https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105086
--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> --- (In reply to Andrew Macleod from comment #2) > Ranger VRP doesn't simulate edges the same way VRP does. It looks like VRP > simulates the back edge twice and the second time notes that the MAX value > is greater than it was before and "projects" the max to +INF to avoid > further simulations and thus executing every instance of the loop. VRP uses optimistic SSA propagation with iteration, to avoid compile-time explosion we do not simulate IV ranges growing [0,0] -> [0, 1] ->...-> [0, n] but instead do the cited projection and rely on SCEV to improve the resulting range. > This allows it to refine the range in the loop better, which ranger VRP > isn't doing as it is still doing a DOM walk and doesn't revisit the node. > ANd I haven't added any sort of similar "projection" logic to the back edge > processing. I think ranger simply doesn't simulate SCCs (but I think that's OK) > I have an alternate question. it looks like when we utilize scev to pick up > ranges we just give up if scev_probably_wraps_p() is true. > > Analyzing # of iterations of loop 1 > exit condition 1 < [4294967273, + , 1] > bounds on difference of bases: 4294967272 ... 4294967272 > result: > # of iterations 23, bounded by 23 > > Statement (exit)if (a_1 > 1) > is executed at most 23 (bounded by 23) + 1 times in loop 1. > > but we neglect to create range for the PHI. We should be able to properly > create a range for this from the SCEV info rather than giving up? It would > be [0,0][4294967273, 4294967295]. Well, we give up if the IV wraps because then the logic we have to compute the IV range doesn't work. I'm talking about bounds_of_var_in_loop which basically computes the range as [base, base + step * niter] with adjustments to create proper ranges for negative step. > And even with the old value_range we could use anti-range and produce > ~[1, 4294967272]? It should use the range as computed by the "iteration", just not use SCEV to refine it. > Is there a practical reason we don't look any closer at wrap cases to see if > they are "simple wraps" or not? I think that would also solve this issue. The only reason is that nobody implemented it. The important thing is to compute that it will wrap exactly once of course.
