On Thu, 17 Dec 2015, Yury Gribov wrote: > That's an interesting one. The original comparison function assumes that > operand_equal_p(a,b) is true iff compare_tree(a, b) == 0. > Unfortunately that's not true (functions are written by different authors). > > This causes subtle violation of transitiveness. > > I believe removing operand_equal_p should preserve the intended semantics > (same approach taken in another comparison function in this file - > comp_dr_with_seg_len_pair). > > Cc-ing Cong Hou and Richard who are the authours.
I don't think the patch is good. compare_tree really doesn't expect equal elements (and it returning zero is bad or a bug). But it's also "lazy" in that it will return 0 when it hopes a further disambiguation inside dr_group_sort_cmp on a different field will eventually lead to a non-zero compare_tree. So eventually if compare_tree returns zero we have to fall back to the final disambiguator using gimple_uid. That said, I'd like to see the testcase where you observe an intransitive comparison. Richard.
