[vm] Avoid local labels in assembly snapshots.

Labels starting with L are treated as local by the assembler
and cause problems down the line. When targeting ARM64 Mach-O
having local labels cause linker to break when trying to
generate compact unwinding information with a cryptic error

    ld: too many compact unwind infos in function <...>

This happens because local labels are not seen as function
boundaries and multiple .cfi_startproc/.cfi_endproc are
mashed into a single function.

Fixes flutter/flutter#102281

TEST=runtime/tests/vm/dart{,_2}/no_local_labels_test.dart

Change-Id: I0171dc08f49c71ccb1ca02b398e01ac241efd9a8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/241962
Reviewed-by: Daco Harkes <dacoharkes@google.com>
Commit-Queue: Slava Egorov <vegorov@google.com>

runtime/tests/vm/dart/no_local_labels_test.dart

@@ -0,0 +1,78 @@
+// Copyright (c) 2022, 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.
+
+// This test verifies that assembly snapshot does not contain local labels.
+// (labels starting with 'L').
+
+import 'dart:io';
+
+import 'package:expect/expect.dart';
+import 'package:path/path.dart' as path;
+
+import 'use_flag_test_helper.dart';
+
+main(List<String> args) async {
+  if (!isAOTRuntime) {
+    return; // Running in JIT: AOT binaries not available.
+  }
+
+  if (Platform.isAndroid) {
+    return; // SDK tree not available on the test device.
+  }
+
+  // These are the tools we need to be available to run on a given platform:
+  if (!File(platformDill).existsSync()) {
+    throw "Cannot run test as $platformDill does not exist";
+  }
+  if (!await testExecutable(genSnapshot)) {
+    throw "Cannot run test as $genSnapshot not available";
+  }
+
+  await withTempDir('no-local-labels-test', (String tempDir) async {
+    final script = path.join(tempDir, 'program.dart');
+    final scriptDill = path.join(tempDir, 'program.dill');
+
+    await File(script).writeAsString('''
+class Local {
+  @pragma('vm:never-inline')
+  void foo() {
+  }
+
+  @pragma('vm:never-inline')
+  void bar() {
+  }
+}
+
+void main(List<String> args) {
+  Local()..foo()..bar();
+}
+''');
+
+    // Compile script to Kernel IR.
+    await run(genKernel, <String>[
+      '--aot',
+      '--platform=$platformDill',
+      '-o',
+      scriptDill,
+      script,
+    ]);
+
+    if (Platform.isWindows) {
+      return; // No assembly generation on Windows.
+    }
+
+    final assembly = path.join(tempDir, 'program.S');
+    await run(genSnapshot, <String>[
+      '--snapshot-kind=app-aot-assembly',
+      '--assembly=$assembly',
+      scriptDill,
+    ]);
+
+    final localLabelRe = RegExp(r'^L[a-zA-Z0-9_\.$]*:$', multiLine: true);
+    final match = localLabelRe.firstMatch(await File(assembly).readAsString());
+    if (match != null) {
+      Expect.isTrue(false, 'unexpected local label found ${match[0]!}');
+    }
+  });
+}

runtime/tests/vm/dart_2/no_local_labels_test.dart

@@ -0,0 +1,80 @@
+// Copyright (c) 2022, 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.
+
+// This test verifies that assembly snapshot does not contain local labels.
+// (labels starting with 'L').
+
+// @dart=2.9
+
+import 'dart:io';
+
+import 'package:expect/expect.dart';
+import 'package:path/path.dart' as path;
+
+import 'use_flag_test_helper.dart';
+
+main(List<String> args) async {
+  if (!isAOTRuntime) {
+    return; // Running in JIT: AOT binaries not available.
+  }
+
+  if (Platform.isAndroid) {
+    return; // SDK tree not available on the test device.
+  }
+
+  // These are the tools we need to be available to run on a given platform:
+  if (!File(platformDill).existsSync()) {
+    throw "Cannot run test as $platformDill does not exist";
+  }
+  if (!await testExecutable(genSnapshot)) {
+    throw "Cannot run test as $genSnapshot not available";
+  }
+
+  await withTempDir('no-local-labels-test', (String tempDir) async {
+    final script = path.join(tempDir, 'program.dart');
+    final scriptDill = path.join(tempDir, 'program.dill');
+
+    await File(script).writeAsString('''
+class Local {
+  @pragma('vm:never-inline')
+  void foo() {
+  }
+
+  @pragma('vm:never-inline')
+  void bar() {
+  }
+}
+
+void main(List<String> args) {
+  Local()..foo()..bar();
+}
+''');
+
+    // Compile script to Kernel IR.
+    await run(genKernel, <String>[
+      '--aot',
+      '--platform=$platformDill',
+      '-o',
+      scriptDill,
+      script,
+    ]);
+
+    if (Platform.isWindows) {
+      return; // No assembly generation on Windows.
+    }
+
+    final assembly = path.join(tempDir, 'program.S');
+    await run(genSnapshot, <String>[
+      '--snapshot-kind=app-aot-assembly',
+      '--assembly=$assembly',
+      scriptDill,
+    ]);
+
+    final localLabelRe = RegExp(r'^L[a-zA-Z0-9_\.$]*:$', multiLine: true);
+    final match = localLabelRe.firstMatch(await File(assembly).readAsString());
+    if (match != null) {
+      Expect.isTrue(false, 'unexpected local label found ${match[0]}');
+    }
+  });
+}

runtime/vm/image_snapshot.cc

@@ -1108,6 +1108,16 @@ void AssemblyImageWriter::Finalize() {
 
 static void AddAssemblerIdentifier(ZoneTextBuffer* printer, const char* label) {
   ASSERT(label[0] != '.');
+  if (label[0] == 'L' && printer->length() == 0) {
+    // Assembler treats labels starting with `L` as local which can cause
+    // some issues down the line e.g. on Mac the linker might fail to encode
+    // compact unwind information because multiple functions end up being
+    // treated as a single function. See https://github.com/flutter/flutter/issues/102281.
+    //
+    // Avoid this by prepending an underscore.
+    printer->AddString("_");
+  }
+
   for (char c = *label; c != '\0'; c = *++label) {
 #define OP(dart_name, asm_name)                                                \
   if (strncmp(label, dart_name, strlen(dart_name)) == 0) {                     \