Re: Reduce function call costs on ELF platforms

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Andres Freund <andres(at)anarazel(dot)de>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Reduce function call costs on ELF platforms
Date: 2021-11-22 22:32:21
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Andres Freund <andres(at)anarazel(dot)de> writes:
> Basically they way we currently build our extensions, the compiler & linker
> assume every symbol inside the extension libraries needs to be interceptable
> by the main binary. Which means that all function calls to symbols visible
> outside the current translation unit need to be made indirectly via the PLT.

Yeah, that would be nice to improve.

> The easier approach for this class of issues is to use the linker option
> -Bsymbolic.

I don't recall details, but we've previously rejected the idea of
trying to use -Bsymbolic widely; apparently it has undesirable
side-effects on some platforms. See commit message for e3d77ea6b
(hopefully there's some detail in the email thread [1]). It sounds
like you're not actually proposing that, but I thought it would be
a good idea to note the hazard here.

> By compiling with -fno-plt, the above becomes:

Does -fno-plt amount to an ABI change? If so, I'm worried that it'd
break the ability to compile extensions with a different compiler.

Also, we have at least some places where there actually are cross-calls
between extensions, eg hstore_perl -> plperl. Do we need to worry about
breaking those?

regards, tom lane


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