Inline Assembly Error: suffix or operands invalid for 'shr'

Inline Assembly Error: suffix or operands invalid for 'shr'

[Rodrigo Dominguez]

Hi,

While debugging a problem with Open64, I ran into a similar problem with
GCC. I created the following unit test program:

#include <stdint.h>

int main(void)
{
    uint32_t    a = 7;
    int8_t      s = -1;

    __asm__ ("shrl %1, %0\n\t"
            : "+r" (a)
            : "c" (-s)
            );

    return a;
}

When assembling this program, 'cc1' emits a 'shrl %ecx, %eax' instruction.
The 'shr' instruction can only take an 8-bit register as the first operand.
The emitted instruction should have been 'shrl %cl, %eax'. Therefore, the
compilation fails with a 'suffix or operands invalid for shr' message.

I have 2 questions:

1. AFAIK, by default, __asm__ chooses a register according to the size of
the operand (int8_t in this case). Is this correct? Where can I find
documentation of this?

2. If #1 is true, why does this fail?

Thank you,

Rodrigo

Re: Inline Assembly Error: suffix or operands invalid for 'shr'

[H.J. Lu]

On Tue, Mar 24, 2009 at 11:02 AM, Rodrigo Dominguez <roddomi@hotmail.com> wrote:
> Hi,
>
> While debugging a problem with Open64, I ran into a similar problem with
> GCC. I created the following unit test program:
>
> #include <stdint.h>
>
> int main(void)
> {
>    uint32_t    a = 7;
>    int8_t      s = -1;
>
>    __asm__ ("shrl %1, %0\n\t"
>            : "+r" (a)
>            : "c" (-s)
>            );
>
>    return a;
> }
>
> When assembling this program, 'cc1' emits a 'shrl %ecx, %eax' instruction.
> The 'shr' instruction can only take an 8-bit register as the first operand.
> The emitted instruction should have been 'shrl %cl, %eax'. Therefore, the
> compilation fails with a 'suffix or operands invalid for shr' message.
>

Please use

   __asm__ ("shrl %b1, %0\n\t"
           : "+r" (a)
           : "c" (-s)
           );

-- 
H.J.

RE: Inline Assembly Error: suffix or operands invalid for 'shr'

[Rodrigo Dominguez]

H.J,

Thanks for replying but this doesn't answer my question. Shouldn't __asm__
generate the right code without using the %b1 trick? The reason I am asking
is I have a 350 line macro which I can't change.

Is there any documentation about __asm__ default behavior regarding this
issue? Something like an Intel/AMD/AT&T manual explaining this?

Thank you,

Rodrigo

> -----Original Message-----
> From: H.J. Lu [mailto:hjl.tools@gmail.com]
> Sent: Tuesday, March 24, 2009 2:09 PM
> To: Rodrigo Dominguez
> Cc: gcc@gcc.gnu.org
> Subject: Re: Inline Assembly Error: suffix or operands invalid for
> 'shr'
> 
> On Tue, Mar 24, 2009 at 11:02 AM, Rodrigo Dominguez
> <roddomi@hotmail.com> wrote:
> > Hi,
> >
> > While debugging a problem with Open64, I ran into a similar problem
> with
> > GCC. I created the following unit test program:
> >
> > #include <stdint.h>
> >
> > int main(void)
> > {
> >    uint32_t    a = 7;
> >    int8_t      s = -1;
> >
> >    __asm__ ("shrl %1, %0\n\t"
> >            : "+r" (a)
> >            : "c" (-s)
> >            );
> >
> >    return a;
> > }
> >
> > When assembling this program, 'cc1' emits a 'shrl %ecx, %eax'
> instruction.
> > The 'shr' instruction can only take an 8-bit register as the first
> operand.
> > The emitted instruction should have been 'shrl %cl, %eax'. Therefore,
> the
> > compilation fails with a 'suffix or operands invalid for shr'
> message.
> >
> 
> Please use
> 
>    __asm__ ("shrl %b1, %0\n\t"
>            : "+r" (a)
>            : "c" (-s)
>            );
> 
> --
> H.J.

RE: Inline Assembly Error: suffix or operands invalid for 'shr'

[Paul Koning]

>>>>> "Rodrigo" == Rodrigo Dominguez <roddomi@hotmail.com> writes:

 Rodrigo> H.J, Thanks for replying but this doesn't answer my
 Rodrigo> question. Shouldn't __asm__ generate the right code without
 Rodrigo> using the %b1 trick? The reason I am asking is I have a 350
 Rodrigo> line macro which I can't change.

GCC doesn't look at the instructions.  You have to tell it what form
of argument the instruction expects, and the %b1 does that.  So the
macro has a bug and you'll have to fix it...

      paul

Re: Inline Assembly Error: suffix or operands invalid for 'shr'

[H.J. Lu]

On Tue, Mar 24, 2009 at 1:18 PM, Rodrigo Dominguez <roddomi@hotmail.com> wrote:
> H.J,
>
> Thanks for replying but this doesn't answer my question. Shouldn't __asm__
> generate the right code without using the %b1 trick? The reason I am asking
> is I have a 350 line macro which I can't change.

Those macros are wrong.

> Is there any documentation about __asm__ default behavior regarding this
> issue? Something like an Intel/AMD/AT&T manual explaining this?

asm statement is compiler specific. It is documented in gcc manual. You
can take a look at *.md in i386 to see how asm statement may be used:

[hjl@gnu-13 i386]$ grep shr i386.md | grep %b
   shr{q}\t{%b2, %0|%0, %b2}"
   shr{l}\t{%b2, %0|%0, %b2}"
   shr{l}\t{%b2, %k0|%k0, %b2}"
   shr{w}\t{%b2, %0|%0, %b2}"
   shr{b}\t{%b2, %0|%0, %b2}"
   shr{b}\t{%b1, %0|%0, %b1}"
[hjl@gnu-13 i386]$

H.J.
---
> Thank you,
>
> Rodrigo
>
>> -----Original Message-----
>> From: H.J. Lu [mailto:hjl.tools@gmail.com]
>> Sent: Tuesday, March 24, 2009 2:09 PM
>> To: Rodrigo Dominguez
>> Cc: gcc@gcc.gnu.org
>> Subject: Re: Inline Assembly Error: suffix or operands invalid for
>> 'shr'
>>
>> On Tue, Mar 24, 2009 at 11:02 AM, Rodrigo Dominguez
>> <roddomi@hotmail.com> wrote:
>> > Hi,
>> >
>> > While debugging a problem with Open64, I ran into a similar problem
>> with
>> > GCC. I created the following unit test program:
>> >
>> > #include <stdint.h>
>> >
>> > int main(void)
>> > {
>> >    uint32_t    a = 7;
>> >    int8_t      s = -1;
>> >
>> >    __asm__ ("shrl %1, %0\n\t"
>> >            : "+r" (a)
>> >            : "c" (-s)
>> >            );
>> >
>> >    return a;
>> > }
>> >
>> > When assembling this program, 'cc1' emits a 'shrl %ecx, %eax'
>> instruction.
>> > The 'shr' instruction can only take an 8-bit register as the first
>> operand.
>> > The emitted instruction should have been 'shrl %cl, %eax'. Therefore,
>> the
>> > compilation fails with a 'suffix or operands invalid for shr'
>> message.
>> >
>>
>> Please use
>>
>>    __asm__ ("shrl %b1, %0\n\t"
>>            : "+r" (a)
>>            : "c" (-s)
>>            );
>>
>> --
>> H.J.
>
>



-- 
H.J.

Re: Inline Assembly Error: suffix or operands invalid for 'shr'

[Chris Lattner]

On Mar 24, 2009, at 11:02 AM, Rodrigo Dominguez wrote:
> When assembling this program, 'cc1' emits a 'shrl %ecx, %eax'  
> instruction.
> The 'shr' instruction can only take an 8-bit register as the first  
> operand.
> The emitted instruction should have been 'shrl %cl, %eax'.  
> Therefore, the
> compilation fails with a 'suffix or operands invalid for shr' message.
>
> I have 2 questions:
>
> 1. AFAIK, by default, __asm__ chooses a register according to the  
> size of
> the operand (int8_t in this case). Is this correct? Where can I find
> documentation of this?

Though several people responded with useful information, no one has  
answered the question.  The issue here is that "-s" has int type, not  
int8_t because of C promotion rules.  Try using (int8_t)(-s).


-Chris