitled this revision from to [TM] Restore default TargetOptions in TargetMachine::resetTargetOptions..
jlebar updated this object.
jlebar added a reviewer: mkuper.
jlebar added subscribers: llvm-commits, majnemer, hfinkel.
jlebar updated this object.Jan 9 2017, 7:34 PM

Shout out to rr [1]. I went from "huh, this seems wrong" to finding the responsible code in about 30 seconds. Put a breakpoint where TargetOptions.UnsafeFPMath is read, hit the breakpoint, watch -l TargetOptions.UnsafeFPMath, reverse continue. Bam.


majnemer accepted this revision.Jan 9 2017, 8:25 PM
majnemer added a reviewer: majnemer.


This revision is now accepted and ready to land.Jan 9 2017, 8:25 PM
This revision now requires review to proceed.Jan 10 2017, 12:03 PM
echristo accepted this revision.Jan 10 2017, 12:06 PM
echristo edited edge metadata.

LGTM, thanks. I was looking for a reproducer for this for a while :)

This revision is now accepted and ready to land.Jan 10 2017, 12:06 PM

Thank you for the reviews, everyone!

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