Re: Using POPCNT and other advanced bit manipulation instructions

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>
Cc: Alvaro Herrera <alvherre(at)2ndquadrant(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 05:24:50
Message-ID: 16168.1550121890@sss.pgh.pa.us
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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.

Alvaro noted that the test programs used by c-compiler.m4 fail
to produce any actual code involving the builtin, because of
compile-time constant folding. This seems pretty unwise.
I see that on my x86_64 compilers, without -mpopcnt,
__builtin_popcnt compiles to a call of some libgcc function
or other. It's conceivable that on an (arguably misconfigured)
platform, these c-compiler.m4 tests would pass yet the build fails
at link because libgcc lacks the needed infrastructure. These tests
should be coded in a way that doesn't allow the call to be optimized
away -- cf comments for PGAC_C_BUILTIN_OP_OVERFLOW.

Also, it's starting to seem like we have enough probes for compiler
builtins that we should fold them to use one set of infrastructure.
There are some like __builtin_constant_p that probably do need their
own custom tests, but these ones that just verify that a call
compiles seem pretty duplicative ...

regards, tom lane

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