Re: [musl] [PATCH v2] add qsort_r.

[Alexander Monakov <amonakov@ispras.ru>]

On Tue, 9 Mar 2021, Rich Felker wrote:

> I tested sh4, sh2/fdpic, rv64, s390x, or1k, m68k, and mips (32-bit)
> and they all do the tail call properly. But mips64 (n64 and n32) both
> fail to. According to the GCC source, it's some thing to allow lazy
> binding. MIPS64 does not use a real PLT, but actually has GOT entries
> that might go through a lazy resolver and that expect %gp (call-saved)
> to be valid on entry.

I think you botched something in your MIPS64 testing, probably making
a direct call instead of an indirect call. An indirect call should not
go to a lazy resolver (because then if you take the address once and
reuse it many times, you risk entering the resolver multiple times).

Here's a Compiler Explorer link demonstrating that indirect call
compiles to a simple jump on MIPS64: https://godbolt.org/z/vroTs9

> musl does not, and will never, do lazy binding, so this is purely
> counterproductive for musl and we should probably teach GCC not to do
> it. The current logic is:
> 
>   /* Sibling calls should not prevent lazy binding.  Lazy-binding stubs
>      require $gp to be valid on entry, so sibcalls can only use stubs
>      if $gp is call-clobbered.  */
>   if (decl

decl will be NULL for an indirect call

>       && TARGET_CALL_SAVED_GP
>       && !TARGET_ABICALLS_PIC0
>       && !targetm.binds_local_p (decl))
>     return false;
> 
> TARGET_CALL_SAVED_GP is rightly true (it's the ABI).
> 
> TARGET_ABICALLS_PIC0 is rightly false (I'm pretty sure that's a bogus
> alt ABI, and defined as TARGET_ABSOLUTE_ABICALLS && TARGET_PLT).
> 
> It probably needs an addition condition && TARGET_LAZY_BINDING that we
> can define as false. Alternatively the issue could just be fixed not
> to go through lazy resolver anywhere.
> 
> I opened a bug for it here:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99491
> 
> 
> Rich
>