On 06/21/16 00:06, Jeff Law wrote:
> On 06/09/2016 10:45 AM, Jakub Jelinek wrote:
>> On Thu, Jun 09, 2016 at 06:43:04PM +0200, Jakub Jelinek wrote:
>>> Yes, I'm all in favor in disabling X constraint for inline asm.
>>> Especially if people actually try to print it as well, rather than
>>> make it
>>> unused. That is a sure path to ICEs.
>>
>> Though, on the other side, even our documentation mentions
>> asm volatile ("mtfsf 255,%1" : "=X"(sum): "f"(fpenv));
>> So perhaps we need to error just in case such an argument is printed?
> Are you thinking to scan the output string for %<N> for the appropriate
> <N>? That shouldn't be too hard. But that's not sufficient to address
> the problem Bernd is trying to tackle AFAICT.
Correct.
And furthermore, the use case with matching X input & output that Marc
wanted to use, seems to be a valid one. Because "+X" allows more
registers than "+g" or "+r", it should create less register pressure.
And although it is probably unpredictable if a register of the
class "ALL_REGISTERS" will work for an assembler instruction, it is
still interesting to print it in an assembler comment.
Bernd.
