[PATCH] Check dominator info in compute_dominance_frontiers

[Tom de Vries]

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?

Thanks,
- Tom
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