On 02/12/2017 12:13 AM, Marc Glisse wrote:
On Tue, 7 Feb 2017, Jeff Law wrote:
* tree-vrp.c (extract_range_from_binary_expr_1): For EXACT_DIV_EXPR,
if the numerator has the range ~[0,0] make the resultant range ~[0,0].
If I understand correctly, for x /[ex] 4 with x!=0, we currently split
~[0,0] into [INT_MIN,-1] and [1,INT_MAX], then apply EXACT_DIV_EXPR
which gives [INT_MIN/4,-1] and [1,INT_MAX/4], and finally compute the
union of those, which prefers [INT_MIN/4,INT_MAX/4] over ~[0,0]. We
could change the union function, but this patch prefers changing the
code elsewhere so that the new behavior is restricted to the
EXACT_DIV_EXPR case (and supposedly the patch will be reverted if we get
a new non-contiguous version of ranges, where union would already work).
Is that it?
That was one of alternate approaches I suggested.
Part of the problem is the conversion of ~[0,0] is imprecise when it's
going to be used in EXACT_DIV_EXPR, which I outlined elsewhere in the
thread. As a result of that imprecision, the ranges we get are
[INT_MIN/4,0] U [0,INT_MAX/4].
If we fix that imprecision so that the conversion yields [INT_MIN,-4] U
[4, INT_MAX] then apply EXACT_DIV_EXPR we get [INT_MIN/4,-1] U
[1,INT_MAX/4], which union_ranges turns into [INT_MIN/4,INT_MAX/4]. We
still end up needing a hack in union_ranges that will look a hell of a
lot like the one we're contemplating for intersect_ranges.
Jeff
