[PATCH 2/3] mips: make sp 16-byte aligned on N64/N32

[Adhemerval Zanella]

On 04/12/2020 07:57, Maciej W. Rozycki wrote:
> On Mon, 30 Nov 2020, Adhemerval Zanella via Libc-alpha wrote:
> 
>>> MIPS N64/N32 ABI request stack pointer be 16-byte alinged
>>>
>>
>> It seems to align what gcc assumes:
>>
>> gcc/config/mips/mips.h
>>
>>  364 #define TARGET_NEWABI               (mips_abi == ABI_N32 || mips_abi == ABI_64)
>>
>> 2525 /* Treat LOC as a byte offset from the stack pointer and round it up
>> 2526    to the next fully-aligned offset.  */
>> 2527 #define MIPS_STACK_ALIGN(LOC) \
>> 2528   (TARGET_NEWABI ? ROUND_UP ((LOC), 16) : ROUND_UP ((LOC), 8))
> 
>  This is correct (the alignment being twice the register width), although 
> in this particular case obviously hardly matters in practice, as the 
> syscall will switch to the kernel stack right away.  It's fine to get it 
> fixed regardless of course, once a correct change has been proposed.

Indeed.

> 
>> (It would be easier that https://www.linux-mips.org/ could show 
>> this information easier, I could only find the old o32 ABI documentation).
> 
>  We largely continue relying on old SGI ABI documentation, although there 
> used to be a wiki hosted by MIPS Technologies (MTI) where such details 
> were summarised.  Sadly with the demise of both companies this stuff is 
> either gone or hard to track down, except for individual people's storage 
> devices.
> 
>  See:
> 
> <https://web.archive.org/web/20180829093347/https://dmz-portal.mips.com/wiki/Main_Page>
> 
> for the old MTI wiki; unfortunately not all stuff has been archived.
> 
>  I can see if I can upload some stuff to linux-mips.org, either the wiki 
> itself or my own FTP area I can link the wiki to, but mind that this site 
> is under threat of disappearance as well and as much as I'd like to I may 
> not be able to stop it.

It would be good if we have a wiki entry referring to all the architecture
and ABI documentation. I will check if I can start one, maybe using the
distro scatter information.

> 
>>> @@ -48,8 +48,8 @@ NESTED (syscall, SZREG, ra)
>>>  
>>>  	REG_L s0, (sp)
>>>  	cfi_restore (s0)
>>> -	PTR_ADDIU sp, SZREG
>>> -	cfi_adjust_cfa_offset (-SZREG)
>>> +	PTR_ADDIU sp, 2 * SZREG
>>> +	cfi_adjust_cfa_offset (-2 * 2 * SZREG)
> 
>  The updated CFA adjustment does not look right to me though.
> 
>   Maciej
>