[ASM] [PowerPC] floating-point registers mnemonics

[Segher Boessenkool]

> On the code below, I'm storing the result of the product in a temporary
> variable (fra).
>
> inline void MTH_Vec3Scale(register MTH_tt_Vector3 *D,register const
> MTH_tt_Vector3 *A, register f32 f)
> {
>     register       f32  fra;
>     asm     (       "lfs   %3,   0(%0)              \n"        // FRA <-
> *A
>                     "\t    fmul   %3,   %3,     %2  \n"       // FRT <-
> FRA
> * FRC
>                     "\t    stfs   %3,   0(%1)       \n"         // *dest =
> FRT
>
>                     "\t    lfs    %3,   4(%0)       \n"
>                     "\t    fmul   %3,   %3,     %2  \n"
>                     "\t    stfs   %3,   4(%1)       \n"
>
>                     "\t    lfs    %3,   8(%0)       \n"
>                     "\t    fmul   %3,   %3,     %2  \n"
>                     "\t    stfs   %3,   8(%1)       \n"
>         /* outputs: */ :
>         /* inputs : */ :"b"(A),"b"(D),"f"(f),"f"(fra)
>         /* clobbers:*/ :"memory");
> }

You write to "fra" (and never read it), so you should list it in
outputs instead of in inputs:

         /* outputs: */ :"=f"(fra)
         /* inputs : */ :"b"(A),"b"(D),"f"(f)
         /* clobbers:*/ :"memory");

(and change all the %0..%3, or use named asm parameters).

> What I would like to do is to use a floating-point register directly, f11
> for instance, to store the results:
> lfs   %3,   0(%0)
> would be come
> lfs    f11,  0(%0)
> for instance.
> I would add "f11" to the list of clobber registers.  But it won't let me,
> complaining that "f11" is unknown register. "fr11" doesn't work either. I
> have given it many tries ...

I don't know, sorry.  One thing you could do is write the whole routine
in asm (in a .s or .S file); it's much easier for bigger code (but you
have to take care of the ABI yourself, usually not a problem since you're
only dealing with leaf fucntions without stack frame).


Segher