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

[David Miller <davem at davemloft dot net>]

From: Steven Bosscher <stevenb.gcc@gmail.com>
Date: Thu, 8 Nov 2012 01:19:11 +0100

> 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.

That would only influence instruction recognition.

> (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.)

Right.

>> 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/

Thanks, I'll fix that.