Wilco <wdijkstr at arm dot com> changed:

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                 CC|                            |wdijkstr at arm dot com

--- Comment #2 from Wilco <wdijkstr at arm dot com> ---
(In reply to Steve Ellcey from comment #0)
> Created attachment 43279 [details]
> Test case
> The example code comes from milc in SPEC2006.
> GCC on x86 or aarch64 generates better code with -O3 than it does with
> -Ofast or '-O3 -ffast-math'.  On x86 compiling with '-mfma -O3' I get 5
> vfmadd231sd instructions, 1 vmulsd instruction and 6 vmovsd.  With '-mfma
> -Ofast' I get 3 vfmadd231sd, 2 vaddsd, 3 vmulsd, and 6 vmovsd.  That is two
> extra instructions.
> The problem seems to be that -Ofast turns on -ffast-math and that enables
> the global reassociation pass (tree-ssa-reassoc.c) and the code changes
> done there create some temporary variables which inhibit the recognition
> and use of fma instructions.
> Using -O3 and -Ofast on aarch64 shows the same change.

I noticed this a while back, the reassociation pass has changed and now we get
far fewer fmas.


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