'A' constraint's behavior not compliant with doc under x86-64

[Zuxy Meng]

Hi,

According to "info gcc", the 'A' constraint denotes the `a' and `d'
registers, as a pair (for instructions that return half the result in
one and half in the other). However, in reality 'A' is treated simply as
a 64-bit long integer in %rax under x86-64. For example, the following
code piece:

#include <stdint.h>

uint64_t rdtsc(void)
{
	uint64_t ret;
	asm ("rdtsc":"=A"(ret));
	return ret;
}

produces:

[zuxy@Rainbow31 bin]$ objdump -d a.o

a.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <rdtsc>:
   0:	0f 31                	rdtsc
   2:	c3                   	retq

I'm using GCC 4.4.3 but I doubt the problem has been there since Day One.
--
Zuxy