From: | David Fetter <david(at)fetter(dot)org> |
---|---|
To: | John Naylor <john(dot)naylor(at)2ndquadrant(dot)com> |
Cc: | Jesse Zhang <sbjesse(at)gmail(dot)com>, PostgreSQL Development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Use compiler intrinsics for bit ops in hash |
Date: | 2020-02-28 15:13:00 |
Message-ID: | 20200228151259.GM13804@fetter.org |
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Lists: | pgsql-hackers |
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(at)fetter(dot)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/
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