From: Noah Misch <noah@leadboat.com>
To: Andres Freund <andres@anarazel.de>
Cc: Alvaro Herrera <alvherre@alvh.no-ip.org>
Cc: Shankaran, Akash <akash.shankaran@intel.com>
Cc: Nathan Bossart <nathandbossart@gmail.com>
Cc: Amonson, Paul D <paul.d.amonson@intel.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Matthias van de Meent <boekewurm+postgres@gmail.com>
Cc: pgsql-hackers@lists.postgresql.org <pgsql-hackers@lists.postgresql.org>
Subject: Re: Popcount optimization using AVX512
Date: Sat, 10 Feb 2024 15:52:38 -0800
Message-ID: <20240210235238.eb@rfd.leadboat.com> (raw)
In-Reply-To: <20240210043323.uofbl6h7twao2mqg@awork3.anarazel.de>
References: <202401250949.xrwkd46bgbpl@alvherre.pgsql>
<202401260642.3rxzgb2xrhkl@alvherre.pgsql>
<20240209182432.7c2peq5cgbdl43um@awork3.anarazel.de>
<20240209232757.5d@rfd.leadboat.com>
<20240210043323.uofbl6h7twao2mqg@awork3.anarazel.de>
On Fri, Feb 09, 2024 at 08:33:23PM -0800, Andres Freund wrote:
> On 2024-02-09 15:27:57 -0800, Noah Misch wrote:
> > On Fri, Feb 09, 2024 at 10:24:32AM -0800, Andres Freund wrote:
> > > On 2024-01-26 07:42:33 +0100, Alvaro Herrera wrote:
> > > > This suggests that finding a way to make the ifunc stuff work (with good
> > > > performance) is critical to this work.
> > >
> > > Ifuncs are effectively implemented as a function call via a pointer, they're
> > > not magic, unfortunately. The sole trick they provide is that you don't
> > > manually have to use the function pointer.
> >
> > The IFUNC creators introduced it so glibc could use arch-specific memcpy with
> > the instruction sequence of a non-pointer, extern function call, not the
> > instruction sequence of a function pointer call.
>
> My understanding is that the ifunc mechanism just avoid the need for repeated
> indirect calls/jumps to implement a single function call, not the use of
> indirect function calls at all. Calls into shared libraries, like libc, are
> indirected via the GOT / PLT, i.e. an indirect function call/jump. Without
> ifuncs, the target of the function call would then have to dispatch to the
> resolved function. Ifuncs allow to avoid this repeated dispatch by moving the
> dispatch to the dynamic linker stage, modifying the contents of the GOT/PLT to
> point to the right function. Thus ifuncs are an optimization when calling a
> function in a shared library that's then dispatched depending on the cpu
> capabilities.
>
> However, in our case, where the code is in the same binary, function calls
> implemented in the main binary directly (possibly via a static library) don't
> go through GOT/PLT. In such a case, use of ifuncs turns a normal direct
> function call into one going through the GOT/PLT, i.e. makes it indirect. The
> same is true for calls within a shared library if either explicit symbol
> visibility is used, or -symbolic, -Wl,-Bsymbolic or such is used. Therefore
> there's no efficiency gain of ifuncs over a call via function pointer.
>
>
> This isn't because ifunc is implemented badly or something - the reason for
> this is that dynamic relocations aren't typically implemented by patching all
> callsites (".text relocations"), which is what you would need to avoid the
> need for an indirect call to something that fundamentally cannot be a constant
> address at link time. The reason text relocations are disfavored is that
> they can make program startup quite slow, that they require allowing
> modifications to executable pages which are disliked due to the security
> implications, and that they make the code non-shareable, as the in-memory
> executable code has to differ from the on-disk code.
>
>
> I actually think ifuncs within the same binary are a tad *slower* than plain
> function pointer calls, unless -fno-plt is used. Without -fno-plt, an ifunc is
> called by 1) a direct call into the PLT, 2) loading the target address from
> the GOT, 3) making an an indirect jump to that address. Whereas a "plain
> indirect function call" is just 1) load target address from variable 2) making
> an indirect jump to that address. With -fno-plt the callsites themselves load
> the address from the GOT.
That sounds more accurate than what I wrote. Thanks.
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To: pgsql-hackers@postgresql.org
Cc: noah@leadboat.com, andres@anarazel.de, alvherre@alvh.no-ip.org, akash.shankaran@intel.com, nathandbossart@gmail.com, paul.d.amonson@intel.com, tgl@sss.pgh.pa.us, boekewurm+postgres@gmail.com, pgsql-hackers@lists.postgresql.org
Subject: Re: Popcount optimization using AVX512
In-Reply-To: <20240210235238.eb@rfd.leadboat.com>
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