Re: [PATCH 1/7] alpha: honor -frounding-math by selecting dynamic rounding

Sam James <[email protected]> Tue, 18 Aug 2026 19:26:47 +0100
Newsgroups	gmane.comp.gcc.patches
Organization	Gentoo
Message-ID	<[email protected]>
Jeffrey Law <[email protected]> writes:

> On 8/14/2026 1:29 PM, Matt Turner wrote:
>> Alpha encodes the rounding mode in the instruction itself, so honoring a
>> rounding mode chosen at run time requires the /d qualifier, which takes the
>> mode from the FPCR.  alpha_fprm defaulted to ALPHA_FPRM_NORM and was only
>> ever changed by an explicit -mfp-rounding-mode, so -frounding-math emitted
>> statically rounded instructions and fesetround was silently ignored.
>>
>> Default alpha_fprm to ALPHA_FPRM_DYN when -frounding-math is in effect and
>> -mfp-rounding-mode was not given, leaving both ordinary code generation and
>> an explicit -mfp-rounding-mode unaffected.  The VAX floating-point
>> instructions have no dynamic rounding qualifier, so -mfloat-vax keeps the
>> static default.
>>
>> This fixes the execution failures of gcc.dg/torture/fp-uint64-convert-double-1.c,
>> fp-uint64-convert-double-2.c and fp-double-convert-float-1.c.
>>
>> gcc/ChangeLog:
>>
>> 	* config/alpha/alpha.cc (alpha_option_override): Set alpha_fprm to
>> 	ALPHA_FPRM_DYN when flag_rounding_math is set, -mfp-rounding-mode
>> 	was not given explicitly and -mfloat-vax is not in effect.
>> 	* doc/invoke.texi (Alpha Options): Document the default rounding
>> 	mode and its interaction with -frounding-math.
> THanks.  I've pushed this to the trunk as well.  My last alpha build
> didn't bootstrap, so something has gone weird, but I suspect it's
> generic and not anything you've done.
>
> The last clean build I had was with:
>
> 7e0bb48cbb2654816ce2714de5b644f4d965d3db
>
> The first failed build with:
>
> df494ca28d75f80b4d0b4e0b93b5d23d2092131a
>
> The failure mode is an ICE in the gimple threader:
> during GIMPLE pass: thread
> ../../../gcc/libcpp/lex.cc: In function 'void _cpp_clean_line(cpp_reader*)':
> ../../../gcc/libcpp/lex.cc:970:1: internal compiler error: Segmentation fault
>   970 | _cpp_clean_line (cpp_reader *pfile)
>       | ^~~~~~~~~~~~~~~
>
> I haven't tried to chase it down yet.  I usually let these things
> simmer a bit since testing/bisecting bootstraps of qemu emulated
> targets takes an absurdly long time.  But if you're interested in
> chasing another issue, this may be good to check.

It's https://gcc.gnu.org/PR126876 which Aldy just fixed but trunk is
kind of rough at the moment, there's https://gcc.gnu.org/PR126856 too.

>
> jeff

sam
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