Re: Popcount optimization using AVX512

On Fri, Mar 29, 2024 at 02:13:12PM -0500, Nathan Bossart wrote:
> * If the compiler understands AVX512 intrinsics, we assume that it also
> knows about the required CPUID and XGETBV intrinsics, and we assume that
> the conditions for TRY_POPCNT_FAST are true.

Bleh, cfbot's 32-bit build is unhappy with this [0]. It looks like it's
trying to build the AVX512 stuff, but TRY_POPCNT_FAST isn't set.

[19:39:11.306] ../src/port/pg_popcount_avx512.c:39:18: warning: implicit declaration of function ‘pg_popcount_fast’; did you mean ‘pg_popcount’? [-Wimplicit-function-declaration]
[19:39:11.306] 39 | return popcnt + pg_popcount_fast(buf, bytes);
[19:39:11.306] | ^~~~~~~~~~~~~~~~
[19:39:11.306] | pg_popcount

There's also a complaint about the inline assembly:

[19:39:11.443] ../src/port/pg_popcount_avx512_choose.c:55:1: error: inconsistent operand constraints in an ‘asm’
[19:39:11.443] 55 | __asm__ __volatile__(" xgetbv\n":"=a"(low), "=d"(high):"c"(xcr));
[19:39:11.443] | ^~~~~~~

I'm looking into this...

> +#if defined(HAVE__GET_CPUID)
> + __get_cpuid_count(7, 0, &exx[0], &exx[1], &exx[2], &exx[3]);
> +#elif defined(HAVE__CPUID)
> + __cpuidex(exx, 7, 0);

Is there any reason we can't use __get_cpuid() and __cpuid() here, given
the sub-leaf is 0?

[0] https://cirrus-ci.com/task/5475113447981056

--
Nathan Bossart
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