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Re: inline newNode()

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Neil Conway <neilc(at)samurai(dot)com>
Cc: PostgreSQL Patches <pgsql-patches(at)postgresql(dot)org>
Subject: Re: inline newNode()
Date: 2002-10-08 01:08:20
Message-ID: 27833.1034039300@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patches
Neil Conway <neilc(at)samurai(dot)com> writes:
> Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:
>> How much did you bloat the code?  There are an awful lot of calls to
>> newNode(), so even though it's not all that large, I'd think the
>> multiplier would be nasty.

> The patch increases the executable from 12844452 to 13005244 bytes,
> when compiled with '-pg -g -O2' and without being stripped.

Okay, not as bad as I feared, but still kinda high.

I believe that most of the bloat comes from the MemSet macro; there's
just not much else in newNode().  Now, the reason MemSet expands to
a fair amount of code is its if-then-else case to decide whether to
call memset() or do an inline loop.  I've looked at the assembler code
for it on a couple of machines, and the loop proper is only about a
third of the code that gets generated.

Ideally, we'd like to eliminate the if-test for inlined newNode calls.
That would buy back a lot of the bloat and speed things up still
further.

Now the tests on _val == 0 and _len <= MEMSET_LOOP_LIMIT and _len being
a multiple of 4 are no problem, since _val and _len are compile-time
constants; these will be optimized away.  What is not optimized away
(on the compilers I've looked at) is the check for _start being
int-aligned.

A brute-force approach is to say "we know _start is word-aligned because
we just got it from palloc, which guarantees MAXALIGNment".  We could
make a variant version of MemSet that omits the alignment check, and use
it here and anywhere else we're sure it's safe.

A nicer approach would be to somehow make use of the datatype of the
first argument to MemSet.  If we could determine at compile time that
it's supposed to point at a type with at least int alignment, then
it'd be possible for the compiler to optimize away this check in a
reasonably safe fashion.  I'm not sure if there's a portable way to
do this, though.  There's no "alignof()" construct in C :-(.
Any ideas?

			regards, tom lane

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