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Re: [RS6000] PR 80938, Don't emit eh_frame for regs that don't need saving


Hi!

On Fri, Aug 11, 2017 at 12:40:11PM +0930, Alan Modra wrote:
> It is possible when using out-of-line register saves or store multiple
> to save some registers unnecessarily, for example one reg in the block
> saved might be unused.  We don't need to emit eh_frame info for those
> registers as that just bloats the eh_frame info, and also can result
> in an ICE when shrink-wrap gives multiple paths through the function
> saving different sets of registers.  All the join points need to have
> identical eh_frame register save state.

It isn't just eh_frame, we might have never noticed if it was.  It is
also the DWARF call frame info used for debugging.

> 	PR target/80938
> 	* config/rs6000/rs6000.c (rs6000_savres_strategy): Revert 2017-08-09.
> 	Don't use store multiple if only one reg needs saving.
> 	(rs6000_frame_related): Don't emit eh_frame for regs that
> 	don't need saving.
> 	(rs6000_emit_epilogue): Likewise.

> +      int count;
> +
> +      for (count = 0, i = info->first_gp_reg_save; i < 32; i++)
> +	if (save_reg_p (i))
> +	  count += 1;

      int count = 0;
      for (i = info->first_gp_reg_save; i < 32; i++)
	if (save_reg_p (i))
	  count++;

> @@ -25681,9 +25683,15 @@ rs6000_frame_related (rtx_insn *insn, rtx reg, HOST_WIDE_INT val,
>  		 register save functions, or store multiple, then omit
>  		 eh_frame info for any user-defined global regs.  If
>  		 eh_frame info is supplied, frame unwinding will
> -		 restore a user reg.  */
> +		 restore a user reg.  Also omit eh_frame info for any
> +		 reg we don't need to save, as that bloats eh_frame
> +		 and can cause problems with shrink wrapping.  Saves
> +		 of r0 are actually saving LR, so don't omit those.   */
>  	      if (!REG_P (SET_SRC (set))
> -		  || !fixed_reg_p (REGNO (SET_SRC (set))))
> +		  || REGNO (SET_SRC (set)) == 0
> +		  || REGNO (SET_SRC (set)) == CR2_REGNO
> +		  || (!fixed_reg_p (REGNO (SET_SRC (set)))
> +		      && save_reg_p (REGNO (SET_SRC (set)))))
>  		RTX_FRAME_RELATED_P (set) = 1;
>  	    }
>        RTX_FRAME_RELATED_P (insn) = 1;

That "0" and "CR2_REGNO" is non-obvious, it needs a big fat comment (it's
not clear that just CR2_REGNO is correct, either).  Oh, and please factor
out "REGNO (SET_SRC (set))".

The rest looks good, but please repost.


Segher


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