Thanks a lot for your answer John! Since then I managed to get this code
working but I still have a few questions, for you or others ;-) :
1.- Is there an official help for the clobbered register list and its use ?
I could not find any relevant section in the GCC manual
2.- Is there a place where I could find up to date examples (all example
around the web I could find use registers in both input and clobbered list,
which GCC no longer supports as far as I understand, and results in the
compilation error : can't find a register in class `CREG' while reloading
`asm')
3.- What is the difference in the clobber list between ?cc? and ?st?? When
do I have to use them ? Only when I modified the flag registers and intend
to read them thereafter or as soon as I use an instruction which might
modify them ?
4.- Is using ?cld? or ?std? instructions considered modifying the flags
register ?
5.- Is the proposed snippet I wrote below as good as it can be ?
Thanks in advance,
Charles
I have made several versions of my assembly code until I got one perfectly
working. I went through several pitfalls :
- an early clobbered registers is not considered as clobbered register? This
means that though I put an ampersand in front of an input operant, GCC would
use it as if it had not changed thereafter?
- an asm block which provides no output variable is optimized away by the
compiler with the ?O2 flag. This happens, even when the ?memory? clobber is
used? You have to use the __volatile__ keyword additionally?
Here is the working version :
{
uint32 dummy_ecx;
uint32 dummy_esi;
uint32 dummy_edi;
__asm__ __volatile__
(
"cld\n\t"
"movl %3, %%esi\n\t"
"movl %4, %%edi\n\t"
"movl %5, %%ecx\n\t"
"rep\n\t"
"movsl"
: "=&c"(dummy_ecx), "=&S"(dummy_esi), "=&D"(dummy_edi)
: "g"(from), "g"(where), "g"(times)
: "memory"
);
}
And here is all the snippets I tried :
////First version (no clobber)
//{
// __asm__ __volatile__
// (
// "cld\n\t"
// "rep\n\t"
// "movsl"
// :
// : "S"(from), "D"(where), "c"(times)
// : "memory"
// );
//}
//Version 2 (cant remember what happened on this one)
//{
// std::size_t ecx_dummy;
// std::size_t esi_dummy;
// std::size_t edi_dummy;
// __asm__ __volatile__
// (
// "cld\n\t"
// "rep\n\t"
// "movsl"
// : "=S"(esi_dummy), "=D"(edi_dummy), "=c"(ecx_dummy)
// : "S"(from), "D"(where), "c"(times)
// : "memory"
// );
//}
//Version 3 (early clobber)
//{
// __asm__
// (
// "cld\n\t"
// "movl %0, %%esi\n\t"
// "movl %1, %%edi\n\t"
// "movl %2, %%ecx\n\t"
// "rep\n\t"
// "movsl"
// :
// : "g"(from), "g"(where), "g"(times)
// : "%esi", "%edi", "%ecx", "memory"
// );
//}
//Version 4 (lacking volatile? should never had suppressed it?)
//{
// uint32 dummy_ecx;
// uint32 dummy_esi;
// uint32 dummy_edi;
// __asm__
// (
// "cld\n\t"
// "movl %3, %%esi\n\t"
// "movl %4, %%edi\n\t"
// "movl %5, %%ecx\n\t"
// "rep\n\t"
// "movsl"
// : "=&c"(dummy_ecx), "=&S"(dummy_esi), "=&D"(dummy_edi)
// : "g"(from), "g"(where), "g"(times)
// : "memory"
// );
//}
//Version 5 (does not compile)
//{
// uint32 dummy_ecx;
// uint32 dummy_esi;
// uint32 dummy_edi;
// __asm__ __volatile__
// (
// "cld\n\t"
// "rep\n\t"
// "movsl"
// : "=&c"(dummy_ecx), "=&S"(dummy_esi), "=&D"(dummy_edi)
// : "S"(from), "D"(where), "c"(times)
// : "memory"
// );
//}
