dom: small compile time optimization with switches

In the process of converting gswitch away from CASE_LABEL_EXPR,
I found a place in dom where we store the whole CASE_LABEL_EXPR.
This place only needed to store the value of the case rather than
the whole case expression. This does that small optimization
and adds a few comments for the next person to understand what
is going on here. It was not obvious at my first read of the code
what it was doing or what error_mark was being used for.

Bootstrapped and tested on x86_64-linux-gnu.

gcc/ChangeLog:

	* tree-ssa-dom.cc (record_edge_info): For switches
	info only store the case low value to be recorded as
	the only value. Add comments.

Signed-off-by: Andrew Pinski <andrew.pinski@oss.qualcomm.com>

diff --git a/gcc/tree-ssa-dom.cc b/gcc/tree-ssa-dom.cc
index 10a1c3f0b21af11829a93644c99441fd74b1ea8c..3be7979f5289f72f8a4ceeb8d973d94aa91be06d 100644 (file)
--- a/gcc/tree-ssa-dom.cc
+++ b/gcc/tree-ssa-dom.cc
@@ -524,6 +524,11 @@ record_edge_info (basic_block bb)
            {
              int i;
               int n_labels = gimple_switch_num_labels (switch_stmt);
+             /* info contains NULL, error_mark_node or a value.
+                error_mark signifies there are multiple values already.
+                A NULL signifies there it is uninitialized.
+                A value signifies that is the only value on that edge
+                to that bb.  */
              tree *info = XCNEWVEC (tree, last_basic_block_for_fn (cfun));
 
              for (i = 0; i < n_labels; i++)
@@ -531,23 +536,27 @@ record_edge_info (basic_block bb)
                  tree label = gimple_switch_label (switch_stmt, i);
                  basic_block target_bb
                    = label_to_block (cfun, CASE_LABEL (label));
+                 /* The default case is a case with multiple values.
+                    If the value is already set then it has multiple values.  */
                  if (CASE_HIGH (label)
                      || !CASE_LOW (label)
                      || info[target_bb->index])
                    info[target_bb->index] = error_mark_node;
                  else
-                   info[target_bb->index] = label;
+                   /* Record the one value that can be on that edge to the
+                      target_bb.  */
+                   info[target_bb->index] = CASE_LOW (label);
                }
 
              FOR_EACH_EDGE (e, ei, bb->succs)
                {
                  basic_block target_bb = e->dest;
-                 tree label = info[target_bb->index];
+                 tree value = info[target_bb->index];
 
-                 if (label != NULL && label != error_mark_node)
+                 if (value != NULL && value != error_mark_node)
                    {
                      tree x = fold_convert_loc (loc, TREE_TYPE (index),
-                                                CASE_LOW (label));
+                                                value);
                      edge_info = new class edge_info (e);
                      edge_info->record_simple_equiv (index, x);
                    }