On Thu, Mar 23, 2023 at 7:37 PM Andrew MacLeod via Gcc-patches <gcc-patches@gcc.gnu.org> wrote: > > Detailed info in the PR. > > As we walk the DOM tree to calculate ranges, any block with multiple > predecessors is processed by evaluating and unioning incoming values. > This catches more complex cases where the dominator node itself may not > carry range adjustments that we care about. > > What was missing was the "quick check" doesn't propagate any info. If > the edge we check is dominated by this block (ie, its a back edge), then > no additional useful information can be provided as it just leads back > to where we currently are. Only edges which are not dominated by the > current block need be checked. > > The issue arose because this "quick check" mechanism gives up on complex > cases and returns VARYING... so any backedge would union the real value > from the dominators with the failed result "VARYING" from that edge, and > we get VARYING instead of the correct result. > > The patch simply checks if the current block dominates the predecessor > of an edge before launching the query. > > Performance impact in negligible. slight slowdown for the check, slight > speedup by doing less work.. its a wash. > > Bootstraps on x86_64-pc-linux-gnu with no regressions. > > Ok for trunk?
LGTM with the testcase Jakub provided. Richard. > Andrew > > PS I have not managed to produce a reduced testcase yet.. If I do I will > supply it. > > >
