Re: [libstdc++] Use __gthread_join in jthread/95989

[Alexandre Oliva <oliva@adacore.com>]

On Mar  3, 2023, Alexandre Oliva <oliva@adacore.com> wrote:

> Hello, Florian,
> On Mar  3, 2023, Florian Weimer <fw@deneb.enyo.de> wrote:

>> * Alexandre Oliva via Gcc-patches:
>>> +    // Make sure it's not optimized out, not even with LTO.
>>> +    asm ("" : : "rm" (depend));

>> If the m constraint is used, this may never emit the symbol name and
>> thus not create a reference after all.

> But that is no longer the pthread symbol itself, it's the symbol of a
> static member function with vague linkage that, because the compiler
> believes the asm statement will reference it, will still be output, and
> it's that function body that will refer to and thus pull in the symbols
> we need.

> Now, hmm, maybe with per-function sections, the compiler will emit it,
> but with section gc, the linker may drop it, so we might lose the needed
> function body.  When performing LTO with an LTO-enabled libstdc++.

Wait, no, you had me going but it's really fine.  What would be in
memory is a *pointer* to the function, so the function would still be
referenced by whatever initialized the pointer, so it wouldn't get GCed.

-- 
Alexandre Oliva, happy hacker                https://FSFLA.org/blogs/lxo/
   Free Software Activist                       GNU Toolchain Engineer
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