On 2/17/2010 6:21 AM, Zuxy Meng wrote:
> 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
>
>
The problem of incompatibility of gcc inline asm for rdtsc between 32-
and 64-bit mode "has been there since Day One". For 64-bit mode, you are
stuck with concatenating the 2 registers explicitly.
Tim Prince
