[Patch, i386] Support BMI and BMI2 targets in multiversioning

[H.J. Lu]

On Mon, Jan 26, 2015 at 11:08 AM, Allan Sandfeld Jensen
<carewolf@gmail.com> wrote:
> On Monday 26 January 2015, H.J. Lu wrote:
>> On Mon, Jan 26, 2015 at 10:53 AM, Allan Sandfeld Jensen
>>
>> <carewolf@gmail.com> wrote:
>> >> >> >> Committed with a bunch of fixes (e.g. missing fold_builtin_cpu
>> >> >> >> part in gcc/config/i386/i386.c, and mv17.C test didn't compile at
>> >> >> >> all due to missing parenthesis).
>> >> >> >
>> >> >> > ... and now with committed ChangeLog and patch.
>> >> >> >
>> >> >> > gcc/ChangeLog:
>> >> >> >     * config/i386/i386.c (get_builtin_code_for_version): Add
>> >> >> >     support for BMI and BMI2 multiversion functions.
>> >> >> >     (fold_builtin_cpu): Add F_BMI and F_BMI2.
>> >> >> >
>> >> >> > libgcc/ChangeLog:
>> >> >> >     * config/i386/cpuinfo.c (enum processor_features): Add
>> >> >> >     FEATURE_BMI and FEATURE_BMI2.
>> >> >> >     (get_available_features): Detect FEATURE_BMI and FEATURE_BMI2.
>> >> >> >
>> >> >> > testsuite/ChangeLog:
>> >> >> >     * gcc.target/i386/funcspec-5.c: Test new multiversion targets.
>> >> >> >     * g++.dg/ext/mv17.C: Test BMI/BMI2 multiversion dispatcher.
>> >> >>
>> >> >> diff --git a/gcc/config/i386/i386.c b/gcc/config/i386/i386.c
>> >> >> index 9ec40cb..441911d 100644
>> >> >> --- a/gcc/config/i386/i386.c
>> >> >> +++ b/gcc/config/i386/i386.c
>> >> >> @@ -34289,15 +34289,18 @@ get_builtin_code_for_version (tree decl,
>> >> >> tree *predica te_list)
>> >> >>
>> >> >>      P_PROC_SSE4_A,
>> >> >>      P_SSE4_1,
>> >> >>      P_SSE4_2,
>> >> >>
>> >> >> -    P_PROC_SSE4_2,
>> >> >>
>> >> >>      P_POPCNT,
>> >> >>
>> >> >> +    P_PROC_SSE4_2,
>> >> >>
>> >> >>      P_AVX,
>> >> >>      P_PROC_AVX,
>> >> >>
>> >> >> +    P_BMI,
>> >> >> +    P_PROC_BMI,
>> >> >>
>> >> >>      P_FMA4,
>> >> >>      P_XOP,
>> >> >>      P_PROC_XOP,
>> >> >>      P_FMA,
>> >> >>      P_PROC_FMA,
>> >> >>
>> >> >> +    P_BMI2,
>> >> >>
>> >> >>      P_AVX2,
>> >> >>      P_PROC_AVX2,
>> >> >>      P_AVX512F,
>> >> >>
>> >> >> This changed the priority of P_POPCNT and caused
>> >> >>
>> >> >> FAIL: g++.dg/ext/mv1.C  -std=gnu++98 execution test
>> >> >> FAIL: g++.dg/ext/mv1.C  -std=gnu++11 execution test
>> >> >> FAIL: g++.dg/ext/mv1.C  -std=gnu++14 execution test
>> >> >>
>> >> >> on Nehalem and Westmere machines:
>> >> >>
>> >> >> mv1.exe:
>> >> >> /export/gnu/import/git/sources/gcc/gcc/testsuite/g++.dg/ext/mv1.C:51:
>> >> >> int main(): Assertion `val == 5' failed.
>> >> >>
>> >> >> since "val" is 6 now.
>> >> >
>> >> > Right. I am not sure why popcnt was prioritized below arch=corei7. The
>> >> > logic is supposed to be that any target that includes an extension is
>> >> > prioritized
>> >>
>> >> I don't understand your question.  popcnt feature is separate from
>> >> -march. Its priority has nothing to do with -march=corei7.
>> >
>> > arch=corei7 implies popcnt. See PTA_NEHALEM in i386.c. The test would
>> > probably work with -march=core2.
>> >
>> > AFAIK The logic of the priorities in multiversioning is that architecture
>> > specific functions are chosen over feature specific, unless the feature
>> > is one that isn't required by the architecture.
>>
>> On SSE4.2 machines, we should choose
>>
>> int __attribute__ ((target("arch=corei7"))) foo ();
>>
>> over
>>
>> int __attribute__ ((target("popcnt"))) foo ();
>>
>> But we shouldn't choose
>>
>> int __attribute__ ((target("arch=corei7"))) foo ();
>>
>> over
>>
>> int __attribute__ ((target("arch=corei7,popcnt"))) foo ();
>
> I guess since they represent the exact same effective ISA, they would have
> equal priority, so that it would likely chose whatever comes last.

I have no strong opinion on this.  But this is a user visible compiler
behavior change.  We should issue a warning/note here.


-- 
H.J.