$ cat 1.c 
#include <inttypes.h>

static uint64_t y = 0x123456789ABCDEF0ULL;

int
main()
{
	register uint64_t x;
	asm volatile ( "movq %1, %0" : "=Yz"(x) : "m"(y) );

	return 0;
}

$ cc -O0 -ggdb -Wall -W -mssse3 -m64 1.c -o 1

$ objdump -d 1
...
0000000000400530 <main>:
  400530:	55                   	push   %rbp
  400531:	48 89 e5             	mov    %rsp,%rbp
  400534:	f3 0f 7e 05 a4 00 10 	movq   1048740(%rip),%xmm0        # 5005e0 <y>
  40053b:	00 
  40053c:	b8 00 00 00 00       	mov    $0x0,%eax
  400541:	c9                   	leaveq 
  400542:	c3                   	retq   
...

$ clang -O0 -ggdb -Wall -W -mssse3 -m64 1.c -o 1
1.c:9:33: error: invalid output constraint '=Yz' in asm
        asm volatile ( "movq %1, %0" : "=Yz"(x) : "m"(y) );
                                       ^
1 error generated.

$ cc --version
cc (GCC) 4.2.2 20070831 prerelease [FreeBSD]
Copyright (C) 2007 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ clang --version
clang version 3.1 (trunk)
Target: amd64-portbld-freebsd8.2
Thread model: posix