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Date: Mon, 15 Mar 2021 22:18:08 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: [PATCH v2] add qsort_r.
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
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