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

[Zuxy Meng <zuxy.meng@xxxxxxxxx>]

于 2010年02月17日 22:28, Tim Prince 写道:
> 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

Thanks Tim. So I guess the documentation needs fixing?

--
Zuxy