[PATCH GCC]Refine comment and set type for partition merged from SCC

[Bin Cheng]

Hi,
When reading the code I found it's could be confusing without comment.
This patch adds comment explaining why we want merge PARALLEL type
partitions in a SCC, even though the result partition can no longer
be executed in parallel.  It also sets type of the result partition
to sequential.
Bootstrap and test on x86_64 and AArch64.  Is it OK?

Thanks,
bin
2017-10-10  Bin Cheng  <bin.ch...@arm.com>

        * tree-loop-distribution.c (break_alias_scc_partitions): Add comment
        and set PTYPE_SEQUENTIAL for merged partition.
diff --git a/gcc/tree-loop-distribution.c b/gcc/tree-loop-distribution.c
index 9ffac53..dc429cf 100644
--- a/gcc/tree-loop-distribution.c
+++ b/gcc/tree-loop-distribution.c
@@ -2062,7 +2062,7 @@ break_alias_scc_partitions (struct graph *rdg,
       auto_vec<enum partition_type> scc_types;
       struct partition *partition, *first;
 
-      /* If all paritions in a SCC has the same type, we can simply merge the
+      /* If all partitions in a SCC have the same type, we can simply merge the
         SCC.  This loop finds out such SCCS and record them in bitmap.  */
       bitmap_set_range (sccs_to_merge, 0, (unsigned) num_sccs);
       for (i = 0; i < num_sccs; ++i)
@@ -2075,6 +2075,10 @@ break_alias_scc_partitions (struct graph *rdg,
              if (pg->vertices[j].component != i)
                continue;
 
+             /* Note we Merge partitions of parallel type on purpose, though
+                the result partition is sequential.  The reason is vectorizer
+                can do more accurate runtime alias check in this case.  Also
+                it results in more conservative distribution.  */
              if (first->type != partition->type)
                {
                  bitmap_clear_bit (sccs_to_merge, i);
@@ -2096,7 +2100,7 @@ break_alias_scc_partitions (struct graph *rdg,
       if (bitmap_count_bits (sccs_to_merge) != (unsigned) num_sccs)
        {
          /* Run SCC finding algorithm again, with alias dependence edges
-            skipped.  This is to topologically sort paritions according to
+            skipped.  This is to topologically sort partitions according to
             compilation time known dependence.  Note the topological order
             is stored in the form of pg's post order number.  */
          num_sccs_no_alias = graphds_scc (pg, NULL, pg_skip_alias_edge);
@@ -2139,6 +2143,8 @@ break_alias_scc_partitions (struct graph *rdg,
              data = (struct pg_vdata *)pg->vertices[k].data;
              gcc_assert (data->id == k);
              data->partition = NULL;
+             /* The result partition of merged SCC must be sequential.  */
+             first->type = PTYPE_SEQUENTIAL;
            }
        }
     }