Caller save mode on MIPS

[Richard Sandiford]

"Fu, Chao-Ying" <fu@mips.com> writes:
>   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!

Sounds like a good idea.

Richard