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0 pointstedunangst3y ago0 comments

Which flag should I check for fast PEXT?

0 comments

dataflow3y ago

See [1], or [2] and [3].

[1] https://github.com/as-com/varint-simd/blob/f4c4af672403cc683...

[2] https://en.wikipedia.org/wiki/X86_Bit_manipulation_instructi...

[3] https://en.wikipedia.org/wiki/List_of_AMD_CPU_microarchitect...

tedunangstOP3y ago

So... ignore the cpuid bit indicating the instruction is present and look at manufacturer string, which is exactly what intel is being lambasted for here.

dataflow3y ago

No, you don't ignore the CPUID feature bit. You use the CPUID to enable the feature by default, but override that default by blacklisting specific models that you know would have undesirable implementations.

So what you would do for PDEP/PEXT would be

  use_pdep = cpuid.supports_bmi2() && !cpuid.is_amd_before_zen3()

and not

  use_pdep = cpuid.is_intel_haswell_or_later()

j / k navigate · click thread line to collapse

0 comments

dataflow3y ago

See [1], or [2] and [3].

[1] https://github.com/as-com/varint-simd/blob/f4c4af672403cc683...

[2] https://en.wikipedia.org/wiki/X86_Bit_manipulation_instructi...

[3] https://en.wikipedia.org/wiki/List_of_AMD_CPU_microarchitect...

tedunangstOP3y ago

So... ignore the cpuid bit indicating the instruction is present and look at manufacturer string, which is exactly what intel is being lambasted for here.

dataflow3y ago

So what you would do for PDEP/PEXT would be

  use_pdep = cpuid.supports_bmi2() && !cpuid.is_amd_before_zen3()

and not

  use_pdep = cpuid.is_intel_haswell_or_later()

j / k navigate · click thread line to collapse