On Sun, Jan 07, 2024 at 04:10:21PM +0100, David Brown wrote:
> On 05/01/2024 19:46, Segher Boessenkool wrote:
> >>However, when I use just "asm ("" : "+X" (x));", I get an error message
> >>"error: inconsistent operand constraints in an 'asm'". I have no idea
> >>why this is an issue.
> >
> >The C constraint means "Any operand whatsoever is allowed." Here you
> >are saying to use it both as input and as output, and GCC does not know
> >how to reload wherever it chose to put it.
>
> It doesn't need to reload it
The error message is emitted during reloading.
> >"+" really creates two operands, and ties them together. Writing
> > asm("oink" : "+X"(bla));
> >is shorthand for
> > asm("oink" : "=X"(bla) : "0"(bla));
> >
>
> I don't know if these are /exactly/ the same, but they are certainly
> similar.
They are the same thing. The "+" syntactic sugar is modified during
gimplification already, almost nothing in the compiler has to deal with
it.
See "gimplify_asm_expr", see the code after
/* An input/output operand. To give the optimizers more
flexibility, split it into separate input and output
operands. */
(Constraints that allow registers get a "0" (or "1" etc.) matching
constraint for the input duplicate, constraints that do not (like "m")
are unmodified. The output duplicate just gets the "+" changed to "=").
Segher
