From: Tom Lane <tgl@sss.pgh.pa.us>
To: Thomas Munro <thomas.munro@enterprisedb.com>
cc: Alvaro Herrera <alvherre@2ndquadrant.com>,
        Andrew Gierth <andrew@tao11.riddles.org.uk>,
        David Rowley <david.rowley@2ndquadrant.com>,
        PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Using POPCNT and other advanced bit manipulation instructions
In-reply-to: <14024.1550119448@sss.pgh.pa.us>
References: <20190213221719.GA15976@alvherre.pgsql>
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 <11319.1550115484@sss.pgh.pa.us>
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 <14024.1550119448@sss.pgh.pa.us>
Comments: In-reply-to Tom Lane <tgl@sss.pgh.pa.us>
	message dated "Wed, 13 Feb 2019 23:44:08 -0500"
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Date: Thu, 14 Feb 2019 00:24:50 -0500
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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