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Message-ID: <20251212150625.GV3520958@port70.net>
Date: Fri, 12 Dec 2025 16:06:25 +0100
From: Szabolcs Nagy <nsz@...t70.net>
To: David Edelsohn <dje.gcc@...il.com>
Cc: musl@...ts.openwall.com,
Alex Rønne Petersen <alex@...xrp.com>
Subject: Re: [PATCH] powerpc64: add nop after b to __dlsym
* David Edelsohn <dje.gcc@...il.com> [2025-12-11 20:35:48 -0500]:
> On Thu, Dec 11, 2025 at 8:10 PM Alex Rønne Petersen <alex@...xrp.com> wrote:
>
> > To avoid errors like this:
> >
> > ld.lld: error:
> > /home/ci/.cache/act/2c84ccdd36a924eb/hostexecutor/zig-global-cache/o/9e32a733e976f6f0194dc11bcf100f7a/libc.a(/home/ci/.cache/act/2c84ccdd36a924eb/hostexecutor/zig-global-cache/o/19fa681ef1630f481aadf66ec88d2634/dlsym.o):(function
> > dlsym: .text+0xc): call to save___dlsym lacks nop, can't restore toc
> > ---
> > src/ldso/powerpc64/dlsym.s | 1 +
> > 1 file changed, 1 insertion(+)
> >
> > diff --git a/src/ldso/powerpc64/dlsym.s b/src/ldso/powerpc64/dlsym.s
> > index a14715fd..73e70db9 100644
> > --- a/src/ldso/powerpc64/dlsym.s
> > +++ b/src/ldso/powerpc64/dlsym.s
> > @@ -8,4 +8,5 @@ dlsym:
> > .localentry dlsym,.-dlsym
> > mflr 5 # The return address is arg3.
> > b __dlsym
> > + nop
> >
>
> The instruction is
>
> b __dlsym
>
> which is a branch -- a tail call. It cannot return. It will never need a
> TOC restore instruction after the branch.
>
> As with the other patch, are you fixing actual bugs or bloating the code to
> remove false-positive linker error messages?
>
> If the target of the call (bl) in the previous patch is known to be local,
> it will never need a TOC restore.
alan modra implies here that the linker
logic is based on relocs (which is
presumably the same for b and bl)
https://sourceware.org/bugzilla/show_bug.cgi?id=26164
i think the linker could check the instruction
at the target location of a reloc in the inputs
and decide how to generate the output based on
that, but maybe that was deemed too complex for
the linker abi (iirc tocs and other stubs
can already cause headaches as they change
section layout late and thus need iterative
adjustments that may not converge).
>
> David
>
>
> > .size dlsym, .-dlsym
> > --
> > 2.51.0
> >
> >
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