From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
Cc: Heikki Linnakangas <hlinnaka@iki.fi>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: pgsql-hackers@postgresql.org
Subject: Re: WIP: Faster Expression Processing v4
Date: Sat, 25 Mar 2017 12:22:15 -0400
Message-ID: <5768.1490458935@sss.pgh.pa.us> (raw)
In-Reply-To: <13918.1490410879@sss.pgh.pa.us>
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More random musing ... have you considered making the jump-target fields
in expressions be relative rather than absolute indexes? That is,
EEO_JUMP would look like
op += (stepno); \
EEO_DISPATCH(); \
instead of
op = &state->steps[stepno]; \
EEO_DISPATCH(); \
I have not carried out a full patch to make this work, but just making
that one change and examining the generated assembly code looks promising.
Instead of this
movslq 40(%r14), %r8
salq $6, %r8
addq 24(%rbx), %r8
movq %r8, %r14
jmp *(%r8)
we get this
movslq 40(%r14), %rax
salq $6, %rax
addq %rax, %r14
jmp *(%r14)
which certainly looks like it ought to be faster. Also, the real reason
I got interested in this at all is that with relative jumps, groups of
steps would be position-independent within the steps array, which would
enable some compile-time tricks that seem impractical with the current
definition.
BTW, now that I've spent a bit of time looking at the generated assembly
code, I'm kind of disinclined to believe any arguments about how we have
better control over branch prediction with the jump-threading
implementation. At least with current gcc (6.3.1 on Fedora 25) at -O2,
what I see is multiple places jumping to the same indirect jump
instruction :-(. It's not a total disaster: as best I can tell, all the
uses of EEO_JUMP remain distinct. But gcc has chosen to implement about
40 of the 71 uses of EEO_NEXT by jumping to the same couple of
instructions that increment the "op" register and then do an indirect
jump :-(.
So it seems that we're at the mercy of gcc's whims as to which instruction
dispatches will be distinguishable to the hardware; which casts a very
dark shadow over any benchmarking-based arguments that X is better than Y
for branch prediction purposes. Compiler version differences are likely
to matter a lot more than anything we do.
regards, tom lane
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To: pgsql-hackers@postgresql.org
Cc: tgl@sss.pgh.pa.us, andres@anarazel.de, hlinnaka@iki.fi, tomas.vondra@2ndquadrant.com
Subject: Re: WIP: Faster Expression Processing v4
In-Reply-To: <5768.1490458935@sss.pgh.pa.us>
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