From f55d18d86a11dfcfa020cfad45b76c0967fea377 Mon Sep 17 00:00:00 2001
From: Vaibhav Gogte <vgogte@google.com>
Date: Wed, 5 Aug 2026 14:26:23 -0700
Subject: [PATCH] Add opt-in library to re-enable generic sharded transfer
 caches.

PiperOrigin-RevId: 959870601
---
 tcmalloc/BUILD                                | 11 ++++++++
 tcmalloc/transfer_cache.h                     |  9 +++++-
 .../want_generic_sharded_transfer_cache.cc    | 28 +++++++++++++++++++
 3 files changed, 47 insertions(+), 1 deletion(-)
 create mode 100644 tcmalloc/want_generic_sharded_transfer_cache.cc

diff --git a/tcmalloc/BUILD b/tcmalloc/BUILD
index f35e78665..96dacddca 100644
--- a/tcmalloc/BUILD
+++ b/tcmalloc/BUILD
@@ -1716,3 +1716,14 @@ cc_library(
     ],
     alwayslink = 1,
 )
+
+cc_library(
+    name = "want_generic_sharded_transfer_cache",
+    srcs = ["want_generic_sharded_transfer_cache.cc"],
+    copts = ["-g0"] + TCMALLOC_DEFAULT_COPTS,
+    visibility = ["//tcmalloc/testing:__pkg__"],
+    deps = [
+        "@com_google_absl//absl/base:core_headers",
+    ],
+    alwayslink = 1,
+)
diff --git a/tcmalloc/transfer_cache.h b/tcmalloc/transfer_cache.h
index 57d84212d..8044f2c18 100644
--- a/tcmalloc/transfer_cache.h
+++ b/tcmalloc/transfer_cache.h
@@ -72,10 +72,17 @@ class StaticForwarder {
                                   std::align_val_t alignment = kAlignment);
 };
 
+extern "C" ABSL_ATTRIBUTE_WEAK bool
+default_want_generic_sharded_transfer_cache();
+
 class ShardedStaticForwarder : public StaticForwarder {
  public:
   static void Init() {
-    use_generic_cache_ = false;
+    use_generic_cache_ =
+        (default_want_generic_sharded_transfer_cache != nullptr &&
+         default_want_generic_sharded_transfer_cache()) &&
+        !IsExperimentActive(
+            Experiment::TEST_ONLY_TCMALLOC_SHARDED_TRANSFER_CACHE);
     // Traditionally, we enable sharded transfer cache for large size
     // classes alone.
     enable_cache_for_large_classes_only_ = IsExperimentActive(
diff --git a/tcmalloc/want_generic_sharded_transfer_cache.cc b/tcmalloc/want_generic_sharded_transfer_cache.cc
new file mode 100644
index 000000000..a9b3fab2c
--- /dev/null
+++ b/tcmalloc/want_generic_sharded_transfer_cache.cc
@@ -0,0 +1,28 @@
+// Copyright 2026 The TCMalloc Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/attributes.h"
+
+namespace tcmalloc {
+namespace tcmalloc_internal {
+
+// This - if linked into a binary - allows generic sharded transfer cache to be
+// enabled.
+extern "C" ABSL_ATTRIBUTE_UNUSED bool
+default_want_generic_sharded_transfer_cache() {
+  return true;
+}
+
+}  // namespace tcmalloc_internal
+}  // namespace tcmalloc