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Re: [patch] For BZ #17328, mark __errno_location with __attribute__((returns_nonnull)) for gcc >=4.9.0
- From: Rich Felker <dalias at libc dot org>
- To: Florian Weimer <fweimer at redhat dot com>
- Cc: Roland McGrath <roland at hack dot frob dot com>, Zack Weinberg <zackw at panix dot com>, Paul Pluzhnikov <ppluzhnikov at gmail dot com>, GLIBC Devel <libc-alpha at sourceware dot org>
- Date: Tue, 3 Mar 2015 09:54:26 -0500
- Subject: Re: [patch] For BZ #17328, mark __errno_location with __attribute__((returns_nonnull)) for gcc >=4.9.0
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- References: <CALoOobOuAEpw+zxRrrDyHB7UVbAZMzreXqpujzZOWNLS7+aRUA at mail dot gmail dot com> <20150301011753 dot GV23507 at brightrain dot aerifal dot cx> <CAPC3xapQBMH+DJdup2Y8_tt6xdcFAnQLB_K8VpT3ouCavvzXXA at mail dot gmail dot com> <CAKCAbMgyAEueVsYB=avfUg0pK49YiY-Qyi8w1qXvfTCzfPbUiQ at mail dot gmail dot com> <20150302224606 dot 9C97F2C3A08 at topped-with-meat dot com> <54F5918B dot 30805 at redhat dot com>
On Tue, Mar 03, 2015 at 11:48:43AM +0100, Florian Weimer wrote:
> On 03/02/2015 11:46 PM, Roland McGrath wrote:
> > Something that we could consider as a middle road is exporting errno with a
> > proper symbol version (it's currently GLIBC_PRIVATE, which nothing residing
> > outside the libc source tree should ever use) and then having errno.h do:
> > extern __thread int errno;
> Can we make the TLS offset part of the ABI instead and somehow teach GCC
> about it? Then there would at least be an unquestionable improvement.
I would really advise against this. Fixed offsets are really nasty
lock-in and don't save much. For the non-PIE main program they save
basically nothing versus a runtime-variable offset. For shared
libraries or PIE, there is some minor saving especially on archs
(32-bit x86) that need to load a GOT register to load the offset;
however, right now such archs are already loading a GOT pointer to
make the function call to __errno_location, so this is not a new cost.
> Right now, we have (in PIC code):
> call __errno_location@PLT
> movl (%rax), %eax
> This turns into:
> leaq errcode@tlsgd(%rip), %rdi
> call __tls_get_addr@PLT
> movl (%rax), %eax
> errno is mostly used on error paths, so this just bloats the code and is
> probably even slower, too.
__tls_get_addr is not needed if you use the initial-exec model. And if
you're using GD with TLSDESC rather than the old GD model, it's also
not this ugly/bloated.
However I still prefer just leaving things alone with
__errno_location. Like you say it's tiny and its internals can be
optimized heavily if needed (e.g. using a constant offset like it is
now). In this case the constant is encoded in libc.so so it can change
from one version to another rather than being permanent ABI lock-in.