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

Zuxy Meng zuxy.meng@gmail.com
Wed Feb 17 17:56:00 GMT 2010


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



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