ARCH/X86: Use UCS function to count leading zeros #10514
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@tvegas1 __builtin_clz is sligtly different when compared to _lzcnt_u32(). One problem with __builtin_clz() is that it is undefined if the input is zero. LZCNT ( _lzcnt_u32()) is a better instruction for counting the number of leading zeros. How do you think we should proceed? Can we add _lzcnt_u32() support in NVC? |
We can file a bug for nvc, this will take time to fix. And we can handle nvc case in |
| __m256i y0, y1, y2, y3; | ||
| /* Handle lengths that fall usually within eager short range */ | ||
| switch (_lzcnt_u32(len)) { | ||
| switch (ucs_count_leading_zero_bits(len)) { |
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If the len is zero the code expects '32' as output, is ucs_count_leading_zero_bits(0) return 32?
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yes you're right, should be fixed now
What?
Use common UCS count leading zeros function (implemented with
__builtin_clz()) to address #10509.Why?
nvc 24.9 seems to have issue with
_lzcnt_u32(). It defines it as__builtin_ia32_lzcnt_u32()but fails to find it at link time. But the_lzcnt_u32()function seems to be likely defined as__builtin_clz()in gcc, and is also properly linked by nvc.