On 2019-Feb-14, Tom Lane wrote:
> Some further thoughts here ...
>
> Does the "lzcnt" runtime probe actually do anything useful?
> On the x86_64 compilers I tried (gcc 8.2.1 and 4.4.7), __builtin_clz
> and __builtin_ctz compile to sequences involving bsrq and bsfq
> regardless of -mpopcnt. It's fairly hard to see how lzcnt would
> buy anything over those sequences even if there were zero overhead
> involved in using it.
Hah, I just realized you have to add -mlzcnt in order for these builtins
to use the lzcnt instructions. It goes from something like
bsrq %rax, %rax
xorq $63, %rax
to
lzcntq %rax, %rax
Significant?
I have this patch now, written before I realized the above; I'll augment
it to cater to this (adding -mlzcnt and a new set of functions --
perhaps a new file "lzcnt.c" or maybe put the lot in pg_popcount.c and
rename it?) and resubmit after an errand I have to run now.
--
Álvaro Herrera https://www.2ndQuadrant.com/
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