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:
[z...@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
--
Summary: 'A' constraint's behavior not compliant with doc under
x86-64
Product: gcc
Version: 4.4.3
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: inline-asm
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: zuxy dot meng at gmail dot com
GCC build triplet: x86_64-redhat-linux
GCC host triplet: x86_64-redhat-linux
GCC target triplet: x86_64-redhat-linux
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43112
