[libc-commits] [libc] [libc] add a simple TTAS spin lock (PR #98846)

[via libc-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-libc

Author: Schrodinger ZHU Yifan (SchrodingerZhu)

<details>
<summary>Changes</summary>

This is to be used in `pthread_spinlock`.

---
Full diff: https://github.com/llvm/llvm-project/pull/98846.diff


2 Files Affected:

- (modified) libc/src/__support/threads/CMakeLists.txt (+9) 
- (added) libc/src/__support/threads/spin_lock.h (+56) 


``````````diff
diff --git a/libc/src/__support/threads/CMakeLists.txt b/libc/src/__support/threads/CMakeLists.txt
index 9ea0b59befe7a..d2e46b8e2574e 100644
--- a/libc/src/__support/threads/CMakeLists.txt
+++ b/libc/src/__support/threads/CMakeLists.txt
@@ -10,6 +10,15 @@ add_header_library(
     sleep.h
 )
 
+add_header_library(
+  spin_lock
+  HDRS
+    spin_lock.h
+  DEPENDS
+    .sleep
+    libc.src.__support.CPP.atomic
+)
+
 if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/${LIBC_TARGET_OS})
   add_subdirectory(${LIBC_TARGET_OS})
 endif()
diff --git a/libc/src/__support/threads/spin_lock.h b/libc/src/__support/threads/spin_lock.h
new file mode 100644
index 0000000000000..8fd6fb960f3ce
--- /dev/null
+++ b/libc/src/__support/threads/spin_lock.h
@@ -0,0 +1,56 @@
+//===-- TTAS Spin Lock ----------------------------------------------------===//
+//
+// 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_THREADS_SPIN_LOCK_H
+#define LLVM_LIBC_SRC___SUPPORT_THREADS_SPIN_LOCK_H
+
+#include "src/__support/CPP/atomic.h"
+#include "src/__support/macros/attributes.h"
+#include "src/__support/threads/sleep.h"
+namespace LIBC_NAMESPACE_DECL {
+class SpinLock {
+  cpp::Atomic<bool> flag;
+
+public:
+  LIBC_INLINE constexpr SpinLock() : flag{false} {}
+  LIBC_INLINE bool try_lock() {
+    return !flag.exchange(true, cpp::MemoryOrder::ACQUIRE);
+  }
+  LIBC_INLINE void lock() {
+    // clang-format off
+    // this compiles to the following on armv9a and x86_64:
+    //         mov     w8, #1            |          .LBB0_1:
+    // .LBB0_1:                          |                  mov     al, 1
+    //         swpab   w8, w9, [x0]      |                  xchg    byte ptr [rdi], al
+    //         tbnz    w9, #0, .LBB0_3   |                  test    al, 1
+    //         b       .LBB0_4           |                  jne     .LBB0_3
+    // .LBB0_2:                          |                  jmp     .LBB0_4
+    //         isb                       |         .LBB0_2:
+    // .LBB0_3:                          |                  pause
+    //         ldrb    w9, [x0]          |         .LBB0_3:       
+    //         tbnz    w9, #0, .LBB0_2   |                  movzx   eax, byte ptr [rdi]
+    //         b       .LBB0_1           |                  test    al, 1
+    // .LBB0_4:                          |                  jne     .LBB0_2
+    //         ret                       |                  jmp     .LBB0_1
+    //                                   |          .LBB0_4:
+    //                                   |                  ret
+    // clang-format on
+    // Notice that inside the busy loop .LBB0_2 and .LBB0_3, only instructions
+    // with load semantics are used. swpab/xchg is only issued in outer loop
+    // .LBB0_1. This is useful to avoid extra writer traffic. The cache
+    // coherence guarantees "write propagation", so even if the inner loop only
+    // reads with relaxed ordering, the thread will evetually see the write.
+    while (!try_lock())
+      while (flag.load(cpp::MemoryOrder::RELAXED))
+        sleep_briefly();
+  }
+  LIBC_INLINE void unlock() { flag.store(false, cpp::MemoryOrder::RELEASE); }
+};
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC___SUPPORT_THREADS_SPIN_LOCK_H

``````````

</details>


https://github.com/llvm/llvm-project/pull/98846