specifier on operand in asm inline

[mp]

Ian Lance Taylor <iant@.com> wrote:
> Mihail Platonov <dofg@.ru> writes:
> 
> > what is the semantics of "r" specifier in subx operand (%r2)?
> > and what is the meaning of connection between this specifier and "J"
> > constraint?
> >     __asm__ __volatile__ (
> > ...
> >             "subx %r2,%3,%%g1\n\t"
> >            : "=&r" ((USItype)(r1)),
> >              "=&r" ((USItype)(r0))
> >            : "%rJ" ((USItype)(x3)),
> >              "rI" ((USItype)(y3)),
> > ...
> 
> The 'r' constraint means that that operand must be a general register.
> The 'J' constraint means that it must be the integer zero.  Using both
> rJ and together means that the operand must be either a general
> register or the integer zero.
thanx for answer anyway, but it is the description of constraints.
i'm trying to find out info about ...
subx op1, op2, op3
... about op1. why 'r' is there and so on.

> 
> The '%' means that the operands are commutative, which seems to me to
> be extremely unlikely for a sub instruction.  Unless this is used in
> some very strange way, that almost has to be a bug.
yealp, defining first operand as rJ _is_ weird.

> 
> Ian