[Bug target/106902] [11/12/13/14 Regression] Program compiled with -O3 -mfma produces different result

["rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106902

--- Comment #27 from rguenther at suse dot de <rguenther at suse dot de> ---
> Am 18.05.2023 um 10:31 schrieb amonakov at gcc dot gnu.org <gcc-bugzilla@gcc.gnu.org>:
> 
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106902
> 
> --- Comment #25 from Alexander Monakov <amonakov at gcc dot gnu.org> ---
> (In reply to Richard Biener from comment #24)
>> As of the patch it looks good, I wonder if we want to check for OPTIMIZE_BOTH
>> though since at least when no extra negations are required the contraction
>> should also be a win when optimizing for size?
> 
> Makes sense, I'll change that (current target hooks always return true for
> fma).
> 
>> Also I wondered about the PROP_gimple_any check - do we get into the
>> gimplification langhook after lowering?  I see we are not resetting the
>> langhook after lowering (only in free-lang-data, but that only runs with
>> LTO).
> 
> Yes, that surprised me. I caught it when analyzing ICE on slp-50.c testcase.
> 
>> We probably at least should gate the langhook invocation in the gimplifier
>> with what you added in the patch or specify whether the gimplifier is
>> invoked from the middle-end via the gimplifier context.
> 
> Perhaps. I'll add a comment that we want to handle -ffp-contract=on strictly
> during initial gimplification, to hash this out further on gcc-patches, if
> necessary.  
> 
>> If we go for c-family only the genericize entry could be another place to
>> handle this.
> 
> That seems less convenient to me. Is IFN_FMA representable as a tree?

Yes, that’s possible.  Let’s see if others have an opinion on the ml.

>> Did you run into any of NON_LVALUE / C_MAYBE_CONST wrappings of the
>> multiplication btw?
> 
> No, I'm not familiar with those, so I didn't try to construct corresponding
> testcases.
> 
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