https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114151
--- Comment #14 from Richard Biener <rguenth at gcc dot gnu.org> --- (In reply to Andrew Macleod from comment #13) > Created attachment 57638 [details] > patch > > Ok, there were 2 issues with simply invoking range_of_stmt, which this new > patch resolves. IF we aren't looking to fix this in GCC 14 right now > anyway, this is the way to go. > > 1) The cache has always tried to provide a global range by pre-folding a > stmt for an estimate using global values. This is a bad idea for PHIs when > SCEV is invoked AND SCEV is calling ranger. This changes it to not > pre-evaluate PHIs, which also saves time when functions have a lot of edges. > Its mostly pointless for PHIs anyway since we're about to do a real > evaluation. > > 2) The cache's entry range propagator was not re-entrant. We didn't > previously need this, but with SCEV (and possible other place) invoking > range_of_expr without context and having range_of_stmt being called, we can > occasionally get layered calls for cache filling (of different ssa-names) > > With those 2 changes, we can now safely invoke range_of_stmt from a > contextless range_of_expr call. > > We would have tripped over this earlier if SCEV or one of those other places > using range_of_expr without context had instead invoked range_of_stmt. That > would have been perfectly reasonable, and would have resulting in these same > issues. We never tripped over it because range_of_stmt is not used much > outside of ranger. That is the primary reason I wanted to track this down. > There were alternative paths to the same end result that would have > triggered these issues. It sounds like this part is a bugfix? > Give this patch a try. it also bootstraps with no regressions. I will queue > it up for stage 1 instead assuming all is good. It seems to work well, it now computes a lot of additional ranges and causes a minor code generation change on the testcase (it doesn't fix the observed regression though). Thanks for working on this. As of things unexplored is whether we can with better range-info lift the constraint on the folding some more. We're turning (A + i * B) * C into (A * C + i * (B * C)) and need to avoid any additional intermediate undefined overflow with this association for i in [0, n] (with n being the number of iterations of the loop where i varies). As said, if the regression is too important to ignore we could choose to leave the bug unfixed for all but the case with A, B and C constant which was the case for the testcase in the original PR.
