[PATCH] Fix assertions along default switch labels (PR tree-optimization/78819)

[Marek Polacek]

Starting with r238761, we register assertions for default switch labels;
these assertions correspond to the anti-range of each case label.  It
means that we can optimize this better:

  switch (i) {
    case 1: case 2: case 3:
      ...
      break;
    default:
      // i is ~[1, 3]
      if (i == 2) // fold to 0
	...
  }

But as this testcase shows, this breaks when the default label shares a label
with another case.  On this testcase, when we reach the switch, we know that
argc is either 1, 2, or 3.  So by the time we reach vrp2, the IR will have
been optimized to

  switch (argc) {
    default:
    case 3:
      // argc will be considered 1 despite the case 3
      break;
    case 2:
      ...
   }

Now, the assertion for argc in default: is that it's ~[2,3].  Since argc had
been proved to be either 1, 2, or 3, we optimized it to 1.  Given the
consecutive case 3: this is incorrect.

So ignore the range when a case label is "shared" with the default case.

Another bug: --param max-vrp-switch-assertions=0 didn't work -- we register
the assertion and then there's this check

  if (--insertion_limit == 0)
    break;

but given the decrement this isn't true so we'd kept going.

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2016-12-16  Marek Polacek  <polacek@redhat.com>

	PR tree-optimization/78819
	* tree-vrp.c (find_switch_asserts): Return if the insertion limit is 0.
	Don't register an assertion if the default case shares a label with
	another case.

	* gcc.dg/tree-ssa/vrp112.c: New test.

diff --git gcc/testsuite/gcc.dg/tree-ssa/vrp112.c gcc/testsuite/gcc.dg/tree-ssa/vrp112.c
index e69de29..fdc6711 100644
--- gcc/testsuite/gcc.dg/tree-ssa/vrp112.c
+++ gcc/testsuite/gcc.dg/tree-ssa/vrp112.c
@@ -0,0 +1,31 @@
+/* PR tree-optimization/78819 */
+/* { dg-do run } */
+/* { dg-options "-O2" } */
+
+__attribute__((noinline, noclone)) void
+foo (int argc)
+{
+  if (argc <= 0 || argc > 3)
+    return;
+
+  switch (argc)
+    {
+    case 1:
+    case 3:
+      if (argc != 3)
+	__builtin_abort ();
+      break;
+    case 2:
+      asm ("");
+      break;
+    default:
+      __builtin_abort ();
+    }
+}
+
+int
+main (void)
+{
+  foo (3);
+  return 0;
+}
diff --git gcc/tree-vrp.c gcc/tree-vrp.c
index 97e9953..405469a 100644
--- gcc/tree-vrp.c
+++ gcc/tree-vrp.c
@@ -6051,10 +6051,17 @@ find_switch_asserts (basic_block bb, gswitch *last)
   /* Now register along the default label assertions that correspond to the
      anti-range of each label.  */
   int insertion_limit = PARAM_VALUE (PARAM_MAX_VRP_SWITCH_ASSERTIONS);
+  if (insertion_limit == 0)
+    return;
+
+  /* We can't do this if the default case shares a label with another case.  */
+  tree default_cl = gimple_switch_default_label (last);
   for (idx = 1; idx < n; idx++)
     {
       tree min, max;
       tree cl = gimple_switch_label (last, idx);
+      if (CASE_LABEL (cl) == CASE_LABEL (default_cl))
+	break;
 
       min = CASE_LOW (cl);
       max = CASE_HIGH (cl);
@@ -6065,6 +6072,8 @@ find_switch_asserts (basic_block bb, gswitch *last)
 	{
 	  tree next_min, next_max;
 	  tree next_cl = gimple_switch_label (last, idx);
+	  if (CASE_LABEL (next_cl) == CASE_LABEL (default_cl))
+	    break;
 
 	  next_min = CASE_LOW (next_cl);
 	  next_max = CASE_HIGH (next_cl);

	Marek