On 12/13/2017 03:06 AM, Richard Biener wrote: > On December 12, 2017 9:50:38 PM GMT+01:00, David Malcolm > <dmalc...@redhat.com> wrote: >> PR tree-optimization/83312 reports a false positive from >> -Warray-bounds. >> The root cause is that VRP both: >> >> (a) updates a GIMPLE_COND to be always false, and >> >> (b) updates an ARRAY_REF in the now-unreachable other path to use an >> ASSERT_EXPR with a negative index: >> def_stmt j_6 = ASSERT_EXPR <j_9, j_9 < 0>; >> >> When vrp_prop::check_all_array_refs () runs, the CFG hasn't yet been >> updated to take account of (a), and so a false positive is emitted >> when (b) is encountered. >> >> This patch fixes the false warning by converting >> vrp_prop::check_all_array_refs >>from a simple iteration over all BBs to use a new dom_walker subclass, >> using the "skip_unreachable_blocks = true" mechanism to avoid analyzing >> (b). >> >> There didn't seem to be a pre-existing way to determine the unique >> out-edge after a GIMPLE_COND (if it has a constant cond), so I added >> a new gimple_cond_get_unique_successor_edge function. Similarly, >> something similar may apply for switches, so I put in a >> gimple_get_unique_successor_edge (though I wasn't able to create a >> reproducer that used a switch). >> >> Successfully bootstrapped®rtested on x86_64-pc-linux-gnu. >> >> OK for trunk? > > I don't like the GIMPLE.c bits a lot. Can't you use the existing known taken > edge helper (too lazy to look up from my phone...) basically splitting out > some code from cond processing in evrp for example? I'm not sure those bits are needed at all. I think the right things will happen if we clear EDGE_EXECUTABLE on the appropriate edge in vrp_folder::fold_predicate_in.
That should cause the block in question to become unreachable (zero preds). So later when we do the domwal dom_walker::walk will see the block as unreachable -- which avoids walking into it and also triggers the call to propagate_unreachable_to_edges which marks the outgoing edges as not executable. I think David's got to go back to some diagnostics/FE stuff, so I'll probably be wrapping this up. Thanks David! jeff
