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Message-Id: <c400e569-3499-44bc-bf57-7cf096d26192@app.fastmail.com>
Date: Fri, 12 Dec 2025 03:38:41 +0100
From: Alex Rønne Petersen <alex@...xrp.com>
To: "David Edelsohn" <dje.gcc@...il.com>, musl@...ts.openwall.com
Subject: Re: [PATCH] powerpc64: add nop after b to __dlsym
On Fri, Dec 12, 2025, at 02:35, David Edelsohn wrote:
> 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.
In this particular case (__dlsym), it looks to me like LLD is clearly wrong to issue this error. I haven't investigated why it does so, but since this only happens when linking a very large executable (the Zig compiler statically linked with LLVM), I vaguely suspect that it's a bad interaction between its relocation and range extension thunk logic.
I want to say that LLD is probably also wrong in the case of __start_c, but I'm less sure on that one.
Unfortunately, a .hidden directive is not sufficient to silence either error.
>
> David
>
>> .size dlsym, .-dlsym
>> --
>> 2.51.0
>>
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