On Mon, Jun 22, 2015 at 10:04 AM, Tom de Vries <tom_devr...@mentor.com> wrote: > Hi, > > during development of a patch I ran into a case where > compute_dominance_frontiers was called with incorrect dominance info. > > The result was a segmentation violation somewhere in the bitmap code while > executing this bitmap_set_bit in compute_dominance_frontiers_1: > ... > if (!bitmap_set_bit (&frontiers[runner->index], > b->index)) > break; > ... > > The segmentation violation happens because runner->index is 0, and > frontiers[0] is uninitialized. > > [ The initialization in update_ssa looks like this: > ... > dfs = XNEWVEC (bitmap_head, last_basic_block_for_fn (cfun)); > FOR_EACH_BB_FN (bb, cfun) > bitmap_initialize (&dfs[bb->index], &bitmap_default_obstack); > compute_dominance_frontiers (dfs); > ... > > FOR_EACH_BB_FN skips over the entry-block and the exit-block, so dfs[0] > (frontiers[0] in compute_dominance_frontiers_1) is not initialized. > > We could add initialization by making the entry/exit-block bitmap_heads > empty and setting the obstack to a reserved obstack bitmap_no_obstack for > which allocation results in an assert. ] > > AFAIU, the immediate problem is not that frontiers[0] is uninitialized, but > that the loop reaches the state of runner->index == 0, due to the incorrect > dominance info. > > The patch adds an assert to the loop in compute_dominance_frontiers_1, to > make the failure mode cleaner and easier to understand. > > I think we wouldn't catch all errors in dominance info with this assert. So > the patch also contains an ENABLE_CHECKING-enabled verify_dominators call at > the start of compute_dominance_frontiers. I'm not sure if: > - adding the verify_dominators call is too costly in runtime. > - the verify_dominators call should be inside or outside the > TV_DOM_FRONTIERS measurement. > - there is a level of ENABLE_CHECKING that is more appropriate for the > verify_dominators call. > > Is this ok for trunk if bootstrap and reg-test on x86_64 succeeds?
I don't think these kind of asserts are good. A segfault is good by itself (so you can just add the comment if you like). Likewise the verify_dominators call is too expensive and misplaced. If then the call belongs in the dom_computed[] == DOM_OK early-out in calculate_dominance_info (eventually also for the case where we end up only computing the fast-query stuff). Richard. > Thanks, > - Tom
