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Message-ID: <541FEC77.3010905@gentoo.org>
Date: Mon, 22 Sep 2014 05:31:35 -0400
From: Joshua Kinard <kumba@gentoo.org>
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Subject: Re: gcc-4.8+ and R10000+
References: <540C165F.7030307@gentoo.org> <loom.20140908T100138-568@post.gmane.org> <540D7A7B.9090905@gentoo.org>
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Message-ID: <20140922093135.Mhgt3iymA5Q70a1V4Vq4KJY-_Gy4HpgFanBpqsljRh0@z>

On 09/08/2014 05:44, Joshua Kinard wrote:
> On 09/08/2014 04:11, Miod Vallat wrote:
>>> Disassembling a statically-built copy of the "sln" binary generated by
>>> glibc's compile phase, there are slight differences in how gcc-4.7 and
>>> gcc-4.8 are compiling the __lll_lock_wait_private function.  The key
>>> differences in the output asm are
>>> this:
>>
>> [...]
>>
>>> gcc-4.8:
>>>    x+4   <START>
>>>          ...
>>>    x+24  bne     v1,v0,<x+56>
>>>          ...
>>>    x+32  0x7c03e83b /* rdhwr */
>>>    x+36  li      a2,2
>>>    x+40  lw      a1,-29832(v1)
>>>    x+44  move    a3,zero
>>>    x+48  li      v0,4238
>>>    x+52  syscall
>>> *  x+56  ll      v0,0(s0)
>>> *  x+60  li      at,2
>>> *  x+64  sc      at,0
>>
>> Note how the sc address is no longer 0(s0). Since the address does
>> not match the address used in the ll instruction, sc will always
>> fail on the R10k.
> 
> That would be a typo on my part.  I typed that out by hand and just missed it.  It should read:
> 
> gcc-4.8:
>    x+4   <START>
>          ...
>    x+24  bne     v1,v0,<x+56>
>          ...
>    x+32  0x7c03e83b /* rdhwr */
>    x+36  li      a2,2
>    x+40  lw      a1,-29832(v1)
>    x+44  move    a3,zero
>    x+48  li      v0,4238
>    x+52  syscall
> *  x+56  ll      v0,0(s0)
> *  x+60  li      at,2
> *  x+64  sc      at,0(s0)
>    x+68  beqzl   at,<x+56>
>    x+72  nop
>    x+76  sync
>    x+80  bnez    v0,<x+32>

I did some more tracing.  It seems the issue with glibc itself stems from the
addition of __atomic_* builtins added generally in gcc-4.7 and
MIPS-specifically in gcc-4.8: