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);
+
