The following testcase FAILs with -fcompare-debug, because reassociate_bb mishandles the case when the last stmt in a bb has zero uses. In that case reassoc_remove_stmt (like gsi_remove) moves the iterator to the next stmt, i.e. gsi_end_p is true, which means the code sets the iterator back to gsi_last_bb. The problem is that the for loop does gsi_prev on that before handling the next statement, which means the former penultimate stmt, now last one, is not processed by reassociate_bb. Now, with -g, if there is at least one debug stmt at the end of the bb, reassoc_remove_stmt moves the iterator to that following debug stmt and we just do gsi_prev and continue with the former penultimate non-debug stmt, now last non-debug stmt. The following patch fixes that by not doing the gsi_prev in this case; there are too many continue; cases, so I didn't want to copy over the gsi_prev to all of them, so this patch uses a bool for that instead. The second gsi_end_p check isn't needed anymore, because when we don't do the undesirable gsi_prev after gsi = gsi_last_bb, the loop !gsi_end_p (gsi) condition will catch the removal of the very last stmt from a bb. 2020-03-28 Jakub Jelinek <jakub@redhat.com> PR tree-optimization/94329 * tree-ssa-reassoc.c (reassociate_bb): When calling reassoc_remove_stmt on the last stmt in a bb, make sure gsi_prev isn't done immediately after gsi_last_bb. * gfortran.dg/pr94329.f90: New test.