Re: [PATCH] MIPS: Fix PR target/98491 (ChangeLog)

[Jeff Law <law@redhat.com>]

On 1/4/21 2:00 PM, Jakub Jelinek wrote:
> On Mon, Jan 04, 2021 at 01:51:59PM -0700, Jeff Law via Gcc-patches wrote:
>>> Sorry, I forgot to include the ChangeLog:
>>>
>>>     gcc/ChangeLog:
>>>     
>>>     2020-12-31  Xi Ruoyao <xry111@mengyan1223.wang>
>>>     
>>>             PR target/98491
>>>             * config/mips/mips.c (mips_symbol_insns): Do not use
>>>               MSA_SUPPORTED_MODE_P if mode is MAX_MACHINE_MODE.
>> So I absolutely agree the current code is wrong as it does an out of
>> bounds array access.
>>
>>
>> Would it be better to instead to change MSA_SUPPORTED_MODE_P to evaluate
>> to zero if MODE is MAX_MACHINE_MODE?  That would protect all the uses of
>> MSA_SUPPORTED_MODE_P.    Something like this perhaps?
> But MAX_MACHINE_MODE is the one past last valid mode, I'm not aware of
> any target that would protect all macros that deal with modes that way.
>
> So, perhaps best would be stop using the MAX_MACHINE_MODE as magic value
> for that function and instead use say VOIDmode that shouldn't normally
> appear either?
I think we have to allow VOIDmode because constants don't necessarily
have modes.   And I certainly agree that using MAX_MACHINE_MODE like
this is ugly and error prone (as we can see from the BZ).

I also couldn't convince myself that the code and comments were actually
consistent, particularly for MSA targets which the comment claims can
never handle constants for ld/st (and thus should be returning 0 for
MAX_MACHINE_MODE).  Though maybe mips_symbol_insns_1 ultimately handles
that correctly.


>
> But I don't really see anything wrong on the mips_symbol_insns above
> change either.
Me neither.  I'm just questioning if bullet-proofing in the
MSA_SUPPORTED_MODE_P would be a better option.  While I've worked in the
MIPS port in the past, I don't really have any significannt experience
with the MSA support.

jeff