> Am 06.05.2026 um 18:02 schrieb Richard Sandiford <[email protected]>:
>
> Richard Biener <[email protected]> writes:
>> The following adds special handling to OMP SIMD vector call costs
>> which were not costed at all and for which a single simple vector
>> stmt isn't appropriate. PR125174 shows that even when AVX imposes
>> more overhead (from also slightly bogus costing) than SSE, when
>> there's two OMP SIMD calls involved doing less of those should trump
>> that.
>>
>> Bootstrap & regtest ongoing on x86_64-unknown-linux-gnu.
>>
>> I've verified this resolves the observed 465.tonto regression. I
>> thought about catching all OMP SIMD vectorized stmts but then
>> realized scalar costing doesn't see this yet so we'll make all
>> vectorizations unprofitable. We cannot handle all calls this
>> way either, as some directly expand to native insns (popcount, etc.).
>> So I fear we have to maintain a positive list. There's 52
>> 'notinbranch' SIMD declatations in glibc 2.38 on x86_64, probably
>> different ones on ARM.
>>
>> Also we of course have no idea about actual cost of the call
>> (but it's expensive). Nor do we have an idea of the scalar
>> vs. vector cost.
>>
>> But as the PR shows, doing "nothing" isn't an option, at least
>> when, like on x86 there's both SSE and AVX variants and the
>> surrounding code would make the SSE variant (appear) cheaper.
>>
>> Any good ideas?
>>
>> Otherwise I'll try to extensively cover all libm builtins
>> (anticipating future SIMD-ification) in the same way, with
>> same costs.
>
> Probably a daft question, but: if the assumption is that OMP SIMD calls
> are very expensive (which I agree is reasonable!), then would there be
> any important cases in which we'd want to pick an SSE loop with OMP SIMD
> calls over an AVX loop with OMP SIMD calls?
>
> I just wonder whether comparing "number of OMP SIMD CALLs / estimated VF"
> would get us most of the way there, falling back to the current cost
> comparison when the ratios are equal.
I think that would work for the case in question, but how does it work for
Comparing against the scalar loop cost?
We could effectively ignore it or simply make it number of calls per iteration.
The quite arbitrary cost for SIMD calls is equally problematic. I guess
there’s simply no perfect way. I do somewhat like the patch for the reason
we’ll keep one thing to compare rather than making the compare effectively “two
dimensional”
Richard
>
> Thanks,
> Richard
>
>>
>> Thanks,
>> Richard.
>>
>> PR target/125174
>> * config/i386/i386.cc (ix86_vector_costs::add_stmt_cost):
>> Cost calls as 10 times FMA.
>> ---
>> gcc/config/i386/i386.cc | 7 +++++++
>> 1 file changed, 7 insertions(+)
>>
>> diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc
>> index e73c2d7f7d0..6b271ac3fca 100644
>> --- a/gcc/config/i386/i386.cc
>> +++ b/gcc/config/i386/i386.cc
>> @@ -26602,6 +26602,13 @@ ix86_vector_costs::add_stmt_cost (int count,
>> vect_cost_for_stmt kind,
>> case CFN_MULH:
>> stmt_cost = ix86_multiplication_cost (ix86_cost, mode);
>> break;
>> + CASE_CFN_SIN:
>> + CASE_CFN_COS:
>> + CASE_CFN_EXP:
>> + stmt_cost = 10 * ix86_vec_cost (mode,
>> + mode == SFmode ? ix86_cost->fmass
>> + : ix86_cost->fmasd);
>> + break;
>> default:
>> break;
>> }
