On Mon, Oct 23, 2017 at 1:29 PM, Richard Sandiford
<richard.sandif...@linaro.org> wrote:
> Most GCC ranges seem to be represented as an offset and a size (rather
> than a start and inclusive end or start and exclusive end). The usual
> test for whether X is in a range is of course:
>
> x >= start && x < start + size
> or:
> x >= start && x - start < size
>
> which means that an empty range of size 0 contains nothing. But other
> range tests aren't as obvious.
>
> The usual test for whether one range is contained within another
> range is:
>
> start1 >= start2 && start1 + size1 <= start2 + size2
>
> while the test for whether two ranges overlap (from ranges_overlap_p) is:
>
> (start1 >= start2 && start1 < start2 + size2)
> || (start2 >= start1 && start2 < start1 + size1)
>
> i.e. the ranges overlap if one range contains the start of the other
> range. This leads to strange results like:
>
> (start X, size 0) is a subrange of (start X, size 0) but
> (start X, size 0) does not overlap (start X, size 0)
>
> Similarly:
>
> (start 4, size 0) is a subrange of (start 2, size 2) but
> (start 4, size 0) does not overlap (start 2, size 2)
>
> It seems like "X is a subrange of Y" should imply "X overlaps Y".
>
> This becomes harder to ignore with the runtime sizes and offsets
> added for SVE. The most obvious fix seemed to be to say that
> an empty range does not overlap anything, and is therefore not
> a subrange of anything.
>
> Using the new definition of subranges didn't seem to cause any
> codegen differences in the testsuite. But there was one change
> with the new definition of overlapping ranges. strncpy-chk.c has:
>
> memset (dst, 0, sizeof (dst));
> if (strncpy (dst, src, 0) != dst || strcmp (dst, ""))
> abort();
>
> The strncpy is detected as a zero-size write, and so with the new
> definition of overlapping ranges, we treat the strncpy as having
> no effect on the strcmp (which is true). The reaching definition
> is the memset instead.
>
> This patch makes ranges_overlap_p return false for zero-sized
> ranges, even if the other range has an unknown size.
Ok.
Thanks,
Richard.
>
> 2017-10-23 Richard Sandiford <richard.sandif...@linaro.org>
>
> gcc/
> * tree-ssa-alias.h (ranges_overlap_p): Return false if either
> range is known to be empty.
>
> Index: gcc/tree-ssa-alias.h
> ===================================================================
> --- gcc/tree-ssa-alias.h 2017-03-28 16:19:22.000000000 +0100
> +++ gcc/tree-ssa-alias.h 2017-10-23 11:47:38.181155696 +0100
> @@ -171,6 +171,8 @@ ranges_overlap_p (HOST_WIDE_INT pos1,
> HOST_WIDE_INT pos2,
> unsigned HOST_WIDE_INT size2)
> {
> + if (size1 == 0 || size2 == 0)
> + return false;
> if (pos1 >= pos2
> && (size2 == (unsigned HOST_WIDE_INT)-1
> || pos1 < (pos2 + (HOST_WIDE_INT) size2)))
