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Re: [PATCH] Sanitize block partitioning under -freorder-blocks-and-partition
- From: Teresa Johnson <tejohnson at google dot com>
- To: Jan Hubicka <hubicka at ucw dot cz>
- Cc: Bernhard Reutner-Fischer <rep dot dot dot nop at gmail dot com>, "gcc-patches at gcc dot gnu dot org" <gcc-patches at gcc dot gnu dot org>, Steven Bosscher <stevenb dot gcc at gmail dot com>, Jeff Law <law at redhat dot com>
- Date: Mon, 5 Aug 2013 06:35:59 -0700
- Subject: Re: [PATCH] Sanitize block partitioning under -freorder-blocks-and-partition
- References: <CAAe5K+U6+xyy95KSeA7+SZ0tUdFt2dmF-vSxNsBsqg53NSyU3Q at mail dot gmail dot com> <CAC1BbcSnwiAfzKqngLvLBGnmPDLNhYdwbyWDMJ++PzsMd=M-3Q at mail dot gmail dot com> <CAAe5K+XM_RCJBmtKSyQCfD86hi2Ls_BCyuZkd2WKSoGUFm84DA at mail dot gmail dot com> <20130802150529 dot GC15776 at kam dot mff dot cuni dot cz> <CAAe5K+WiR02Rs1jYMFRF2F8ey60UO7LwRa8WWq7coQ5Pq8HhiQ at mail dot gmail dot com>
On Fri, Aug 2, 2013 at 9:48 PM, Teresa Johnson <tejohnson@google.com> wrote:
> On Fri, Aug 2, 2013 at 8:05 AM, Jan Hubicka <hubicka@ucw.cz> wrote:
>>>
>>> 2013-08-01 Teresa Johnson <tejohnson@google.com>
>>> Steven Bosscher <steven@gcc.gnu.org>
>>>
>>> * cfgrtl.c (fixup_bb_partition): New routine.
>>> (commit_edge_insertions): Invoke fixup_partitions.
>>> (find_partition_fixes): New routine.
>>> (fixup_partitions): Ditto.
>>> (verify_hot_cold_block_grouping): Update comments.
>>> (rtl_verify_edges): Invoke find_partition_fixes.
>>> (rtl_verify_bb_pointers): Update comments.
>>> (rtl_verify_bb_layout): Ditto.
>>> * basic-block.h (fixup_partitions): Declare.
>>> * cfgcleanup.c (try_optimize_cfg): Invoke fixup_partitions.
>>> * bb-reorder.c (sanitize_dominator_hotness): New function.
>>> (find_rarely_executed_basic_blocks_and_crossing_edges): Invoke
>>> sanitize_dominator_hotness.
>>>
>>> Index: cfgrtl.c
>>> ===================================================================
>>> --- cfgrtl.c (revision 201281)
>>> +++ cfgrtl.c (working copy)
>>> @@ -1341,6 +1341,34 @@ fixup_partition_crossing (edge e)
>>> }
>>> }
>>>
>>> +/* Called when block BB has been reassigned to a different partition,
>>> + to ensure that the region crossing attributes are updated. */
>>> +
>>> +static void
>>> +fixup_bb_partition (basic_block bb)
>>> +{
>>> + edge e;
>>> + edge_iterator ei;
>>> +
>>> + /* Now need to make bb's pred edges non-region crossing. */
>>> + FOR_EACH_EDGE (e, ei, bb->preds)
>>> + {
>>> + fixup_partition_crossing (e);
>>> + }
>>> +
>>> + /* Possibly need to make bb's successor edges region crossing,
>>> + or remove stale region crossing. */
>>> + FOR_EACH_EDGE (e, ei, bb->succs)
>>> + {
>>> + if ((e->flags & EDGE_FALLTHRU)
>>> + && BB_PARTITION (bb) != BB_PARTITION (e->dest)
>>> + && e->dest != EXIT_BLOCK_PTR)
>>> + force_nonfallthru (e);
>>> + else
>>> + fixup_partition_crossing (e);
>>> + }
>>> +}
>>
>> Is there particular reason why preds can not be fallhtrus and why
>> force_nonfallthru edge does not need partition crossing fixup?
>> (if so, perhpas it could be mentioned in the description, if not,
>> I think force_nonfallthru path has to check if new BB was introduced
>> and do the right thing on the edge.
>
> I need to clarify the comments in this routine, because without the
> context of how this is called it isn't clear. This routine is only
> called when we detect a hot bb that is now dominated by a cold bb and
> needs to become cold. Therefore, its preds will no longer be region
> crossing (any non-dominating blocks that were previously hot would
> have been marked cold in the caller for the same reason, so we will
> not end up adjusting the region crossing-ness or fallthrough-ness of
> those pred edges). Any that were region crossing before but aren't any
> longer could not have been fall through (as Steven noted, you can't
> have a fall through across a partition boundary). I will add some
> better comments here.
>
> Regarding the call to force_nonfallthru, that routine calls
> fixup_partition_crossing as needed, and I will update the comment to
> reflect that too.
Patch with updated comments below. Ok for trunk?
Thanks,
Teresa
2013-08-05 Teresa Johnson <tejohnson@google.com>
Steven Bosscher <steven@gcc.gnu.org>
* cfgrtl.c (fixup_bb_partition): New routine.
(commit_edge_insertions): Invoke fixup_partitions.
(find_partition_fixes): New routine.
(fixup_partitions): Ditto.
(verify_hot_cold_block_grouping): Update comments.
(rtl_verify_edges): Invoke find_partition_fixes.
(rtl_verify_bb_pointers): Update comments.
(rtl_verify_bb_layout): Ditto.
* basic-block.h (fixup_partitions): Declare.
* cfgcleanup.c (try_optimize_cfg): Invoke fixup_partitions.
* bb-reorder.c (sanitize_dominator_hotness): New function.
(find_rarely_executed_basic_blocks_and_crossing_edges): Invoke
sanitize_dominator_hotness.
Index: cfgrtl.c
===================================================================
--- cfgrtl.c (revision 201461)
+++ cfgrtl.c (working copy)
@@ -1341,6 +1341,43 @@ fixup_partition_crossing (edge e)
}
}
+/* Called when block BB has been reassigned to the cold partition,
+ because it is now dominated by another cold block,
+ to ensure that the region crossing attributes are updated. */
+
+static void
+fixup_new_cold_bb (basic_block bb)
+{
+ edge e;
+ edge_iterator ei;
+
+ /* This is called when a hot bb is found to now be dominated
+ by a cold bb and therefore needs to become cold. Therefore,
+ its preds will no longer be region crossing. Any non-dominating
+ preds that were previously hot would also have become cold
+ in the caller for the same region. Any preds that were previously
+ region-crossing will be adjusted in fixup_partition_crossing. */
+ FOR_EACH_EDGE (e, ei, bb->preds)
+ {
+ fixup_partition_crossing (e);
+ }
+
+ /* Possibly need to make bb's successor edges region crossing,
+ or remove stale region crossing. */
+ FOR_EACH_EDGE (e, ei, bb->succs)
+ {
+ /* We can't have fall-through edges across partition boundaries.
+ Note that force_nonfallthru will do any necessary partition
+ boundary fixup by calling fixup_partition_crossing itself. */
+ if ((e->flags & EDGE_FALLTHRU)
+ && BB_PARTITION (bb) != BB_PARTITION (e->dest)
+ && e->dest != EXIT_BLOCK_PTR)
+ force_nonfallthru (e);
+ else
+ fixup_partition_crossing (e);
+ }
+}
+
/* Attempt to change code to redirect edge E to TARGET. Don't do that on
expense of adding new instructions or reordering basic blocks.
@@ -1979,6 +2016,14 @@ commit_edge_insertions (void)
{
basic_block bb;
+ /* Optimization passes that invoke this routine can cause hot blocks
+ previously reached by both hot and cold blocks to become dominated only
+ by cold blocks. This will cause the verification below to fail,
+ and lead to now cold code in the hot section. In some cases this
+ may only be visible after newly unreachable blocks are deleted,
+ which will be done by fixup_partitions. */
+ fixup_partitions ();
+
#ifdef ENABLE_CHECKING
verify_flow_info ();
#endif
@@ -2173,6 +2218,101 @@ get_last_bb_insn (basic_block bb)
return end;
}
+/* Sanity check partition hotness to ensure that basic blocks in
+ the cold partition don't dominate basic blocks in the hot partition.
+ If FLAG_ONLY is true, report violations as errors. Otherwise
+ re-mark the dominated blocks as cold, since this is run after
+ cfg optimizations that may make hot blocks previously reached
+ by both hot and cold blocks now only reachable along cold paths. */
+
+static vec<basic_block>
+find_partition_fixes (bool flag_only)
+{
+ basic_block bb;
+ vec<basic_block> bbs_in_cold_partition = vNULL;
+ vec<basic_block> bbs_to_fix = vNULL;
+
+ /* Callers check this. */
+ gcc_checking_assert (crtl->has_bb_partition);
+
+ FOR_EACH_BB (bb)
+ if ((BB_PARTITION (bb) == BB_COLD_PARTITION))
+ bbs_in_cold_partition.safe_push (bb);
+
+ if (bbs_in_cold_partition.is_empty ())
+ return vNULL;
+
+ bool dom_calculated_here = !dom_info_available_p (CDI_DOMINATORS);
+
+ if (dom_calculated_here)
+ calculate_dominance_info (CDI_DOMINATORS);
+
+ while (! bbs_in_cold_partition.is_empty ())
+ {
+ bb = bbs_in_cold_partition.pop ();
+ /* Any blocks dominated by a block in the cold section
+ must also be cold. */
+ basic_block son;
+ for (son = first_dom_son (CDI_DOMINATORS, bb);
+ son;
+ son = next_dom_son (CDI_DOMINATORS, son))
+ {
+ /* If son is not yet cold, then mark it cold here and
+ enqueue it for further processing. */
+ if ((BB_PARTITION (son) != BB_COLD_PARTITION))
+ {
+ if (flag_only)
+ error ("non-cold basic block %d dominated "
+ "by a block in the cold partition", son->index);
+ else
+ BB_SET_PARTITION (son, BB_COLD_PARTITION);
+ bbs_to_fix.safe_push (son);
+ bbs_in_cold_partition.safe_push (son);
+ }
+ }
+ }
+
+ if (dom_calculated_here)
+ free_dominance_info (CDI_DOMINATORS);
+
+ return bbs_to_fix;
+}
+
+/* Perform cleanup on the hot/cold bb partitioning after optimization
+ passes that modify the cfg. */
+
+void
+fixup_partitions (void)
+{
+ basic_block bb;
+
+ if (!crtl->has_bb_partition)
+ return;
+
+ /* Delete any blocks that became unreachable and weren't
+ already cleaned up, for example during edge forwarding
+ and convert_jumps_to_returns. This will expose more
+ opportunities for fixing the partition boundaries here.
+ Also, the calculation of the dominance graph during verification
+ will assert if there are unreachable nodes. */
+ delete_unreachable_blocks ();
+
+ /* If there are partitions, do a sanity check on them: A basic block in
+ a cold partition cannot dominate a basic block in a hot partition.
+ Fixup any that now violate this requirement, as a result of edge
+ forwarding and unreachable block deletion. */
+ vec<basic_block> bbs_to_fix = find_partition_fixes (false);
+
+ /* Do the partition fixup after all necessary blocks have been converted to
+ cold, so that we only update the region crossings the minimum number of
+ places, which can require forcing edges to be non fallthru. */
+ while (! bbs_to_fix.is_empty ())
+ {
+ bb = bbs_to_fix.pop ();
+ fixup_new_cold_bb (bb);
+ }
+}
+
/* Verify, in the basic block chain, that there is at most one switch
between hot/cold partitions. This condition will not be true until
after reorder_basic_blocks is called. */
@@ -2219,7 +2359,8 @@ verify_hot_cold_block_grouping (void)
/* Perform several checks on the edges out of each block, such as
the consistency of the branch probabilities, the correctness
of hot/cold partition crossing edges, and the number of expected
- successor edges. */
+ successor edges. Also verify that the dominance relationship
+ between hot/cold blocks is sane. */
static int
rtl_verify_edges (void)
@@ -2382,6 +2523,14 @@ rtl_verify_edges (void)
}
}
+ /* If there are partitions, do a sanity check on them: A basic block in
+ a cold partition cannot dominate a basic block in a hot partition. */
+ if (crtl->has_bb_partition && !err)
+ {
+ vec<basic_block> bbs_to_fix = find_partition_fixes (true);
+ err = !bbs_to_fix.is_empty ();
+ }
+
/* Clean up. */
return err;
}
@@ -2515,7 +2664,7 @@ rtl_verify_bb_pointers (void)
and NOTE_INSN_BASIC_BLOCK
- verify that no fall_thru edge crosses hot/cold partition boundaries
- verify that there are no pending RTL branch predictions
- - verify that there is a single hot/cold partition boundary after bbro
+ - verify that hot blocks are not dominated by cold blocks
In future it can be extended check a lot of other stuff as well
(reachability of basic blocks, life information, etc. etc.). */
@@ -2761,7 +2910,8 @@ rtl_verify_bb_layout (void)
- check that all insns are in the basic blocks
(except the switch handling code, barriers and notes)
- check that all returns are followed by barriers
- - check that all fallthru edge points to the adjacent blocks. */
+ - check that all fallthru edge points to the adjacent blocks
+ - verify that there is a single hot/cold partition boundary after bbro */
static int
rtl_verify_flow_info (void)
Index: basic-block.h
===================================================================
--- basic-block.h (revision 201461)
+++ basic-block.h (working copy)
@@ -797,6 +797,7 @@ extern bool contains_no_active_insn_p (const_basic
extern bool forwarder_block_p (const_basic_block);
extern bool can_fallthru (basic_block, basic_block);
extern void emit_barrier_after_bb (basic_block bb);
+extern void fixup_partitions (void);
/* In cfgbuild.c. */
extern void find_many_sub_basic_blocks (sbitmap);
Index: cfgcleanup.c
===================================================================
--- cfgcleanup.c (revision 201461)
+++ cfgcleanup.c (working copy)
@@ -2807,10 +2807,21 @@ try_optimize_cfg (int mode)
df_analyze ();
}
+ if (changed)
+ {
+ /* Edge forwarding in particular can cause hot blocks previously
+ reached by both hot and cold blocks to become dominated only
+ by cold blocks. This will cause the verification
below to fail,
+ and lead to now cold code in the hot section. This is not easy
+ to detect and fix during edge forwarding, and in some cases
+ is only visible after newly unreachable blocks are deleted,
+ which will be done in fixup_partitions. */
+ fixup_partitions ();
+
#ifdef ENABLE_CHECKING
- if (changed)
- verify_flow_info ();
+ verify_flow_info ();
#endif
+ }
changed_overall |= changed;
first_pass = false;
Index: bb-reorder.c
===================================================================
--- bb-reorder.c (revision 201461)
+++ bb-reorder.c (working copy)
@@ -1444,6 +1444,57 @@ fix_up_crossing_landing_pad (eh_landing_pad old_lp
ei_next (&ei);
}
+
+/* Ensure that no cold bbs dominate hot bbs along the dominance or
+ post-dominance DIR, for example as a result of edge weight insanities.
+ Returns the updated value of COLD_BB_COUNT and adds newly-hot bbs
+ to BBS_IN_HOT_PARTITION. */
+
+static unsigned int
+sanitize_dominator_hotness (enum cdi_direction dir, unsigned int cold_bb_count,
+ vec<basic_block> *bbs_in_hot_partition)
+{
+ /* Callers check this. */
+ gcc_checking_assert (cold_bb_count);
+
+ bool dom_calculated_here = !dom_info_available_p (dir);
+
+ if (dom_calculated_here)
+ calculate_dominance_info (dir);
+
+ /* Keep examining hot bbs while we still have some left to check
+ and there are remaining cold bbs. */
+ vec<basic_block> hot_bbs_to_check = bbs_in_hot_partition->copy ();
+ while (! hot_bbs_to_check.is_empty ()
+ && cold_bb_count)
+ {
+ basic_block bb = hot_bbs_to_check.pop ();
+ basic_block dom_bb = get_immediate_dominator (dir, bb);
+
+ /* If bb's immediate dominator is also hot (or unpartitioned,
+ e.g. the entry block) then it is ok. If it is cold, it
+ needs to be adjusted. */
+ if (BB_PARTITION (dom_bb) != BB_COLD_PARTITION)
+ continue;
+
+ /* We have a hot bb with an immediate dominator that is cold.
+ The dominator needs to be re-marked hot. */
+ BB_SET_PARTITION (dom_bb, BB_HOT_PARTITION);
+ cold_bb_count--;
+
+ /* Now we need to examine newly-hot dom_bb to see if it is also
+ dominated by a cold bb. */
+ bbs_in_hot_partition->safe_push (dom_bb);
+ hot_bbs_to_check.safe_push (dom_bb);
+ }
+
+ if (dom_calculated_here)
+ free_dominance_info (dir);
+
+ return cold_bb_count;
+}
+
+
/* Find the basic blocks that are rarely executed and need to be moved to
a separate section of the .o file (to cut down on paging and improve
cache locality). Return a vector of all edges that cross. */
@@ -1455,16 +1506,42 @@ find_rarely_executed_basic_blocks_and_crossing_edg
basic_block bb;
edge e;
edge_iterator ei;
+ unsigned int cold_bb_count = 0;
+ vec<basic_block> bbs_in_hot_partition = vNULL;
/* Mark which partition (hot/cold) each basic block belongs in. */
FOR_EACH_BB (bb)
{
if (probably_never_executed_bb_p (cfun, bb))
- BB_SET_PARTITION (bb, BB_COLD_PARTITION);
+ {
+ BB_SET_PARTITION (bb, BB_COLD_PARTITION);
+ cold_bb_count++;
+ }
else
- BB_SET_PARTITION (bb, BB_HOT_PARTITION);
+ {
+ BB_SET_PARTITION (bb, BB_HOT_PARTITION);
+ bbs_in_hot_partition.safe_push (bb);
+ }
}
+ /* Ensure that no cold bbs dominate hot bbs. This could happen as a result of
+ several different possibilities. One is that there are edge
weight insanities
+ due to optimization phases that do not properly update basic block profile
+ counts. The second is that the entry of the function may not be
hot, because
+ it is entered fewer times than the number of profile training
runs, but there
+ is a loop inside the function that causes blocks within the function to be
+ above the threshold for hotness. Then do the same along the post-dominator
+ tree (which could have additional changes required after fixing up
+ dominators). */
+ if (cold_bb_count)
+ cold_bb_count = sanitize_dominator_hotness (CDI_DOMINATORS,
+ cold_bb_count,
+ &bbs_in_hot_partition);
+ if (cold_bb_count)
+ sanitize_dominator_hotness (CDI_POST_DOMINATORS,
+ cold_bb_count,
+ &bbs_in_hot_partition);
+
/* The format of .gcc_except_table does not allow landing pads to
be in a different partition as the throw. Fix this by either
moving or duplicating the landing pads. */
>
>>
>>> +/* Sanity check partition hotness to ensure that basic blocks in
>>> + the cold partition don't dominate basic blocks in the hot partition.
>>> + If FLAG_ONLY is true, report violations as errors. Otherwise
>>> + re-mark the dominated blocks as cold, since this is run after
>>> + cfg optimizations that may make hot blocks previously reached
>>> + by both hot and cold blocks now only reachable along cold paths. */
>>
>> With profile, I suppose we can have cold blocks dominating hot blocks when the
>> hot blocks is in loop whose trip count is high enough. Indeed for partitioning
>> reasons it does not make sense to push those into different section.
>>
>> I also wonder, if we finally get the pass stable, can we enable it by default
>> and offline probably cold blocks w/o profile? Primarily blocks reachable only
>> by EH + blocks leading to a crash or throw(). For C++ those should be common
>> enough to make a difference...
>
> Yep, as soon as PR57451 is fixed, which I hope to get to next week,
> then I am going to send a patch to turn this on by default, at least
> with profile feedback, which is where I've been doing performance
> tuning. But you are right that there are some cases where it should be
> beneficial without profile data as well.
>
> Thanks,
> Teresa
>
>>
>> Honza
>
>
>
> --
> Teresa Johnson | Software Engineer | tejohnson@google.com | 408-460-2413
--
Teresa Johnson | Software Engineer | tejohnson@google.com | 408-460-2413