On Tue, 3 Oct 2017, Jakub Jelinek wrote:
> The qsort cmp transitivity checks may fail with the sort_by_operand_rank
> comparator, because if one of the operands has stmt_to_insert and the
> other doesn't (but likely also if one SSA_NAME is (D) and the other is not),
> we fallthrough into SSA_NAME_VERSION comparison, but if both aren't (D) and
> both don't have stmt_to_insert, we use different comparison rules.
When looking at the code I've noticed another issue, so the fix seems
incomplete (but I don't know if the other problem can/should be fixed
independently), see below:
> --- gcc/tree-ssa-reassoc.c.jj 2017-10-02 17:33:14.270282226 +0200
> +++ gcc/tree-ssa-reassoc.c 2017-10-02 18:18:07.328611132 +0200
> @@ -514,36 +514,37 @@ sort_by_operand_rank (const void *pa, co
> && TREE_CODE (oea->op) == SSA_NAME
> && TREE_CODE (oeb->op) == SSA_NAME)
> {
The containing if statement condition reads:
if (oeb->rank == oea->rank
&& TREE_CODE (oea->op) == SSA_NAME
&& TREE_CODE (oeb->op) == SSA_NAME)
so as far as I can tell it's possible to have items A, B, C such that
all have same rank, A and C correspond to SSA_NAMEs and compare C < A,
but B does not, so comparisons of A or C against B fall down to
return oeb->id > oea->id ? 1 : -1;
and ultimately result in A < B < C < A.
Alexander
