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

[Rich Felker <dalias@libc.org>]

On Tue, Mar 16, 2021 at 02:49:59AM +0300, Alexander Monakov wrote:
> 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).

No, it's even stupider. Somehow when I deleted the $TARGET-gcc to
replace with mips64-*-gcc I also deleted the -O2 right after it, so I
was looking at -O0...

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

Yes, confirmed that now. However direct call indeed does not tail-call.

> > 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

*nod* ...

> 
> >       && 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

...but this bug is still valid, as it's undesirable for this to happen
with direct calls too.

Rich