[Bug inline-asm/43112] New: 'A' constraint's behavior not compliant with doc under x86-64

[Bug inline-asm/43112] New: 'A' constraint's behavior not compliant with doc under x86-64

[zuxy dot meng at gmail dot com]

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


-- 
           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

[Bug inline-asm/43112] 'A' constraint's behavior not compliant with doc under x86-64

[jakub at gcc dot gnu dot org]

------- Comment #1 from jakub at gcc dot gnu dot org  2010-02-18 08:56 -------
You just misunderstand it.  You'd need a 128-bit int __attribute__((mode (TI)))
value if you want it to mean a pair, because for -m64 registers are 64-bit.
If you use smaller precision type, "A" constraint acts just as a constraint
allowing both ax and dx registers and it is up to the compiler which one it
chooses.
rdtsc returns result in %eax and %edx, even for 64-bit code, so you need to use
unsigned int low, high;
"=a" (low), "=d" (high)
and then combine that into a 64-bit value.


-- 

jakub at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43112