[vm] Internal-only String.intern.

TEST=ci
Bug: https://github.com/dart-lang/sdk/issues/50648
Change-Id: I02e89c0def9913f12bf7fdd2ef8f3ff6cba231e3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/278808
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

runtime/lib/string.cc

@@ -245,6 +245,12 @@ DEFINE_NATIVE_ENTRY(StringBase_joinReplaceAllResult, 0, 4) {
   return result.ptr();
 }
 
+DEFINE_NATIVE_ENTRY(StringBase_intern, 0, 1) {
+  const String& receiver =
+      String::CheckedHandle(zone, arguments->NativeArgAt(0));
+  return Symbols::New(thread, receiver);
+}
+
 DEFINE_NATIVE_ENTRY(OneByteString_substringUnchecked, 0, 3) {
   const String& receiver =
       String::CheckedHandle(zone, arguments->NativeArgAt(0));

runtime/tests/vm/dart/string_intern_test.dart

@@ -0,0 +1,24 @@
+// 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.
+
+import "dart:_internal" show intern;
+import "package:expect/expect.dart";
+
+@pragma("vm:never-inline")
+String genString(int i) => "abc-${i}-xyz";
+
+main() {
+  int random = Object().hashCode;
+  var a = genString(random);
+  var b = genString(random);
+  Expect.notIdentical(a, b);
+
+  var internedA = intern(a);
+  Expect.equals(a, internedA);
+  // Likely, but not guarenteed: Expect.identical(a, internedA);
+  var internedB = intern(b);
+  Expect.equals(b, internedB);
+  // Likely, but not guarenteed: Expect.identical(a, internedB);
+  Expect.identical(internedA, internedB);
+}

runtime/tests/vm/dart/weak_canonical_string_table_test.dart

@@ -0,0 +1,17 @@
+// 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.
+
+// VMOptions=--old_gen_heap_size=20
+
+import "dart:_internal" show intern;
+
+@pragma("vm:never-inline")
+use(x) => x;
+
+main() {
+  const MB = 1 << 20;
+  for (var i = 0; i < 20 * MB; i++) {
+    use(intern((i.toString()))); // Should not hit OutOfMemory
+  }
+}

runtime/vm/bootstrap_natives.h

@@ -125,6 +125,7 @@ namespace dart {
   V(StringBase_createFromCodePoints, 3)                                        \
   V(StringBase_substringUnchecked, 3)                                          \
   V(StringBase_joinReplaceAllResult, 4)                                        \
+  V(StringBase_intern, 1)                                                      \
   V(StringBuffer_createStringFromUint16Array, 3)                               \
   V(OneByteString_substringUnchecked, 3)                                       \
   V(OneByteString_allocateFromOneByteList, 3)                                  \

sdk/lib/_internal/vm/lib/internal_patch.dart

@@ -431,3 +431,6 @@ class FinalizerEntry {
   @FfiNative<Void Function(Handle, IntPtr)>('FinalizerEntry_SetExternalSize')
   external void setExternalSize(int externalSize);
 }
+
+@pragma("vm:external-name", "StringBase_intern")
+external String intern(String str);