Re: [wide-int] Make shifted_mask more forgiving

[Kenneth Zadeck <zadeck@naturalbridge.com>]

this is fine with me.

kenny

On 11/02/2013 07:52 AM, Richard Sandiford wrote:
> r201806 added some extra checks to the handling of CONCAT when expanding
> assignments.  This was to fix an ICE on gcc.dg/pr48335-2.c for ppc.
>
> I tripped over this because it also causes the assembly output for
> the test to change.  I tried backing out the patch and the ICE was
> from the assert in wide-int.cc:shifted_mask:
>
>    gcc_assert (start < 4 * MAX_BITSIZE_MODE_ANY_INT);
>
> However, in this case, the mask was being generated by mask_rtx,
> which specifically allows inputs that are out of range of the mode:
>
> /* Return a constant integer mask value of mode MODE with BITSIZE ones
>     followed by BITPOS zeros, or the complement of that if COMPLEMENT.
>     The mask is truncated if necessary to the width of mode MODE.  The
>     mask is zero-extended if BITSIZE+BITPOS is too small for MODE.  */
>
> So I don't think these inputs should trigger an ICE, even if in this
> particularly case they're conceptually wrong for some reason.
>
> Since wide-int.cc:shifted_mask already handles zero-width masks
> and cases where the end bit is out of range, I think we should
> handle the start being out of range too.
>
> It might be that 201806 is a good thing anyway (no idea either way),
> but I think it should go directly on mainline first if so.
>
> Tested on powerpc64-linux-gnu and x86_64-linux-gnu.  It fixes the
> pr48335-2.c assembly differences.  OK to install?
>
> Thanks,
> Richard
>
>
> Index: gcc/expr.c
> ===================================================================
> --- gcc/expr.c	2013-11-02 11:15:32.739837369 +0000
> +++ gcc/expr.c	2013-11-02 11:15:42.562911693 +0000
> @@ -4765,14 +4765,12 @@ expand_assignment (tree to, tree from, b
>   		   && (bitpos == 0 || bitpos == mode_bitsize / 2))
>   	    result = store_expr (from, XEXP (to_rtx, bitpos != 0), false,
>   				 nontemporal);
> -	  else if (bitpos + bitsize <= mode_bitsize / 2
> -		   && bitpos+bitsize <= mode_bitsize)
> +	  else if (bitpos + bitsize <= mode_bitsize / 2)
>   	    result = store_field (XEXP (to_rtx, 0), bitsize, bitpos,
>   				  bitregion_start, bitregion_end,
>   				  mode1, from,
>   				  get_alias_set (to), nontemporal);
> -	  else if (bitpos >= mode_bitsize / 2
> -		   && bitpos+bitsize <= mode_bitsize)
> +	  else if (bitpos >= mode_bitsize / 2)
>   	    result = store_field (XEXP (to_rtx, 1), bitsize,
>   				  bitpos - mode_bitsize / 2,
>   				  bitregion_start, bitregion_end,
> @@ -4791,12 +4789,8 @@ expand_assignment (tree to, tree from, b
>   	    }
>   	  else
>   	    {
> -	      HOST_WIDE_INT extra = 0;
> -              if (bitpos+bitsize > mode_bitsize)
> -                extra = bitpos+bitsize - mode_bitsize;
>   	      rtx temp = assign_stack_temp (GET_MODE (to_rtx),
> -					    GET_MODE_SIZE (GET_MODE (to_rtx))
> -					    + extra);
> +					    GET_MODE_SIZE (GET_MODE (to_rtx)));
>   	      write_complex_part (temp, XEXP (to_rtx, 0), false);
>   	      write_complex_part (temp, XEXP (to_rtx, 1), true);
>   	      result = store_field (temp, bitsize, bitpos,
> Index: gcc/wide-int.cc
> ===================================================================
> --- gcc/wide-int.cc	2013-11-02 11:15:32.740837376 +0000
> +++ gcc/wide-int.cc	2013-11-02 11:15:42.562911693 +0000
> @@ -748,18 +748,16 @@ unsigned int
>   wi::shifted_mask (HOST_WIDE_INT *val, unsigned int start, unsigned int width,
>   		  bool negate, unsigned int prec)
>   {
> -  gcc_assert (start < 4 * MAX_BITSIZE_MODE_ANY_INT);
> -
> -  if (start + width > prec)
> -    width = prec - start;
> -  unsigned int end = start + width;
> -
> -  if (width == 0)
> +  if (start >= prec || width == 0)
>       {
>         val[0] = negate ? -1 : 0;
>         return 1;
>       }
>   
> +  if (width > prec - start)
> +    width = prec - start;
> +  unsigned int end = start + width;
> +
>     unsigned int i = 0;
>     while (i < start / HOST_BITS_PER_WIDE_INT)
>       val[i++] = negate ? -1 : 0;