[libc-commits] [libc] [libc][arc4random][patch 2/n] add ABA protected MPMCStack (PR #151361)
via libc-commits
libc-commits at lists.llvm.org
Fri Aug 8 06:17:43 PDT 2025
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-libc
Author: Schrodinger ZHU Yifan (SchrodingerZhu)
<details>
<summary>Changes</summary>
---
Full diff: https://github.com/llvm/llvm-project/pull/151361.diff
5 Files Affected:
- (modified) libc/src/__support/CMakeLists.txt (+24)
- (added) libc/src/__support/aba_ptr.h (+83)
- (added) libc/src/__support/mpmc_stack.h (+107)
- (modified) libc/test/integration/src/__support/CMakeLists.txt (+15)
- (added) libc/test/integration/src/__support/mpmc_stack_test.cpp (+119)
``````````diff
diff --git a/libc/src/__support/CMakeLists.txt b/libc/src/__support/CMakeLists.txt
index 2196d9e23bba7..c9d89cf6fc286 100644
--- a/libc/src/__support/CMakeLists.txt
+++ b/libc/src/__support/CMakeLists.txt
@@ -398,6 +398,30 @@ add_header_library(
libc.src.__support.macros.attributes
)
+add_header_library(
+ aba_ptr
+ HDRS
+ aba_ptr.h
+ DEPENDS
+ libc.hdr.types.size_t
+ libc.src.__support.common
+ libc.src.__support.threads.sleep
+)
+
+add_header_library(
+ mpmc_stack
+ HDRS
+ mpmc_stack.h
+ DEPENDS
+ libc.src.__support.aba_ptr
+ libc.src.__support.common
+ libc.src.__support.CPP.atomic
+ libc.src.__support.CPP.new
+ libc.src.__support.CPP.optional
+ libc.src.__support.CPP.type_traits
+)
+
+
add_subdirectory(FPUtil)
add_subdirectory(OSUtil)
add_subdirectory(StringUtil)
diff --git a/libc/src/__support/aba_ptr.h b/libc/src/__support/aba_ptr.h
new file mode 100644
index 0000000000000..632cc466c295b
--- /dev/null
+++ b/libc/src/__support/aba_ptr.h
@@ -0,0 +1,83 @@
+//===-- Transactional Ptr for ABA prevention --------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC___SUPPORT_TAGGED_POINTER_H
+#define LLVM_LIBC_SRC___SUPPORT_TAGGED_POINTER_H
+
+#include "hdr/types/size_t.h"
+#include "src/__support/common.h"
+#include "src/__support/threads/sleep.h"
+
+#ifdef __GCC_HAVE_SYNC_COMPARE_AND_SWAP_16
+#define LIBC_ABA_PTR_IS_ATOMIC true
+#else
+#define LIBC_ABA_PTR_IS_ATOMIC false
+#endif
+
+namespace LIBC_NAMESPACE_DECL {
+
+template <class T, bool IsAtomic> struct AbaPtrImpl {
+ union Impl {
+ struct alignas(2 * alignof(void *)) Atomic {
+ T *ptr;
+ size_t tag;
+ } atomic;
+ struct Mutex {
+ T *ptr;
+ bool locked;
+ } mutex;
+ } impl;
+
+ LIBC_INLINE constexpr AbaPtrImpl(T *ptr)
+ : impl(IsAtomic ? Impl{.atomic{ptr, 0}} : Impl{.mutex{ptr, false}}) {}
+
+ /// User must guarantee that operation is redoable.
+ template <class Op> LIBC_INLINE void transaction(Op &&op) {
+ if constexpr (IsAtomic) {
+ for (;;) {
+ typename Impl::Atomic snapshot, next;
+ __atomic_load(&impl.atomic, &snapshot, __ATOMIC_RELAXED);
+ next.ptr = op(snapshot.ptr);
+ // Wrapping add for unsigned integers.
+ next.tag = snapshot.tag + 1;
+ if (__atomic_compare_exchange(&impl.atomic, &snapshot, &next, true,
+ __ATOMIC_ACQ_REL, __ATOMIC_RELAXED))
+ return;
+ }
+ } else {
+ // Acquire the lock.
+ while (__atomic_exchange_n(&impl.mutex.locked, true, __ATOMIC_ACQUIRE))
+ while (__atomic_load_n(&impl.mutex.locked, __ATOMIC_RELAXED))
+ LIBC_NAMESPACE::sleep_briefly();
+
+ impl.mutex.ptr = op(impl.mutex.ptr);
+ // Release the lock.
+ __atomic_store_n(&impl.mutex.locked, false, __ATOMIC_RELEASE);
+ }
+ }
+
+ LIBC_INLINE T *get() const {
+ if constexpr (IsAtomic) {
+ // Weak micro-architectures typically reguards simultaneous partial word
+ // loading and full word loading as a race condition. While there are
+ // implementations that uses racy read anyway, we still load the whole
+ // word to avoid any complications.
+ typename Impl::Atomic snapshot;
+ __atomic_load(&impl.atomic, &snapshot, __ATOMIC_RELAXED);
+ return snapshot.ptr;
+ } else {
+ return impl.mutex.ptr;
+ }
+ }
+};
+
+template <class T> using AbaPtr = AbaPtrImpl<T, LIBC_ABA_PTR_IS_ATOMIC>;
+} // namespace LIBC_NAMESPACE_DECL
+
+#undef LIBC_ABA_PTR_IS_ATOMIC
+#endif
diff --git a/libc/src/__support/mpmc_stack.h b/libc/src/__support/mpmc_stack.h
new file mode 100644
index 0000000000000..df235c2c1dfac
--- /dev/null
+++ b/libc/src/__support/mpmc_stack.h
@@ -0,0 +1,107 @@
+//===-- Simple Lock-free MPMC Stack -----------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC___SUPPORT_MPMC_STACK_H
+#define LLVM_LIBC_SRC___SUPPORT_MPMC_STACK_H
+
+#include "src/__support/CPP/atomic.h"
+#include "src/__support/CPP/new.h"
+#include "src/__support/CPP/optional.h"
+#include "src/__support/aba_ptr.h"
+
+namespace LIBC_NAMESPACE_DECL {
+template <class T> class MPMCStack {
+ struct Node {
+ cpp::Atomic<size_t> visitor;
+ Node *next;
+ T value;
+
+ LIBC_INLINE Node(T val) : visitor(0), next(nullptr), value(val) {}
+ };
+ AbaPtr<Node> head;
+
+public:
+ static_assert(cpp::is_copy_constructible<T>::value,
+ "T must be copy constructible");
+ LIBC_INLINE constexpr MPMCStack() : head(nullptr) {}
+ LIBC_INLINE bool push(T value) {
+ AllocChecker ac;
+ Node *new_node = new (ac) Node(value);
+ if (!ac)
+ return false;
+ head.transaction([new_node](Node *old_head) {
+ new_node->next = old_head;
+ return new_node;
+ });
+ return true;
+ }
+ LIBC_INLINE bool push_all(T values[], size_t count) {
+ struct Guard {
+ size_t count;
+ Node **allocated;
+ LIBC_INLINE Guard(Node *allocated[]) : count(0), allocated(allocated) {}
+ LIBC_INLINE ~Guard() {
+ for (size_t i = 0; i < count; ++i)
+ delete allocated[i];
+ }
+ LIBC_INLINE void add(Node *node) { allocated[count++] = node; }
+ LIBC_INLINE void clear() { count = 0; }
+ };
+ // Variable sized array is a GNU extension.
+ __extension__ Node *allocated[count];
+ {
+ Guard guard(allocated);
+ for (size_t i = 0; i < count; ++i) {
+ AllocChecker ac;
+ Node *new_node = new (ac) Node(values[i]);
+ if (!ac)
+ return false;
+ guard.add(new_node);
+ if (i != 0)
+ new_node->next = allocated[i - 1];
+ }
+ guard.clear();
+ }
+ head.transaction([&allocated, count](Node *old_head) {
+ allocated[0]->next = old_head;
+ return allocated[count - 1];
+ });
+ return true;
+ }
+ LIBC_INLINE cpp::optional<T> pop() {
+ cpp::optional<T> res = cpp::nullopt;
+ Node *node = nullptr;
+ head.transaction([&](Node *current_head) -> Node * {
+ if (!current_head) {
+ res = cpp::nullopt;
+ return nullptr;
+ }
+ node = current_head;
+ node->visitor.fetch_add(1);
+ res = cpp::optional<T>{node->value};
+ Node *next = node->next;
+ node->visitor.fetch_sub(1);
+ return next;
+ });
+ // On a successful transaction, a node is popped by us. So we must delete
+ // it. When we are at here, no one else can acquire
+ // new reference to the node, but we still need to wait until other threads
+ // inside the transaction who may potentially be holding a reference to the
+ // node.
+ if (res) {
+ // Spin until the node is no longer in use.
+ while (node->visitor.load() != 0)
+ LIBC_NAMESPACE::sleep_briefly();
+ delete node;
+ }
+ return res;
+ }
+};
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif
diff --git a/libc/test/integration/src/__support/CMakeLists.txt b/libc/test/integration/src/__support/CMakeLists.txt
index b5b6557e8d689..93f54083f3c00 100644
--- a/libc/test/integration/src/__support/CMakeLists.txt
+++ b/libc/test/integration/src/__support/CMakeLists.txt
@@ -2,3 +2,18 @@ add_subdirectory(threads)
if(LIBC_TARGET_OS_IS_GPU)
add_subdirectory(GPU)
endif()
+
+add_libc_integration_test_suite(libc-support-integration-tests)
+
+add_integration_test(
+ mpmc_stack_test
+ SUITE
+ libc-support-integration-tests
+ SRCS
+ mpmc_stack_test.cpp
+ DEPENDS
+ libc.src.__support.mpmc_stack
+ libc.src.__support.threads.thread
+ libc.src.pthread.pthread_create
+ libc.src.pthread.pthread_join
+)
diff --git a/libc/test/integration/src/__support/mpmc_stack_test.cpp b/libc/test/integration/src/__support/mpmc_stack_test.cpp
new file mode 100644
index 0000000000000..9166a816a74fe
--- /dev/null
+++ b/libc/test/integration/src/__support/mpmc_stack_test.cpp
@@ -0,0 +1,119 @@
+#include "src/__support/CPP/atomic.h"
+#include "src/__support/mpmc_stack.h"
+#include "src/pthread/pthread_create.h"
+#include "src/pthread/pthread_join.h"
+#include "test/IntegrationTest/test.h"
+
+using namespace LIBC_NAMESPACE;
+
+void smoke_test() {
+ MPMCStack<int> stack;
+ for (int i = 0; i <= 100; ++i)
+ if (!stack.push(i))
+ __builtin_trap();
+ for (int i = 100; i >= 0; --i)
+ if (*stack.pop() != i)
+ __builtin_trap();
+ if (stack.pop())
+ __builtin_trap(); // Should be empty now.
+}
+
+void multithread_test() {
+ constexpr static size_t NUM_THREADS = 5;
+ constexpr static size_t NUM_PUSHES = 100;
+ struct State {
+ MPMCStack<size_t> stack;
+ cpp::Atomic<size_t> counter = 0;
+ cpp::Atomic<bool> flags[NUM_PUSHES];
+ } state;
+ pthread_t threads[NUM_THREADS];
+ for (size_t i = 0; i < NUM_THREADS; ++i) {
+ LIBC_NAMESPACE::pthread_create(
+ &threads[i], nullptr,
+ [](void *arg) -> void * {
+ State *state = static_cast<State *>(arg);
+ for (;;) {
+ size_t current = state->counter.fetch_add(1);
+ if (current >= NUM_PUSHES)
+ break;
+ if (!state->stack.push(current))
+ __builtin_trap();
+ }
+ while (auto res = state->stack.pop())
+ state->flags[res.value()].store(true);
+ return nullptr;
+ },
+ &state);
+ }
+ for (pthread_t thread : threads)
+ LIBC_NAMESPACE::pthread_join(thread, nullptr);
+ while (cpp::optional<size_t> res = state.stack.pop())
+ state.flags[res.value()].store(true);
+ for (size_t i = 0; i < NUM_PUSHES; ++i)
+ if (!state.flags[i].load())
+ __builtin_trap();
+}
+
+void multithread_push_all_test() {
+ constexpr static size_t NUM_THREADS = 4;
+ constexpr static size_t BATCH_SIZE = 10;
+ constexpr static size_t NUM_BATCHES = 20;
+ struct State {
+ MPMCStack<size_t> stack;
+ cpp::Atomic<size_t> counter = 0;
+ cpp::Atomic<bool> flags[NUM_THREADS * BATCH_SIZE * NUM_BATCHES];
+ } state;
+ pthread_t threads[NUM_THREADS];
+
+ for (size_t i = 0; i < NUM_THREADS; ++i) {
+ LIBC_NAMESPACE::pthread_create(
+ &threads[i], nullptr,
+ [](void *arg) -> void * {
+ State *state = static_cast<State *>(arg);
+ size_t values[BATCH_SIZE];
+
+ for (size_t batch = 0; batch < NUM_BATCHES; ++batch) {
+ // Prepare batch of values
+ for (size_t j = 0; j < BATCH_SIZE; ++j) {
+ size_t current = state->counter.fetch_add(1);
+ values[j] = current;
+ }
+
+ // Push all values in batch
+ if (!state->stack.push_all(values, BATCH_SIZE))
+ __builtin_trap();
+ }
+
+ // Pop and mark all values
+ while (auto res = state->stack.pop()) {
+ size_t value = res.value();
+ if (value < NUM_THREADS * BATCH_SIZE * NUM_BATCHES)
+ state->flags[value].store(true);
+ }
+ return nullptr;
+ },
+ &state);
+ }
+
+ for (pthread_t thread : threads)
+ LIBC_NAMESPACE::pthread_join(thread, nullptr);
+
+ // Pop any remaining values
+ while (cpp::optional<size_t> res = state.stack.pop()) {
+ size_t value = res.value();
+ if (value < NUM_THREADS * BATCH_SIZE * NUM_BATCHES)
+ state.flags[value].store(true);
+ }
+
+ // Verify all values were processed
+ for (size_t i = 0; i < NUM_THREADS * BATCH_SIZE * NUM_BATCHES; ++i)
+ if (!state.flags[i].load())
+ __builtin_trap();
+}
+
+TEST_MAIN() {
+ smoke_test();
+ multithread_test();
+ multithread_push_all_test();
+ return 0;
+}
``````````
</details>
https://github.com/llvm/llvm-project/pull/151361
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