Re: [PATCH 4/8] determine the existence of private futexes at the first thread creation

[Jens Gustedt <jens.gustedt@inria.fr>]

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Hello Rich,

On Fri, 23 Jun 2017 18:08:23 -0400 Rich Felker <dalias@libc.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@libc.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

in the assembler (instead of GOP stuff), this is actually going through
such a complicated mechanism? But then the same penalty applies for
members of the "libc" object, doesn't it? E.g __lock accesses
"libc.threads_minus_1" which results in something like

	movl	12+__libc(%rip), %eax

In any case, all of this is probably not so important in view of the
system call that is happening right after. So let's just drop it.

What would you think of a patch that just cleans up the 128 vs
FUTEX_PRIVATE issue? Just to improve readability?

Also there is this missing volatile in __get_locale.

Thanks
Jens

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