if i do'nt specify output operands in strcpy, will it
make any differnce,
following code works fine on my x86 machine:
inline void nstrcpy(char *src, char *dst){
__asm__ __volatile__ ( "1: lodsb \n\t"
"stosb\n\n"
"testb %%al, %%al \n\t"
"jne 1b"
:
: "S"(src) , "D" (dst)
);
}
please let me know if above code has some problem
Thanx
PC
--- Momchil Velikov <velco@xxxxxxxxx> wrote:
> Vijay Subramanian wrote:
> > On 18/01/06, Rajaram Suryanarayanan
> <rajaram_linux@xxxxxxxxx> wrote:
> >> Can anybody pls explain me step by step what is
> done by the assembly code
> >>below ? Particularly, I want to know how "src" and
> "dest" get substituted in
> >>the assembly code.
> >>
> >> static inline char * strcpy(char * dest,const
> char *src)
> >> {
> >> int d0, d1, d2;
> >> __asm__ __volatile__(
> >> "1:\tlodsb\n\t"
> >> "stosb\n\t"
> >> "testb %%al,%%al\n\t"
> >> "jne 1b"
> >> : "=&S" (d0), "=&D" (d1), "=&a" (d2)
> >> :"0" (src),"1" (dest) : "memory");
> >> return dest;
> >> }
> > I am not sure why we need d0, d1 and d2. Can
> anyone give a hint? Thanks.
>
> The insn takes an input esi and edi and produces
> output in (i.e. modifies) esi,
> edi, eax and memory.
>
> Thus we have to specify that inputs must be copied
> to esi and edi
>
> : <outputs>
> : "S" (src), "D" (dst)
> : <clobbers>
>
> then specify that the insn modifies ESI and EDI
>
> : "=S" (<var>), "=D (<var>)"
> : "S" (src), "D" (dst)
> : <clobbers>
>
> but this is invalid, because it tells GCC to
> allocate two registers from the
> each of S and D classes, but both contain a single
> register, moreover the insn
> in fact modifies its inputs, hence we must tell that
> corresponding inputs and
> outputs are in fact in the same register [1]:
>
> : "=S" (<var>), "=D" (<var>)
> : "0" (src), "1" (dst)
> : <clobbers>
>
> Now we have to tell where the outputs to go. As we
> are not really interested in
> the values of the ESI and EDI, we can specify
> arbitrary temporary variables.
> Note that we can't use the variables ``src'' and
> ``dest'' we already have [2],
> because the compiler will copy ESI and EDI to them
> and we do not want it for
> ```dest''. Thus:
>
> : "=S" (tmp0), "=D" (tmp1)
> : "0" (src), "1" (dst)
> : <clobbers>
>
> and finally
>
> : "=S" (tmp0), "=D" (tmp1), "=a" (tmp2)
> : "0" (src), "1" (dst)
> : "memory"
>
> Note that in this particular case there's no need
> for the ``&'' constraint. It
> prevents the same register from being allocated both
> for input and an output
> operand, but in this case we explictly assign
> concrete registers, so this cannot
> happen.
>
> ~velco
>
> [1] Note that this could theoretically expressed
> also as:
>
> : "=2" (<var>), "=3" (<var>)
> : "S" (src), "S" (dst)
> : <clobbers>
>
> but this is INVALID as gcc allows number constraints
> only on input operands.
>
> [2] well, we can use ``src'', because it is dead
> after the insn
>
> --
> Kernelnewbies: Help each other learn about the Linux
> kernel.
> Archive:
> http://mail.nl.linux.org/kernelnewbies/
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>
>
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