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

[Xi Ruoyao]

Hi Jeff and Jakub,

On 2021-01-04 14:19 -0700, Jeff Law wrote:
> 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.

I can't understand the comment either.  To me it looks like it's possible to
remove this "if (MSA_SUPPORTED_P (mode)) return 0;"

CC Robert to get some help.
-- 
Xi Ruoyao <xry111@mengyan1223.wang>
School of Aerospace Science and Technology, Xidian University