Re: Using POPCNT and other advanced bit manipulation instructions

From: Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
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 22:28:38
Message-ID: 20190214222838.GA10839@alvherre.pgsql
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On 2019-Feb-14, Tom Lane wrote:

> 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.

I see ... makes sense.

That leads me to the attached patch. It creates a new file
pg_popcount.c which is the only one compiled with -mpopcnt (if
available); if there's no compiler switch to enable POPCNT, we just
don't compile the file. I'm not sure that's kosher -- in particular I'm
not sure if it can fail when POPCNT is enabled by other flags and
-mpopcnt is not needed at all. I think our c-compiler.m4 stuff is a bit
too simplistic there: it just assumes that -mpopcnt is always required.
But what if the user passes it in CFLAGS?

I left CPUID alone for the CLZ/CTZ builtins. So we either use the
table, or the builtins. We never try the instructions.

--
Álvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Attachment Content-Type Size
popcount-2.patch text/x-diff 17.1 KB

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