[9fans] gcc x86 assembly help

["Russ Cox" <rsc@plan9.bell-labs.com>]

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