On Tue, 2 Apr 2013, Jakub Jelinek wrote: > On Tue, Apr 02, 2013 at 02:57:13PM +0200, Richard Biener wrote: > > On Tue, 2 Apr 2013, Jakub Jelinek wrote: > > > On the (undefined behavior) testcase below, we end up with > > > then_bb ending with __builtin_unreachable () at the tree level, therefore > > > no successor at the RTL level, and else_bb being EXIT_BLOCK_PTR (i.e. > > > conditional return before a bb with undefined behavior at the end). > > > Trying to optimize that into a conditional execution of the then_bb insns > > > doesn't work, we can't merge the else_bb with then_bb and test_bb in this > > > case, plus it doesn't look like something that would be desirable to do > > > (conditional return is surely better). > > > > > > Fixed thusly, ok for trunk/4.8? > > > > I wonder if > > > > /* Make sure IF, THEN, and ELSE, blocks are adjacent. Actually, we get > > the > > first condition for free, since we've already asserted that there's a > > fallthru edge from IF to THEN. Likewise for the && and || blocks, > > since > > we checked the FALLTHRU flag, those are already adjacent to the last > > IF > > block. */ > > /* ??? As an enhancement, move the ELSE block. Have to deal with > > BLOCK notes, if by no other means than backing out the merge if they > > exist. Sticky enough I don't want to think about it now. */ > > next = then_bb; > > if (else_bb && (next = next->next_bb) != else_bb) > > return FALSE; > > if ((next = next->next_bb) != join_bb && join_bb != EXIT_BLOCK_PTR) > > { > > if (else_bb) > > join_bb = NULL; > > else > > return FALSE; > > } > > > > somehow tries to guard against join_bb == EXIT_BLOCK_PTR but fails. > > Thus, why not do that explicitely here instead of just in the > > single case you cover? (I can't see why join_bb could not be > > set to EXIT_BLOCK_PTR in some weird case) > > From my reading of the code, it can handle the normal case where > join_bb is EXIT_BLOCK_PTR just fine, provided that single_succ (then_bb) > == join_bb and !else_bb || single_succ (else_bb) == join_bb. > The ICE is there only because of the extra optimization I've tweaked, > the problem is there that then_bb has no successors and join_bb is > EXIT_BLOCK_PTR, so while then_bb can be successfully merged together with > test_bb, it has no successor and as join_bb is EXIT_BLOCK_PTR, we just give > up. If then_bb has no successor and join_bb isn't EXIT_BLOCK_PTR, we'd > normally do: > else if (EDGE_COUNT (join_bb->preds) < 2 > && join_bb != EXIT_BLOCK_PTR) > { > /* We can merge the JOIN cleanly and update the dataflow try > again on this pass.*/ > merge_blocks (combo_bb, join_bb); > num_true_changes++; > } > and all is fine, and if then_bb (and else_bb if it exists) has a single > successor of join_bb, all is fine too.
Ah, I see. The patch is ok then. Thanks, Richard.