RE: [Patch MIPS] Enable TARGET_IRA_CHANGE_PSEUDO_ALLOCNO_CLASS hook

[Robert Suchanek <Robert dot Suchanek at imgtec dot com>]

Hi Matthew,

> /* LRA will allocate an FPR for an integer mode pseudo instead of spilling
>    to memory if an FPR is present in the allocno class.  It is rare that
>    we actually need to place an integer mode value in an FPR so where
>    possible limit the allocation to GR_REGS.  This will slightly pessimize
>    code that involves integer to/from float conversions as these will have
>    to reload into FPRs in LRA. Such reloads are sometimes eliminated and
>    sometimes only partially eliminated.  We choose to take this penalty
>    in order to eliminate usage of FPRs in code that does not use floating
>    point data.
> 
>    This change has a similar effect to increasing the cost of FPR->GPR
>    register moves for integer modes so that they are higher than the cost
>    of memory but changing the allocno class is more reliable.
> 
>    This is also similar to forbidding integer mode values in FPRs entirely
>    but this would lead to an inconsistency in the integer to/from float
>    instructions that say integer mode values must be placed in FPRs.  */
> 
> I'm keen to get the description of this right so please feel free to change
> it further if it isn't clear (or correct).

This description is definitely more accurate.  At first glance, I wasn't sure
whether to include the bit about partial elimination since it caused by post
reload passes but the introduction of reloads should be avoided in the first place.

> I don't know if this change will lead to classic reload being unusable for
> MIPS. I'm not worried about that but I think it is probably wise to
> remove classic reload support for MIPS now; we are dependent on LRA for
> several features already.

Indeed.  I think it's the right time to drop the classic reload and resolve
LRA issues if they come up. 

> Do you have any details on when we are left with suboptimal code for
> int->float conversions? I'd like to keep a record of them in this thread
> or in the comment so we know what is left to fix.

I opened a bug report https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66204
and just realised that I had never CCed Vlad.  Hopefully, this is the last
remaining issue related to integers and floating-point registers.

> 
> > +  if (INTEGRAL_MODE_P (PSEUDO_REGNO_MODE (regno)) && allocno_class ==
> > ALL_REGS)
> > +    return GR_REGS;
> > +  return allocno_class;
> > +}
> > +
> 
> Trim the extra trailing newline.
> 
> OK to commit if you are happy with the comment.

I'll update the comment, do a quick regression and commit it.

Regards,
Robert