specifier on operand in asm inline

specifier on operand in asm inline

[Mihail Platonov]

hi.

the function described ( http://www.nabble.com/file/p13135052/t.c t.c ) is
from linux sources, sparc dependent headers.

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)),
...

thnx for answers.
with best regards,
mihail
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Re: specifier on operand in asm inline

[Ian Lance Taylor]

Mihail Platonov <dofg@ya.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.

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.

Ian

Re: 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

Re: specifier on operand in asm inline

[Ian Lance Taylor]

mp <dofg@ya.ru> writes:

> 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.

That is the question I tried to answer.  So I don't know what you are
asking.  What type of answer are you looking for?

Ian

Re: specifier on operand in asm inline

[David Daney]

Ian Lance Taylor wrote:
> mp <dofg@ya.ru> writes:
> 
>> 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.
> 
> That is the question I tried to answer.  So I don't know what you are
> asking.  What type of answer are you looking for?

Not being mp, I could only guess that the question might be:  What is 
the purpose of constraints?

Some documentation on constraints can be found here:

http://gcc.gnu.org/onlinedocs/gccint/Constraints.html#Constraints


David Daney

Re: specifier on operand in asm inline

[dofg]

Ian Lance Taylor <iant@google.com> wrote:
> mp <dofg@ya.ru> writes:
> 
> > 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.
> 
> That is the question I tried to answer.  So I don't know what you are
> asking.  What type of answer are you looking for?
you gave the description of the constraints
: "=&r" ((USItype)(r1)),
  "=&r" ((USItype)(r0))
: "%rJ" ((USItype)(x3)),
  "rI" ((USItype)(y3)),
i am pointing to
"subx %r2,%3,%%g1\n\t"
more definitely, to %r2. operand of subx. 'r' here is also a
constraint, telling compiler to use register?

> 
> Ian

mp

Re: specifier on operand in asm inline

[Ian Lance Taylor]

dofg <dofg@ya.ru> writes:

> you gave the description of the constraints
> : "=&r" ((USItype)(r1)),
>   "=&r" ((USItype)(r0))
> : "%rJ" ((USItype)(x3)),
>   "rI" ((USItype)(y3)),
> i am pointing to
> "subx %r2,%3,%%g1\n\t"
> more definitely, to %r2. operand of subx. 'r' here is also a
> constraint, telling compiler to use register?

Oh, sorry.  That's not a constraint.  That's an operand modifier.

For SPARC %r2 means that if the argument is the integer zero, it
should print %g0.  Otherwise it should print the operand as usual.

Ian

Re: specifier on operand in asm inline

[mp]

Ian Lance Taylor <iant@google.com> wrote:

> dofg <dofg@ya.ru> writes:
> 
> > you gave the description of the constraints
> > : "=&r" ((USItype)(r1)),
> >   "=&r" ((USItype)(r0))
> > : "%rJ" ((USItype)(x3)),
> >   "rI" ((USItype)(y3)),
> > i am pointing to
> > "subx %r2,%3,%%g1\n\t"
> > more definitely, to %r2. operand of subx. 'r' here is also a
> > constraint, telling compiler to use register?
> 
> Oh, sorry.  That's not a constraint.  That's an operand modifier.
> 
> For SPARC %r2 means that if the argument is the integer zero, it
> should print %g0.  Otherwise it should print the operand as usual.
huge thanks.

i'm curious of other operand modifiers. can you advice any docs on them?

> 
> Ian

mp

Re: specifier on operand in asm inline

[Ian Lance Taylor]

mp <dofg@ya.ru> writes:

> > For SPARC %r2 means that if the argument is the integer zero, it
> > should print %g0.  Otherwise it should print the operand as usual.
> huge thanks.
> 
> i'm curious of other operand modifiers. can you advice any docs on them?

See the gcc internals manual on "Output Templates and Operand
Substitution."

Ian