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
Check dominator info in compute_dominance_frontiers
2015-06-22 Tom de Vries <t...@codesourcery.com>
* cfganal.c (compute_dominance_frontiers_1): Add assert.
(compute_dominance_frontiers): Verify dominators if ENABLE_CHECKING.
---
gcc/cfganal.c | 9 +++++++++
1 file changed, 9 insertions(+)
diff --git a/gcc/cfganal.c b/gcc/cfganal.c
index b8d67bc..0e0e2bb 100644
--- a/gcc/cfganal.c
+++ b/gcc/cfganal.c
@@ -1261,6 +1261,11 @@ compute_dominance_frontiers_1 (bitmap_head *frontiers)
domsb = get_immediate_dominator (CDI_DOMINATORS, b);
while (runner != domsb)
{
+ /* If you're running into this assert, the dominator info is
+ incorrect. Try enabling the verify_dominators call at the
+ start of compute_dominance_frontiers. */
+ gcc_assert (runner != ENTRY_BLOCK_PTR_FOR_FN (cfun));
+
if (!bitmap_set_bit (&frontiers[runner->index],
b->index))
break;
@@ -1276,6 +1281,10 @@ compute_dominance_frontiers_1 (bitmap_head *frontiers)
void
compute_dominance_frontiers (bitmap_head *frontiers)
{
+#if ENABLE_CHECKING
+ verify_dominators (CDI_DOMINATORS);
+#endif
+
timevar_push (TV_DOM_FRONTIERS);
compute_dominance_frontiers_1 (frontiers);
--
1.9.1
