[vm] Exclude current_context_var from ValidatePhis

current_context_var is special cased in the liveness analysis and never
pruned even though it does not have uses by instructions. The lack of
pruning caused the assert to fire. This CL adds the special casing to
this assert as well.

Closes: dart-lang/sdk#45855

TEST=tests/language/vm/regress_45855_test.dart

Change-Id: Ica74a43bd2449dd994639c686253449146216458
Fixed: 45855
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/197541
Commit-Queue: Daco Harkes <dacoharkes@google.com>
Reviewed-by: Alexander Markov <alexmarkov@google.com>

Author: dcharkes

runtime/vm/compiler/backend/flow_graph.cc

@@ -1613,6 +1613,10 @@ void FlowGraph::ValidatePhis() {
     return;
   }
 
+  // Current_context_var is never pruned, it is artificially kept alive, so
+  // it should not be checked here.
+  const intptr_t current_context_var_index = CurrentContextEnvIndex();
+
   for (intptr_t i = 0, n = preorder().length(); i < n; ++i) {
     BlockEntryInstr* block_entry = preorder()[i];
     Instruction* last_instruction = block_entry->last_instruction();
@@ -1624,7 +1628,7 @@ void FlowGraph::ValidatePhis() {
       if (successor->phis() != NULL) {
         for (intptr_t j = 0; j < successor->phis()->length(); ++j) {
           PhiInstr* phi = (*successor->phis())[j];
-          if (phi == nullptr) {
+          if (phi == nullptr && j != current_context_var_index) {
             // We have no phi node for the this variable.
             // Double check we do not have a different value in our env.
             // If we do, we would have needed a phi-node in the successsor.

runtime/vm/compiler/backend/flow_graph.h

@@ -651,8 +651,8 @@ class LivenessAnalysis : public ValueObject {
   const GrowableArray<BlockEntryInstr*>& postorder_;
 
   // Live-out sets for each block.  They contain indices of variables
-  // that are live out from this block: that is values that were either
-  // defined in this block or live into it and that are used in some
+  // that are live out from this block. That is values that were (1) either
+  // defined in this block or live into it, and (2) that are used in some
   // successor block.
   GrowableArray<BitVector*> live_out_;
 

tests/language/vm/regress_45855_test.dart

@@ -0,0 +1,34 @@
+// Copyright (c) 2021, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+//
+// VMOptions=--optimization-counter-threshold=100 --deterministic
+//
+// The Dart Project Fuzz Tester (1.89).
+// Program generated as:
+//   dart dartfuzz.dart --seed 928581289 --no-fp --ffi --no-flat
+//
+// Minimized.
+
+@pragma('vm:never-inline')
+void main2(bool? boolParam) {
+  for (int i = 0; i < 200; i++) {
+    final bool1 = boolParam ?? false;
+    if (bool1) {
+      () {
+        // Force creating a new current context.
+        i.toString();
+      };
+      // Force having multiple paths to the exit, popping the current context.
+      break;
+    }
+  }
+}
+
+void main() {
+  // Test OSR.
+  main2(null);
+
+  // Test non-OSR.
+  main2(null);
+}

tests/language_2/vm/regress_45855_test.dart

@@ -0,0 +1,34 @@
+// Copyright (c) 2021, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+//
+// VMOptions=--optimization-counter-threshold=100 --deterministic
+//
+// The Dart Project Fuzz Tester (1.89).
+// Program generated as:
+//   dart dartfuzz.dart --seed 928581289 --no-fp --ffi --no-flat
+//
+// Minimized.
+
+@pragma('vm:never-inline')
+void main2(bool boolParam) {
+  for (int i = 0; i < 200; i++) {
+    final bool1 = boolParam ?? false;
+    if (bool1) {
+      () {
+        // Force creating a new current context.
+        i.toString();
+      };
+      // Force having multiple paths to the exit, popping the current context.
+      break;
+    }
+  }
+}
+
+void main() {
+  // Test OSR.
+  main2(null);
+
+  // Test non-OSR.
+  main2(null);
+}