Re: [PATCH] Add extensive commentary to sparc's "U" constraint.

[Steven Bosscher <stevenb dot gcc at gmail dot com>]

On Wed, Nov 7, 2012 at 11:39 PM, David Miller wrote:
> One idea that occurred to me was perhaps to extend
> define_register_constraint such that an extra condition can be
> specified.  So for sparc's constraint "U" it would evaluate to
> GENERAL_REGS but also express the condition that the hard register
> must be even.

I haven't looked at the details of this all, but there are ports that
use define_predicate to request an even-numbered register. See e.g.
frv and v850. I'm not sure if/how the RA takes predicates into account
when selecting a register.

(bfin uses define_register_constraints, but it has separate register
classes for the even-numbered registers, so apparently that's not for
multi-word hardregs like your case.)


> diff --git a/gcc/config/sparc/constraints.md b/gcc/config/sparc/constraints.md
> index 2f8c6ad..440dc57 100644
> --- a/gcc/config/sparc/constraints.md
> +++ b/gcc/config/sparc/constraints.md
> @@ -130,7 +130,43 @@
>        (match_code "mem")
>        (match_test "memory_ok_for_ldd (op)")))
>
> -;; Not needed in 64-bit mode
> +;; This awkward register constraint is necessary because it is not
> +;; possible to express the "must be even numbered regsiter" condition

s/regsiter/register/

Ciao!
Steven