Re: [RFA] [PATCH 4/4] Ignore reads of "dead" memory locations in DSE

[Richard Sandiford <richard dot sandiford at linaro dot org>]

Jeff Law <law@redhat.com> writes:
> @@ -468,6 +468,36 @@ maybe_trim_partially_dead_store (ao_ref *ref, sbitmap live, gimple *stmt)
>      }
>  }
>  
> +/* Return TRUE if USE_REF reads bytes from LIVE where live is
> +   derived from REF, a write reference.
> +
> +   While this routine may modify USE_REF, it's passed by value, not
> +   location.  So callers do not see those modifications.  */
> +
> +static bool
> +live_bytes_read (ao_ref use_ref, ao_ref *ref, sbitmap live)
> +{
> +  /* We have already verified that USE_REF and REF hit the same object.
> +     Now verify that there's actually an overlap between USE_REF and REF.  */
> +  if (ranges_overlap_p (use_ref.offset, use_ref.size, ref->offset, ref->size))
> +    {
> +      normalize_ref (&use_ref, ref);
> +
> +      /* If USE_REF covers all of REF, then it will hit one or more
> +	 live bytes.   This avoids useless iteration over the bitmap
> +	 below.  */
> +      if (use_ref.offset <= ref->offset
> +	  && use_ref.offset + use_ref.size >= ref->offset + ref->size)
> +	return true;
> +
> +      /* Now check if any of the remaining bits in use_ref are set in LIVE.  */
> +      unsigned int start = (use_ref.offset - ref->offset) / BITS_PER_UNIT;
> +      unsigned int end  = (use_ref.offset + use_ref.size) / BITS_PER_UNIT;
> +      return bitmap_bit_in_range_p (live, start, end);
> +    }
> +  return true;
> +}

When rebasing the SVE changes on top of this, I wasn't sure why the
function returned true rather than false when there's no overlap.
Is that deliberate?  It might be worth a comment if so.

Thanks,
Richard