Date: Fri, 23 Jun 2017 18:08:23 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: [PATCH 4/8] determine the existence of private futexes at the first thread creation On Fri, Jun 23, 2017 at 11:48:25PM +0200, Jens Gustedt wrote: > Hello Rich, > > On Fri, 23 Jun 2017 13:05:35 -0400 Rich Felker <dalias@...c.org> wrote: > > > This was intentional, the idea being that a 100% predictable branch in > > a path where a syscall is being made anyway is much less expensive > > than a GOT address load that gets hoisted all the way to the top of > > the function and affects even code paths that don't need to make the > > syscall. Whether it was a choice that makes sense overall, I'm not > > sure, but that was the intent. > > So if we can avoid going through GOT, this would be better? > I'd just add ATTR_LIBC_VISIBILITY to the variable, and then this > should go away the same way as it is done for the libc object. It's not going through the GOT that's costly, but actually getting the GOT address, which is used for both accesses through the GOT and GOT-relative addressing. On several archs including i386, PC-relative addressing is not directly available and requires hacks to load the PC into a GPR, and these usually take some cycles themselves and spill out of the free call-clobbered registers so that additional stack shuffling is needed. Rich
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