[libc] avoid cmpxchg on the fastpath of callonce

[SchrodingerZhu]

Avoid cmpxchg operation if the function has already been called.
The destination operand of cmpxchg may receive a write cycle without regard to the result of the comparison

[llvmbot]

@llvm/pr-subscribers-libc

Author: Schrodinger ZHU Yifan (SchrodingerZhu)

Changes

Avoid cmpxchg operation if the function has already been called.
The destination operand of cmpxchg may receive a write cycle without regard to the result of the comparison

---

Full diff: https://github.com/llvm/llvm-project/pull/91748.diff

1 Files Affected:

• (modified) libc/src/__support/threads/linux/callonce.cpp (+7)

diff --git a/libc/src/__support/threads/linux/callonce.cpp b/libc/src/__support/threads/linux/callonce.cpp
index 1c29db5f5c1a1..b48a514a44875 100644
--- a/libc/src/__support/threads/linux/callonce.cpp
+++ b/libc/src/__support/threads/linux/callonce.cpp
@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/__support/threads/callonce.h"
+#include "src/__support/macros/optimization.h"
 #include "src/__support/threads/linux/futex_utils.h"
 
 namespace LIBC_NAMESPACE {
@@ -21,6 +22,12 @@ int callonce(CallOnceFlag *flag, CallOnceCallback *func) {
 
   FutexWordType not_called = NOT_CALLED;
 
+  // Avoid cmpxchg operation if the function has already been called.
+  // The destination operand of cmpxchg may receive a write cycle without
+  // regard to the result of the comparison
+  if (LIBC_LIKELY(futex_word->load(cpp::MemoryOrder::RELAXED) == FINISH))
+    return 0;
+
   // The call_once call can return only after the called function |func|
   // returns. So, we use futexes to synchronize calls with the same flag value.
   if (futex_word->compare_exchange_strong(not_called, START)) {