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fneg (xor X -2147483648) -> X fold is missing in IR #43592

Open Quuxplusone opened 4 years ago

Quuxplusone commented 4 years ago
Bugzilla Link PR44622
Status NEW
Importance P normal
Reported by Nikita Kniazev (nok.raven@gmail.com)
Reported on 2020-01-22 07:29:59 -0800
Last modified on 2020-03-31 06:34:10 -0700
Version trunk
Hardware All All
CC htmldeveloper@gmail.com, kamleshbhalui@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, spatel+llvm@rotateright.com
Fixed by commit(s)
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Blocks
Blocked by
See also PR45306 
define float @foo(float %0) {
  %2 = fneg float %0
  %3 = bitcast float %2 to i32
  %4 = xor i32 %3, -2147483648
  %5 = bitcast i32 %4 to float
  %6 = fneg float %5
  %7 = bitcast float %6 to i32
  %8 = xor i32 %7, -2147483648
  %9 = bitcast i32 %8 to float
  ret float %9
}

Adding fast flag to fneg does not help.

https://godbolt.org/z/o9WQYp
Quuxplusone commented 4 years ago 
I'm not sure if the addition of 'fneg' to the IR changed things, but bypassing
a this kinds of transforms in IR was intentional because LLVM supports targets
that are not IEEE754-compliant.

For reference:
http://lists.llvm.org/pipermail/llvm-dev/2016-April/098098.html
https://reviews.llvm.org/D19391
Quuxplusone commented 4 years ago 
Before posting the bug report I have checked langref if IR float is IEEE-754,
and it made me think that it is.

> The binary format of half, float, double, and fp128 correspond to the
> IEEE-754-2008 specifications for binary16, binary32, binary64, and
> binary128 respectively.

https://llvm.org/docs/LangRef.html#floating-point-types
Quuxplusone commented 4 years ago 
(In reply to Nikita Kniazev from comment #2)
> Before posting the bug report I have checked langref if IR float is IEEE-754,
> and it made me think that it is.
>
> > The binary format of half, float, double, and fp128 correspond to the
> > IEEE-754-2008 specifications for binary16, binary32, binary64, and
> > binary128 respectively.
>
> https://llvm.org/docs/LangRef.html#floating-point-types

That text looks familiar. :)
https://reviews.llvm.org/rL328105

I need to review the earlier discussion, but IIRC, it wasn't enough that the
format was as expected. We also had to be target-aware of how the operations
are executed/approximated.