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[libc] avoid cmpxchg on the fastpath of callonce #91748

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Merged
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May 10, 2024
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[libc] avoid cmpxchg on the fastpath of callonce #91748

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7 changes: 7 additions & 0 deletions libc/src/__support/threads/linux/callonce.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -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 {
Expand All @@ -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)) {
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