From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
cc: Alvaro Herrera <alvherre@2ndquadrant.com>,
        Thomas Munro <thomas.munro@enterprisedb.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: <20190214185905.kwvrjy6qu52xdxyh@alap3.anarazel.de>
References: <16168.1550121890@sss.pgh.pa.us>
 <20190214184713.GA6080@alvherre.pgsql>
 <20190214185905.kwvrjy6qu52xdxyh@alap3.anarazel.de>
Comments: In-reply-to Andres Freund <andres@anarazel.de>
	message dated "Thu, 14 Feb 2019 10:59:05 -0800"
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Date: Thu, 14 Feb 2019 16:45:38 -0500
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Andres Freund <andres@anarazel.de> writes:
> On 2019-02-14 15:47:13 -0300, Alvaro Herrera wrote:
>> 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

> I'm confused how this is a general count leading zero operation? Did you
> use constants or something that allowed ot infer a range in the test? If
> so the compiler probably did some optimizations allowing it to do the
> above.

No.  If you compile

int myclz(unsigned long long x)
{
  return __builtin_clzll(x);
}

at -O2, on just about any x86_64 gcc, you will get

myclz:
.LFB1:
        .cfi_startproc
        bsrq    %rdi, %rax
        xorq    $63, %rax
        ret
        .cfi_endproc

			regards, tom lane