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Message-ID: <20140511161943.GR26358@brightrain.aerifal.cx>
Date: Sun, 11 May 2014 12:19:43 -0400
From: Rich Felker <dalias@...c.org>
To: Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>
Cc: musl@...ts.openwall.com
Subject: Re: Broken GCC versions: 4.8.2 and 4.9.0
On Sun, May 11, 2014 at 06:10:20PM +0200, Thomas Petazzoni wrote:
> Dear Rich Felker,
>
> On Sat, 10 May 2014 21:05:03 -0400, Rich Felker wrote:
>
> > It's come to my attention that GCC versions 4.8.2 and 4.9.0 are
> > performing invalid optimizations that result in a broken musl
> > libc.a/libc.so. It's not clear yet whether there's a good workaround,
> > or whether we should attempt to work around the problem, so for now,
> > please just be aware that these versions of GCC cannot be used to
> > compile musl. Using them to compile programs against musl should not
> > be a problem. I'll post more details later. The short version is that
> > it's making incorrect assumptions about the reachability of global
> > variables that have a local weak definition and an external strong
> > one.
>
> Hum, interesting. I've recently tested gcc 4.8.2 + musl on ARM, and gcc
> 4.9.0 + musl on i386, and I could boot a minimal musl+Busybox system
> under Qemu perfectly fine. Maybe the problem you refer to only affects
> certain parts of libc.a/libc.so?
I've filed the bug report which you can see here:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=61144
Something like the following command should confirm whether your build
is affected:
nm src/stdio/fflush.o | grep stdout
For broken gcc versions, there is no output. For non-broken ones, you
should see something like:
00000000 V __stdout_used
Note that fflush(0) is just one place where the behavior may be
affected by the invalid optimization. There are at least several
others, though at a glance the others are more likely to be affected
only with heavy inlining (but in principle, they "should" be affected
even without inlining as long as inter-procedural optimizations are
performed).
Rich
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