On Mon, Jun 06, 2016 at 09:27:56PM +0200, Marc Glisse wrote:
> The last one would miss floating point registers (no 2 platforms use the
> same letter for those, hence my quest for something more generic).
>
> The goal of the experiment is described in PR59159 (for which "+X" is
> unlikely to be the right answer, in particular because it is meaningless for
> constants). I don't know in what context people use the "X" constraint, or
> even better "=X"...
X constraint has been added mainly for uses in match_scratch like:
(clobber (match_scratch:SI 2 "=X,X,X,&r"))
or when the predicate takes care of everything and it is not needed to
specify anything further:
[(set (match_operand:SWI12 0 "push_operand" "=X")
(match_operand:SWI12 1 "nonmemory_no_elim_operand" "rn"))]
Using it in inline asm generally has resulted in lots of issues, including
ICEs etc., so nothing I'd recommend to use.
Jakub
