Date: Fri, 23 Jun 2017 20:53:48 -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 Sat, Jun 24, 2017 at 01:42:20AM +0200, Jens Gustedt wrote: > Hello Rich, > > On Fri, 23 Jun 2017 18:08:23 -0400 Rich Felker <dalias@...c.org> wrote: > > > 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. > > So you are saying that when I add ATTR_LIBC_VISIBILITY > and see something like > > movslq __futex_private(%rip), %rsi i386 does not have (%rip). x86_64 is one of the archs with very efficient PC-relative addressing. > What would you think of a patch that just cleans up the 128 vs > FUTEX_PRIVATE issue? Just to improve readability? That seems like a good thing. Regarding the mutex flag, I thought some places we depended on the 128 shared mutex flag being the inverse of the FUTEX_PRIVATE flag (so ^128 translates between them); if this is the case, do you have an elegant way to make it work? > Also there is this missing volatile in __get_locale. Nice catch. Rich
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