Hello,
On Wed, 28 Apr 2021, Florian Weimer via Binutils wrote:
> > commit 954b63d4c8645f86e40c7ef6c6d60acd2bf019de
> > Author: Alan Modra <amo...@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.
