Improve backward threading with switches.

We've been essentially using find_taken_edge_switch_expr() in the
backward threader, but this is suboptimal because said function only
works with singletons.  VRP has a much smarter find_case_label_range
that works with ranges.

Tested on x86-64 Linux with:

	a) Bootstrap & regtests.

	b) Verifying we get more threads than before.

	c) Asserting that the new code catches everything the old one
	code caught (over a set of bootstrap .ii files).

gcc/ChangeLog:

	* tree-ssa-threadbackward.c
	(back_threader::find_taken_edge_switch): Use find_case_label_range
	instead of find_taken_edge.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/vrp106.c: Adjust for threading.
	* gcc.dg/tree-ssa/vrp113.c: Same.

1 files changed, 4 insertions, 4 deletions

diff --git a/gcc/tree-ssa-threadbackward.c b/gcc/tree-ssa-threadbackward.c
index 6c1b159..456effc 100644
--- a/gcc/tree-ssa-threadbackward.c
+++ b/gcc/tree-ssa-threadbackward.c
@@ -195,11 +195,11 @@ back_threader::find_taken_edge_switch (const vec<basic_block> &path,
   if (r.varying_p ())
     return NULL;
 
-  tree val;
-  if (r.singleton_p (&val))
-    return ::find_taken_edge (gimple_bb (sw), val);
+  tree label = find_case_label_range (sw, &r);
+  if (!label)
+    return NULL;
 
-  return NULL;
+  return find_edge (gimple_bb (sw), label_to_block (cfun, CASE_LABEL (label)));
 }
 
 // Same as find_taken_edge, but for paths ending in a GIMPLE_COND.