[llvm] Always use pthread_rwlock on Apple platforms (PR #70151)

[via llvm-commits]

https://github.com/cooperp created https://github.com/llvm/llvm-project/pull/70151

pthread_rwlock is faster than std::shared_mutex, especially in heavily threaded code such as the Swift compiler.

rdar://117445844

>From 161d9beeb2443325d0cae16d50d9dda3245c5f62 Mon Sep 17 00:00:00 2001
From: Peter Cooper <peter_cooper at apple.com>
Date: Tue, 24 Oct 2023 18:07:54 -0700
Subject: [PATCH] Always use pthread_rwlock on Apple platforms.  Its faster
 than std::shared_mutex.

rdar://117445844
---
 llvm/include/llvm/Support/RWMutex.h | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/llvm/include/llvm/Support/RWMutex.h b/llvm/include/llvm/Support/RWMutex.h
index 04cf3eb3d3aa7a7..32987c3b98f1cbc 100644
--- a/llvm/include/llvm/Support/RWMutex.h
+++ b/llvm/include/llvm/Support/RWMutex.h
@@ -19,12 +19,9 @@
 #include <mutex>
 #include <shared_mutex>
 
-// std::shared_timed_mutex is only available on macOS 10.12 and later.
-#if defined(__APPLE__) && defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__)
-#if __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ < 101200
+#if defined(__APPLE__)
 #define LLVM_USE_RW_MUTEX_IMPL
 #endif
-#endif
 
 namespace llvm {
 namespace sys {