[PATCH] Fix PR middle-end/80775, -O3 produces ice in group_case_labels_stmt

[Peter Bergner]

The test case in PR80775 exposes a problem in handling two separate
case labels that lead to the same block that contains a call to
__builtin_unreachable().   The current code handles the first label
and deletes the associated edge/block, but trips up when we see the
second case label that now points to a removed block.  This patch moves
the deletion of the unreachable case statement to after the merging
of consecutive case labels, which easily fixes the issue.  It also has
a side benefit of reducing the number of calls to gimple_seq_unreachable_p()
when we have consecutive case labels that point to unreachable blocks.

This bootstrapped and regtested with no regressions on both powerpc64le-linux
and x86_64-linux.  Is this ok for trunk?

Peter

gcc/
        PR middle-end/80775
        * tree-cfg.c: Move deletion of unreachable case statements to after
        the merging of consecutive case labels.

gcc/testsuite/
        PR middle-end/80775
        * gcc.dg/pr80775.c: New test.

Index: gcc/tree-cfg.c
===================================================================
--- gcc/tree-cfg.c      (revision 248119)
+++ gcc/tree-cfg.c      (working copy)
@@ -1661,7 +1661,7 @@ void
 group_case_labels_stmt (gswitch *stmt)
 {
   int old_size = gimple_switch_num_labels (stmt);
-  int i, j, new_size = old_size;
+  int i, j, base_index, new_size = old_size;
   basic_block default_bb = NULL;
 
   default_bb = label_to_block (CASE_LABEL (gimple_switch_default_label 
(stmt)));
@@ -1678,16 +1678,9 @@ group_case_labels_stmt (gswitch *stmt)
       gcc_assert (base_case);
       base_bb = label_to_block (CASE_LABEL (base_case));
 
-      /* Discard cases that have the same destination as the default case
-        or if their destination block is unreachable.  */
-      if (base_bb == default_bb
-         || (EDGE_COUNT (base_bb->succs) == 0
-             && gimple_seq_unreachable_p (bb_seq (base_bb))))
+      /* Discard cases that have the same destination as the default case.  */
+      if (base_bb == default_bb)
        {
-         edge e;
-         if (base_bb != default_bb
-             && (e = find_edge (gimple_bb (stmt), base_bb)) != NULL)
-           remove_edge_and_dominated_blocks (e);
          gimple_switch_set_label (stmt, i, NULL_TREE);
          i++;
          new_size--;
@@ -1697,7 +1690,7 @@ group_case_labels_stmt (gswitch *stmt)
       base_high = CASE_HIGH (base_case)
          ? CASE_HIGH (base_case)
          : CASE_LOW (base_case);
-      i++;
+      base_index = i++;
 
       /* Try to merge case labels.  Break out when we reach the end
         of the label vector or when we cannot merge the next case
@@ -1723,6 +1716,18 @@ group_case_labels_stmt (gswitch *stmt)
          else
            break;
        }
+
+      /* Discard cases that have an unreachable destination block.  */
+      if (EDGE_COUNT (base_bb->succs) == 0
+         && gimple_seq_unreachable_p (bb_seq (base_bb)))
+       {
+         edge base_edge = find_edge (gimple_bb (stmt), base_bb);
+         if (base_edge != NULL)
+           remove_edge_and_dominated_blocks (base_edge);
+         gimple_switch_set_label (stmt, base_index, NULL_TREE);
+         new_size--;
+         i++;
+       }
     }
 
   /* Compress the case labels in the label vector, and adjust the
Index: gcc/testsuite/gcc.dg/pr80775.c
===================================================================
--- gcc/testsuite/gcc.dg/pr80775.c      (nonexistent)
+++ gcc/testsuite/gcc.dg/pr80775.c      (working copy)
@@ -0,0 +1,21 @@
+/* PR middle-end/80775 ICE: -O3 produces ice in group_case_labels_stmt.  */
+/* { dg-do compile }  */
+/* { dg-options "-O3" }  */
+
+typedef struct { short a; } b;
+b c[10];
+int d, e, f, g, h;
+int
+i (void)
+{
+  f = 0;
+  for (; f < e; f++) {
+    switch (g) {
+    case 1:
+      d = 1;
+    case 2:
+      c[2 + f].a = 1;
+    }
+    h += c[f].a;
+  }
+}