Re: [committed] Add support for MIPS16e save/restore instructions

[Nigel Stephens <nigel at mips dot com>]

Nigel Stephens wrote:
> Richard Sandiford wrote:
> > It remains to be seen whether:
> >
> >   /* In mips16 mode, we permit the $t temporary registers to be used
> >      for reload.  We prohibit the unused $s registers, since they
> >      are caller saved, and saving them via a mips16 register would
> >      probably waste more time than just reloading the value.  */
> >   if (TARGET_MIPS16)
> >     {
> >       fixed_regs[18] = call_used_regs[18] = 1;
> >       fixed_regs[19] = call_used_regs[19] = 1;
> >       fixed_regs[20] = call_used_regs[20] = 1;
> >       fixed_regs[21] = call_used_regs[21] = 1;
> >       fixed_regs[22] = call_used_regs[22] = 1;
> >       fixed_regs[23] = call_used_regs[23] = 1;
> >       fixed_regs[26] = call_used_regs[26] = 1;
> >       fixed_regs[27] = call_used_regs[27] = 1;
> >       fixed_regs[30] = call_used_regs[30] = 1;
> >     }
> >
> > still makes sense when GENERATE_MIPS16E_SAVE_RESTORE, since saving and
> > restoring these registers is now so cheap.
> >   
>
> Yes, we thought so too, but when we tried that in gcc3.4 we got less 
> than impressive results for code size. IIRC this was due to excessive 
> reloads -- because "spilling" to registers wasn't supported in the 
> same way as spilling to stack, instead variables would get allocated 
> to the s2-8 registers and then need reloading to MIPS16 registers 
> before each use. Could be worth trying again in gcc 4 though, in case 
> any improvements to the register allocator help us here.


In fact we got better code by disabling use of the $t registers also, viz:

 if (TARGET_MIPS16)
   {
     fixed_regs[8] = call_used_regs[8] = 1;
     fixed_regs[9] = call_used_regs[9] = 1;
     fixed_regs[10] = call_used_regs[10] = 1;
     fixed_regs[11] = call_used_regs[11] = 1;
     fixed_regs[12] = call_used_regs[12] = 1;
     fixed_regs[13] = call_used_regs[13] = 1;
     fixed_regs[14] = call_used_regs[14] = 1;
     fixed_regs[15] = call_used_regs[15] = 1;
     fixed_regs[18] = call_used_regs[18] = 1;
     fixed_regs[19] = call_used_regs[19] = 1;
     fixed_regs[20] = call_used_regs[20] = 1;
     fixed_regs[21] = call_used_regs[21] = 1;
     fixed_regs[22] = call_used_regs[22] = 1;
     fixed_regs[23] = call_used_regs[23] = 1;
     fixed_regs[30] = call_used_regs[30] = 1;
   }

Nigel