Date: Tue, 16 Oct 2018 13:59:06 -0700
From: Andres Freund <andres@anarazel.de>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-hackers@postgresql.org,
	Thomas Munro <thomas.munro@enterprisedb.com>
Subject: Re: Large writable variables
Message-ID: <20181016205906.hb2maf5l65kyo7ah@alap3.anarazel.de>
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On 2018-10-16 16:36:12 -0400, Tom Lane wrote:
> Andres Freund <andres@anarazel.de> writes:
> > Attached is a patch that shrinks fmgr_builtins by 25%. That seems
> > worthwhile, it's pretty frequently accessed, making it more dense is
> > helpful.  Unless somebody protests soon, I'm going to apply that...
> 
> Hah.  I'm pretty sure that struct *was* set up with an eye to padding ...
> on 32-bit machines.

Possible, the new layout should work just as well there, luckily.


> This does make it shorter on 64-bit, but also
> makes the size not a power of 2, which might add a few cycles to
> array indexing calculations.  Might be worth checking whether that's
> going to be an issue anywhere.

I can't imagine that that outweight the cost of additional cache misses
on any platform where performance matters.  On x86 I assume indexing
into an array with 24byte stride, will normally be just two leas (lea
eax, [eax + eax * 2]; lea eax, [ebx + eax * 8]; where eax initially is
the index, and ebx the array base).  Indexing also plays less of a role
than in the past, because previously we did a binary search, but now we
normally look up the index via fmgr_builtin_oid_index.


> What's the point of the extra const decoration on funcName?  ISTM
> the whole struct should be const, or not.

The whole array is const.  There's cases where that allows the compiler
more freedom - but I guess it's really a bit redundant here.

Greetings,

Andres Freund