Date: Fri, 28 Feb 2020 16:13:00 +0100
From: David Fetter <david@fetter.org>
To: John Naylor <john.naylor@2ndquadrant.com>
Cc: Jesse Zhang <sbjesse@gmail.com>,
	PostgreSQL Development <pgsql-hackers@postgresql.org>
Subject: Re: Use compiler intrinsics for bit ops in hash
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On Thu, Feb 27, 2020 at 02:41:49PM +0800, John Naylor wrote:
> On Thu, Feb 27, 2020 at 1:56 PM David Fetter <david@fetter.org> wrote:
> > [v6 set]
> 
> Hi David,
> 
> In 0002, the pg_bitutils functions have a test (input > 0), and the
> new callers ceil_log2_* and next_power_of_2_* have asserts. That seems
> backward to me.

To me, too, now that you mention it.  My thinking was a little fuzzed
by trying to accommodate platforms with intrinsics where clz is
defined for 0 inputs.

> I imagine some callers of bitutils will already know the value > 0,
> and it's probably good to keep that branch out of the lowest level
> functions. What do you think?

I don't know quite how smart compilers and CPUs are these days, so
it's unclear to me how often that branch would actually happen.

Anyhow, I'll get a revised patch set out later today.

Best,
David.
-- 
David Fetter <david(at)fetter(dot)org> http://fetter.org/
Phone: +1 415 235 3778

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