[Bug inline-asm/15312] New: "+m" constraint gives strange warnings

[gcc-bugzilla at gcc dot gnu dot org]

When compiling the attached code with gcc-3.4, the following warnings
are now displayed:

xxx.c: In function `__down_read_trylock':
xxx.c:9: warning: read-write constraint does not allow a register
xxx.c:9: warning: read-write constraint does not allow a register

This does not happen with gcc-3.3; and I believe no warnings were generated for
earlier gcc-3.x's.

The attached code is an excerpt from include/asm-i386/rwsem.h in the Linux
kernel.

Environment:
System: Linux warthog.cambridge.redhat.com 2.4.22-1.2149.nptl #1 Wed Jan 7 12:57:33 EST 2004 i686 athlon i386 GNU/Linux
Architecture: i686

	
host: i686-pc-linux-gnu
build: i686-pc-linux-gnu
target: i686-pc-linux-gnu
configured with: ./configure --prefix=/opt/gcc-34 : (reconfigured) ./configure --prefix=/opt/gcc-34

How-To-Repeat:
	

Compile the following bit of code with an x86 compiler capable of
handling the CMPXCHG instruction (i486+). No special flags are required.

struct rw_semaphore {
	signed long		count;
};

int __down_read_trylock(struct rw_semaphore *sem)
{
	signed long result, tmp;

	__asm__ __volatile__(
		"# beginning __down_read_trylock\n\t"
		"  movl      %0,%1\n\t"
		"1:\n\t"
		"  movl	     %1,%2\n\t"
		"  addl      %3,%2\n\t"
		"  jle	     2f\n\t"
"lock;"		"  cmpxchgl  %2,%0\n\t"
		"  jnz	     1b\n\t"
		"2:\n\t"
		"# ending __down_read_trylock\n\t"
		: "+m"(sem->count), "=&a"(result), "=&r"(tmp)
		: "i"(0x00000001)
		: "memory", "cc"
		);

	return result >= 0 ? 1 : 0;
}

-- 
           Summary: "+m" constraint gives strange warnings
           Product: gcc
           Version: 3.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: inline-asm
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: dhowells at redhat dot com
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15312