Re: Using POPCNT and other advanced bit manipulation instructions

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>
Cc: Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk>, David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: Using POPCNT and other advanced bit manipulation instructions
Date: 2019-02-14 18:54:32
Message-ID: 25934.1550170472@sss.pgh.pa.us
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Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> writes:
> 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'd bet a fair amount of money that we'd be better off *not* using
lzcnt, even if available, because then we could just expose things
along this line:

static inline int
pg_clz(...)
{
#ifdef HAVE__BUILTIN_CLZ
return __builtin_clz(x);
#else
handwritten implementation;
#endif
}

Avoiding a function call (that has to indirect through a pointer) probably
saves much more than the difference between lzcnt and the other way.

The tradeoff might be different for popcount, though, especially since
it looks like __builtin_popcount() is not nearly as widely available
as the clz/ctz builtins.

regards, tom lane

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