[Bug inline-asm/87733] local register variable not honored with earlyclobber

[bergner at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87733

--- Comment #25 from Peter Bergner <bergner at gcc dot gnu.org> ---
(In reply to Segher Boessenkool from comment #23)
> Before RA we have asm inputs
>                     [
>                         (reg/v:SI 196 [ n ])
>                         (reg/v:SI 4 $4 [ r4 ])
>                         (reg/v:SI 5 $5 [ r5 ])
>                         (reg/v:SI 6 $6 [ r6 ])
>                         (reg/v:SI 7 $7 [ r7 ])
>                         (reg/v:SI 8 $8 [ r8 ])
>                         (reg/v:SI 9 $9 [ r9 ])
>                     ]
> (and $2 an earlyclobber).  But then IRA decides
> 
> Disposition:
>     0:r195 l0     2    1:r196 l0     2

Vlad can verify, but I don't think IRA takes early clobber constraints into
consideration when computing conflicts, so I'm not surprised pseudo 196 (ie, n)
is assigned hard register $2.  In that case, LRA will notice the illegal
constraint usage and will insert reloads. 


> Peter, do you know which patch fixed this (is that the one you quoted
> above?),
> and if it could feasibly be backported separately?

Yes, I think the patch I mentioned above should stop LRA from spilling the hard
register assigned by the user in %0 and should instead spill pseudo 196 by
reassigning it to another hard register.  Whether it's feasible to backport,
maybe?  :-)   The lra-constraints.c hunk applies to gcc-8 (with fuzz), so maybe
that's enough to fix this?  Can someone try applying that hunk and does it fix
this?