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Date: Tue, 16 Mar 2021 02:49:59 +0300 (MSK)
From: Alexander Monakov <amonakov@...ras.ru>
To: musl@...ts.openwall.com
cc: Markus Wichmann <nullplan@....net>
Subject: Re: [PATCH v2] add qsort_r.
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
>
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