Re: Popcount optimization using AVX512

On Fri, Mar 29, 2024 at 10:59:40AM -0500, Nathan Bossart wrote:
> It might be nice if we conditionally built pg_popcount_avx512.o in autoconf
> builds, too, but AFAICT we still need to wrap most of that code with
> macros, so I'm not sure it's worth the trouble.  I'll take another look at
> this...

If we assumed that TRY_POPCNT_FAST would be set and either
HAVE__GET_CPUID_COUNT or HAVE__CPUIDEX would be set whenever
USE_AVX512_POPCNT_WITH_RUNTIME_CHECK is set, we could probably remove the
surrounding macros and just compile pg_popcount_avx512.c conditionally
based on USE_AVX512_POPCNT_WITH_RUNTIME_CHECK.  However, the surrounding
code seems to be pretty cautious about these assumptions (e.g., the CPUID
macros are checked before setting TRY_POPCNT_FAST), so this would stray
from the nearby precedent a bit.

A counterexample is the CRC32C code.  AFAICT we assume the presence of
CPUID in that code (and #error otherwise).  I imagine its probably safe to
assume the compiler understands CPUID if it understands AVX512 intrinsics,
but that is still mostly a guess.

-- 
Nathan Bossart
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