Re: [9fans] gcc x86 assembly help

[Martin Harriss <martin@Princeton.EDU>]

Russ Cox wrote:
> 
> Ishwar has been trying to compile drawterm on a
> Linux 2.4 kernel.  The drawterm code (like the Inferno
> code before it) depends on the fact that there are
> different TSS registers in each process in Linux,
> but this changed in 2.4 so the Linux-specific code
> broke.
> 
> He's now trying to use the pthread-specific code
> (which works well under FreeBSD and Irix), but gets
> a weird message from gcc about "impossible register
> constraint in asm" on the x86 test-and-set code.
> 
> typedef struct Lock Lock;
> struct Lock {
>         int val;
> };
> 
> int
> canlock(Lock *l)
> {
>         int     v;
> 
>         __asm__(        "movl   $1, %%eax\n\t"
>                         "xchgl  %%eax,(%%ebx)"
>                         : "=a" (v)
>                         : "ebx" (&l->val)
>         );
>         switch(v) {
>         case 0:  return 1;
>         case 1:  return 0;
>         default:        print("canlock: corrupted 0x%lux\n", v);
>         }
>         return 0;
> }
> 
> Does anyone with more gcc inline assembly experience
> know what's wrong with this (or what changed in gcc
> in the latest Linux distributions to make it start
> rejecting this)?
> 
> Thanks.
> Russ

I think the problem may be the "ebx" constraint.  I've never seen it
written like that -- I thought that for the [e]bx register the
constraint was simply "b".  The current manual seems to confirm that.
Can you try "b" and see if you have any luck?

Martin

Reference: http://gcc.gnu.org/onlinedocs/gcc-2.95.3/gcc_16.html#SEC175