Re: combination of read/write and earlyclobber constraint modifier

[Marc Glisse <marc dot glisse at inria dot fr>]

On Wed, 2 Jul 2014, Tom de Vries wrote:

> On 02-07-14 09:02, Marc Glisse wrote:
> > Still, the meaning of +&, in inline asm for instance, seems relatively 
> > clear, no?
>
> I can't find any testsuite examples using this construct.
>
> Furthermore, I'd expect the same semantics and restrictions for constraints 
> in rtl templates and inline asm.
>
> So I'm not sure what you mean.

Coming back to your original question:

> An earlyclobber operand is defined as 'modified before the instruction is 
> finished using the input operands'. AFAIU that would indeed exclude the 
> possibility that the earlyclobber operand is an input/output operand it 
> self, but perhaps I misunderstand.
>
> So my question is: is the combination of '&' and '+' supported ? If so, 
> what is the exact semantics ? If not, should we warn or give an error ?

An earlyclobber operand X prevents *other* input operands from using the 
same register, but that does not include X itself (if it is using +) or 
operands explicitly using a matching constraint for X. At least that's how 
I understand it.

--
Marc Glisse