Caller save mode on MIPS

[Fu, Chao-Ying]

Hi All,

  From testing, I found out that the whole width of a MIPS 
integer/floating-point register
is saved and restored around a call.  This may hurt the performance.

Ex:
fu@debian6:/disk/fu/dev/test$ cat add2.c
void test2(float);

float test(float a, float b)
{
  test2(a*b);
  return a;
}

fu@debian6:/disk/fu/dev/test$ gcc -S -O2 add2.c -mips64r2 -mabi=n32

(# Or )

fu@debian6:/disk/fu/dev/test$ gcc -S -O2 add2.c -mips32r2 -mfp64

fu@debian6:/disk/fu/dev/test$ grep f0 add2.s
        mov.s   $f0,$f12 <----
        sdc1    $f0,24($sp) <----
        ldc1    $f0,24($sp) <----

The 64-bit $f0 is saved and restored via sdc1 and ldc1.  However, using lwc1 
and swc1 should be ok and faster.

  From http://gcc.gnu.org/ml/gcc-patches/2001-02/msg01480.html,
the patch defines HARD_REGNO_CALLER_SAVE_MODE to return proper mode for i386.
For MIPS, we may have:
Ex:
#define HARD_REGNO_CALLER_SAVE_MODE(REGNO, NREGS, MODE)                 \
   ((MODE) == VOIDmode ? choose_hard_reg_mode ((REGNO), (NREGS), false) \
   : (MODE))

  Any feedback about adding HARD_REGNO_CALLER_SAVE_MODE to MIPS?  Thanks!

Regards,
Chao-ying