[LLVMdev] Question to use inline assemble in X86

[Heyu Zhu]

After look at documents of inline assemble,  i have known its reason.

Thanks!

2009/12/30, Chris Lattner <clattner at apple.com>:
>
>
> On Dec 29, 2009, at 3:09 AM, Heyu Zhu wrote:
>
> > Hi everyone,
> >
> > I try to add an instruction to x86. The instruction is a multiply-add
> instruction
> > MULADD A, B, C; //A = A + B * C.
> > I use the instruction by inline assemble as below
> >
> > int x, y, z;
> > .....  ....
> > x = 0;
> > asm("MULADD %0, %1, %2":"=r"(x):"0"(x), "r"(y), "r"(z));
> > ..... ....
> >
> > The backend does allocate registers %edx, %edi, %esi for x,y, z
> respectively,
> > but its assemble output is
> >         MULADD %edx, %edx, %esi
> > I expects it could output:
> >         MULADD %edx, %edi, %esi
> >
> > What's  matter with my constraints of inline asm?
> >
>
> The constraint tells the compiler what registers are valid to use.  "r"
> means that it can use any register.  You need to tell the compiler what
> registers need to be used, because it doesn't look inside the asm
> string.  Please take a look at the GCC documentation for more details:
> http://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#Extended-Asm
> http://gcc.gnu.org/onlinedocs/gcc/Constraints.html#Constraints
>
> -Chris
>
>
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