Re: [PATCH RFC] PR 47862: Fix caller-save spill of vectors on PowerPC

[Pat Haugen <pthaugen at linux dot vnet dot ibm dot com>]

On 03/01/2011 09:06 PM, Alan Modra wrote:
> On Tue, Mar 01, 2011 at 05:22:15PM -0600, Pat Haugen wrote:
> > >  Are there alternative solutions to this such that we'll use scalar
> > >  FP store/load insns for SF/DF mode spill and vector store/load insns
> > >  for vector spill?
> I made a similar fix a while ago in rs6000/e500.h (which your patch
> will clash with, BTW, needs an undef somewhere).  Going from memory, I
> believe you can use something like
>
> #define HARD_REGNO_CALLER_SAVE_MODE(REGNO, NREGS, MODE)	\
>    (TARGET_VSX						\
>     &&  ((MODE) == VOIDmode || (MODE) == V2DFmode)	\
>     &&  FP_REGNO_P (REGNO)				\
>     ? V2DFmode						\
>     : choose_hard_reg_mode ((REGNO), (NREGS), false))
>
> This will allocate enough space for the vector regs, but only
> save/restore using FP insns if the reg is used in a FP mode.

I don't see how this can result in shortening of the save/restore length. 
caller-save.c:save_call_clobbered_regs() initally sets save_mode[] to the prior 
computed widest mode (V2DFmode in this case), and the following code will only 
attempt to increase it.

                  mode = HARD_REGNO_CALLER_SAVE_MODE
                    (r, nregs, PSEUDO_REGNO_MODE (regno));
                  if (GET_MODE_BITSIZE (mode)
                      > GET_MODE_BITSIZE (save_mode[r]))
                    save_mode[r] = mode;

Modifying the above code to always update save_mode[r] to the mode of the pseudo 
would surely do the trick.