How to use "y" constraint in GCC inline powerpc assembly ?

How to use "y" constraint in GCC inline powerpc assembly ?

[Christophe Leroy]

Hi Segher,

I'd like to use cr instead of gpr to return error condition from 
__get_user().

I saw in GCC doc 
(https://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html) that it is 
possible to use "y" as constraint to refer to "Any condition register 
field, cr0…cr7".

I tried the test below, but it fails with "error: impossible register 
constraint in 'asm'"

How does "y" has to be used ?

int test(char *p)
{
	struct {
		int r:1;
	} res = {0};

	asm("crnot %0 * 4 + eq, %0 * 4 + eq": "=&y"(res.r));

	if (res.r)
		return -14;
	return 0;
}

Christophe

Re: How to use "y" constraint in GCC inline powerpc assembly ?

[Segher Boessenkool]

Hi!

On Sat, Apr 18, 2020 at 08:28:53AM +0000, Christophe Leroy wrote:
> I'd like to use cr instead of gpr to return error condition from 
> __get_user().
> 
> I saw in GCC doc 
> (https://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html) that it is 
> possible to use "y" as constraint to refer to "Any condition register 
> field, cr0…cr7".
> 
> I tried the test below, but it fails with "error: impossible register 
> constraint in 'asm'"
> 
> How does "y" has to be used ?

The same as "x".  You cannot really use these constraints in asm, it's
internal only.  I'll remove it from the inline asm documentation.  Thanks!

(You should put the cr fields you use in an inline asm in its clobber
list, i.e. "cr0" or "cr7").


Segher