Re: [PING**2] [PATCH] Fix asm X constraint (PR inline-asm/59155)

[Jeff Law <law@redhat.com>]

On 07/21/2016 10:29 AM, Bernd Edlinger wrote:
>>> But I think we have a use case where "X" means really more possible
>>> registers (i.e. includes ss2, mmx etc.) than "g" (only general
>>> registers).  Otherwise, in the test cases of pr59155 we would not
>>> have any benefit for using "+X" instead of "+g" or "+r".
>>>
>>> Does that sound reasonable?
>> If it's the case that the real benefit of +X is that it's allowing more
>> registers, then that argues that the backend ought to be providing
>> another (larger) register class.
>>
>
> X allows more different registers than r, and it is already documented.
> In the cases where it is already used, the patch should not break
> anything.  I would not understand, why we should forbid the use of X and
> waste another letter of the alphabet for a slightly modified version
> of X.
Doing so essentially allows us to deprecate "X" to used by target 
patterns only -- where what's acceptable is limited by the operand 
predicates.  Those limits ultimately protect the rest of the routines 
from having to handle arbitrary RTL.

Meanwhile asms can use the new letter to say "I'll take any register of 
any class".  Which is, AFAICT, what's desired here.

jeff