From: Michael Matz <matz@suse.de>
To: Florian Weimer <fweimer@redhat.com>
Cc: Andreas Schwab <schwab@linux-m68k.org>,
bug-gnulib@gnu.org,
"H.J. Lu via Libc-alpha" <libc-alpha@sourceware.org>,
Bruno Haible <bruno@clisp.org>,
Binutils <binutils@sourceware.org>
Subject: Re: Undefined use of weak symbols in gnulib
Date: Wed, 28 Apr 2021 13:15:09 +0000 (UTC) [thread overview]
Message-ID: <alpine.LSU.2.20.2104281310370.4716@wotan.suse.de> (raw)
In-Reply-To: <87h7jqkdrn.fsf@oldenburg.str.redhat.com>
Hello,
On Wed, 28 Apr 2021, Florian Weimer via Binutils wrote:
> > commit 954b63d4c8645f86e40c7ef6c6d60acd2bf019de
> > Author: Alan Modra <amodra@gmail.com>
> > Date: Wed Apr 19 01:26:57 2017 +0930
> >
> > Implement -z dynamic-undefined-weak
> >
> > -z nodynamic-undefined-weak is only implemented for x86. (The sparc
> > backend has some support code but doesn't enable the option by
> > including ld/emulparams/dynamic_undefined_weak.sh, and since the
> > support looks like it may be broken I haven't enabled it.) This patch
> > adds the complementary -z dynamic-undefined-weak, extends both options
> > to affect building of shared libraries as well as executables, and
> > adds support for the option on powerpc.
>
> I'm not sure if this option is compatible with all compilers on POWER.
>
> The old binutils behavior allowed the compiler to optimize this common
> pattern
>
> void f1 (void) __attribute__ ((weak));
>
> void
> f2 (void)
> {
> if (f1 != 0)
> f1 ();
> }
>
> into an unconditional call to f1 because the linker would replace the
> call with a NOP if the symbol is undefined, reproducing the effect of
> the if condition. However, GCC 10 does not appear to perform this
> optimization on powerpc64le.
GCC assumes addresses of functions to be non-null, _except when declared
weak_. Over the years we fixed problem in that aspect always into that
direction (hence the above testcase should always be emitted with an
actual test, otherwise it would be considered a bug worth fixing). That
would be consistent with making undefined weak symbols dynamic in the ELF
case, so that the ultimate test survives until runtime.
I don't know what other compilers are doing, of course.
Ciao,
Michael.
next prev parent reply other threads:[~2021-04-28 13:15 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-27 5:53 Undefined use of weak symbols in gnulib Florian Weimer via Libc-alpha
2021-04-27 6:50 ` Paul Eggert
2021-04-27 6:58 ` Florian Weimer via Libc-alpha
2021-04-27 7:13 ` Paul Eggert
2021-04-27 7:24 ` Andreas Schwab
2021-04-27 11:06 ` Florian Weimer via Libc-alpha
2021-04-28 0:09 ` Bruno Haible
2021-04-28 2:10 ` H.J. Lu via Libc-alpha
2021-04-28 2:13 ` H.J. Lu via Libc-alpha
2021-05-05 20:31 ` Fangrui Song
2021-04-28 8:35 ` Florian Weimer via Libc-alpha
2021-04-28 13:15 ` Michael Matz [this message]
2021-04-28 7:44 ` Florian Weimer via Libc-alpha
2021-04-28 14:48 ` Bruno Haible
2021-04-28 17:44 ` Florian Weimer via Libc-alpha
2021-07-17 14:38 ` Bruno Haible
2021-07-17 14:55 ` Florian Weimer via Libc-alpha
2021-07-17 16:39 ` Bruno Haible
2021-07-27 20:02 ` Joseph Myers
2021-07-27 20:19 ` Florian Weimer via Libc-alpha
2021-07-27 23:38 ` Paul Eggert
2021-04-27 23:22 ` Bruno Haible
2021-04-27 23:47 ` Bruno Haible
2021-04-28 7:57 ` Florian Weimer via Libc-alpha
2021-04-28 14:40 ` Bruno Haible
2021-04-28 17:43 ` Florian Weimer via Libc-alpha
2021-04-29 15:15 ` Bruno Haible
2021-04-30 9:55 ` Florian Weimer via Libc-alpha
2021-04-29 6:33 ` Ben Pfaff via Libc-alpha
2021-05-03 1:44 ` Alan Modra via Libc-alpha
2021-07-12 10:04 ` Michael Hudson-Doyle via Libc-alpha
2021-07-12 15:03 ` Florian Weimer via Libc-alpha
2021-07-12 15:30 ` Matthias Klose
2021-07-12 15:37 ` Florian Weimer via Libc-alpha
2021-07-13 0:22 ` Michael Hudson-Doyle via Libc-alpha
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