extended asm and register clobbers

extended asm and register clobbers

[ishare]

On Sat, Mar 30, 2013 at 02:04:04PM -0700, dw wrote:
> I read this line from the docs:
> 
> "If you refer to a particular hardware register from the assembler
> code, you probably have to list the register after the third colon
> to tell the compiler the register's value is modified."
> 
> My own observation shows that this is true.  However, attempting to
> add the register in question to the clobber list is returning a
> compile error.
> 
> The asm (essentially memset):
> 
>     __CRT_INLINE VOID __stosb(PBYTE Dest, BYTE Data, SIZE_T Count)
>     {
>       __asm__
>       (
>         "cld; rep; stosb"
>         :
>         : "D" (Dest), "a" (Data), "c" (Count)
>         : "edi", "memory", "cc"
>       );
>     }
> 
     It is probably because the compile finds the input will modify some regs before 
     save its value , in case its value is important for others .      
    
     try to save original edi on stack ,then use it ,then pop it ,like recovering from an irq.
   
> The error:
> 
> error: can't find a register in class 'DIREG' while reloading 'asm'
> error: 'asm' operand has impossible constraints
> 
> Without the edi clobber, this c++ code:
> 
>    __stosb((PBYTE)&c, 0, sizeof(c));
>    __stosb((PBYTE)&c, 0, sizeof(c));




> 
> generates this asm:
> 
>   402cd3:    cld
>   402cd4:    rep stos BYTE PTR es:[rdi],al
>   402cd6:    cld
>   402cd7:    rep stos BYTE PTR es:[rdi],al
> 
> Since rdi is not clobbered, gcc doesn't reload it between calls
> (likewise with rcx).
> 
> While I might be able to fake the compiler out by specifying outputs
> (probably need the volatile qualifier too), I don't really want to
> change Dest, I just want to use it as an input.
> 
> What's the right way to go here?
> 
> dw