8292660: C2: blocks made unreachable by NeverBranch-to-Goto conversion are removed incorrectly

Reviewed-by: kvn, roland

src/hotspot/share/opto/block.cpp

@@ -151,6 +151,10 @@ bool Block::contains(const Node *n) const {
   return _nodes.contains(n);
 }
 
+bool Block::is_trivially_unreachable() const {
+  return num_preds() <= 1 && !head()->is_Root() && !head()->is_Start();
+}
+
 // Return empty status of a block.  Empty blocks contain only the head, other
 // ideal nodes, and an optional trailing goto.
 int Block::is_Empty() const {
@@ -170,7 +174,7 @@ int Block::is_Empty() const {
   }
 
   // Unreachable blocks are considered empty
-  if (num_preds() <= 1) {
+  if (is_trivially_unreachable()) {
     return success_result;
   }
 
@@ -608,19 +612,6 @@ void PhaseCFG::convert_NeverBranch_to_Goto(Block *b) {
   for (int k = 1; dead->get_node(k)->is_Phi(); k++) {
     dead->get_node(k)->del_req(j);
   }
-  // If the fake exit block becomes unreachable, remove it from the block list.
-  if (dead->num_preds() == 1) {
-    for (uint i = 0; i < number_of_blocks(); i++) {
-      Block* block = get_block(i);
-      if (block == dead) {
-        _blocks.remove(i);
-      } else if (block->_pre_order > dead->_pre_order) {
-        // Enforce contiguous pre-order indices (assumed by PhaseBlockLayout).
-        block->_pre_order--;
-      }
-    }
-    _number_of_blocks--;
-  }
 }
 
 // Helper function to move block bx to the slot following b_index. Return
@@ -954,6 +945,46 @@ void PhaseCFG::fixup_flow() {
   } // End of for all blocks
 }
 
+void PhaseCFG::remove_unreachable_blocks() {
+  ResourceMark rm;
+  Block_List unreachable;
+  // Initialize worklist of unreachable blocks to be removed.
+  for (uint i = 0; i < number_of_blocks(); i++) {
+    Block* block = get_block(i);
+    assert(block->_pre_order == i, "Block::pre_order does not match block index");
+    if (block->is_trivially_unreachable()) {
+      unreachable.push(block);
+    }
+  }
+  // Now remove all blocks that are transitively unreachable.
+  while (unreachable.size() > 0) {
+    Block* dead = unreachable.pop();
+    // When this code runs (after PhaseCFG::fixup_flow()), Block::_pre_order
+    // does not contain pre-order but block-list indices. Ensure they stay
+    // contiguous by decrementing _pre_order for all elements after 'dead'.
+    // Block::_rpo does not contain valid reverse post-order indices anymore
+    // (they are invalidated by block insertions in PhaseCFG::fixup_flow()),
+    // so there is no need to update them.
+    for (uint i = dead->_pre_order + 1; i < number_of_blocks(); i++) {
+      get_block(i)->_pre_order--;
+    }
+    _blocks.remove(dead->_pre_order);
+    _number_of_blocks--;
+    // Update the successors' predecessor list and push new unreachable blocks.
+    for (uint i = 0; i < dead->_num_succs; i++) {
+      Block* succ = dead->_succs[i];
+      Node* head = succ->head();
+      for (int j = head->req() - 1; j >= 1; j--) {
+        if (get_block_for_node(head->in(j)) == dead) {
+          head->del_req(j);
+        }
+      }
+      if (succ->is_trivially_unreachable()) {
+        unreachable.push(succ);
+      }
+    }
+  }
+}
 
 // postalloc_expand: Expand nodes after register allocation.
 //

src/hotspot/share/opto/block.hpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1997, 2021, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1997, 2022, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -321,6 +321,9 @@ public:
   // Check whether the node is in the block.
   bool contains (const Node *n) const;
 
+  // Whether the block is not root-like and does not have any predecessors.
+  bool is_trivially_unreachable() const;
+
   // Return the empty status of a block
   enum { not_empty, empty_with_goto, completely_empty };
   int is_Empty() const;
@@ -610,6 +613,10 @@ class PhaseCFG : public Phase {
   void remove_empty_blocks();
   Block *fixup_trap_based_check(Node *branch, Block *block, int block_pos, Block *bnext);
   void fixup_flow();
+  // Remove all blocks that are transitively unreachable. Such blocks can be
+  // found e.g. after PhaseCFG::convert_NeverBranch_to_Goto(). This function
+  // assumes post-fixup_flow() block indices (Block::_pre_order, Block::_rpo).
+  void remove_unreachable_blocks();
 
   // Insert a node into a block at index and map the node to the block
   void insert(Block *b, uint idx, Node *n) {

src/hotspot/share/opto/compile.cpp

@@ -2969,6 +2969,7 @@ void Compile::Code_Gen() {
       cfg.set_loop_alignment();
     }
     cfg.fixup_flow();
+    cfg.remove_unreachable_blocks();
   }
 
   // Apply peephole optimizations

test/hotspot/jtreg/compiler/loopopts/TestMultipleInfiniteLoops.java

@@ -0,0 +1,56 @@
+/*
+ * Copyright (c) 2022, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/**
+ * @test
+ * @bug 8292660
+ * @summary Test that blocks made unreachable after processing multiple infinite
+ *          loops in the block ordering phase are removed correctly.
+ *
+ * @run main/othervm -Xcomp -XX:CompileOnly=compiler.loopopts.TestMultipleInfiniteLoops::test
+ *                   compiler.loopopts.TestMultipleInfiniteLoops
+ */
+
+package compiler.loopopts;
+
+public class TestMultipleInfiniteLoops {
+
+    static int foo;
+
+    static void test() {
+        int i = 5, j;
+        while (i > 0) {
+            for (j = i; 1 > j; ) {
+                switch (i) {
+                case 4:
+                    foo = j;
+                }
+            }
+            i++;
+        }
+    }
+
+    public static void main(String[] args) {
+        test();
+    }
+}