[llvm] 1b43650 - [Support] Fix data race with “safe libc++”

[Vitaly Buka via llvm-commits]

Author: Vitaly Buka
Date: 2023-02-21T18:43:37-08:00
New Revision: 1b43650c419fd7448af1a9a097e062d1f64ca485

URL: https://github.com/llvm/llvm-project/commit/1b43650c419fd7448af1a9a097e062d1f64ca485
DIFF: https://github.com/llvm/llvm-project/commit/1b43650c419fd7448af1a9a097e062d1f64ca485.diff

LOG: [Support] Fix data race with “safe libc++”

Otherwise NFC.

Added: 
    

Modified: 
    llvm/lib/Support/Parallel.cpp

Removed: 
    


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diff  --git a/llvm/lib/Support/Parallel.cpp b/llvm/lib/Support/Parallel.cpp
index c256d256be4f..f762e7abc833 100644
--- a/llvm/lib/Support/Parallel.cpp
+++ b/llvm/lib/Support/Parallel.cpp
@@ -55,7 +55,10 @@ class ThreadPoolExecutor : public Executor {
     Threads.reserve(ThreadCount);
     Threads.resize(1);
     std::lock_guard<std::mutex> Lock(Mutex);
-    Threads[0] = std::thread([this, ThreadCount, S] {
+    // Use operator[] before creating the thread to avoid data race in .size()
+    // in “safe libc++” mode.
+    auto& Thread0 = Threads[0];
+    Thread0 = std::thread([this, ThreadCount, S] {
       for (unsigned I = 1; I < ThreadCount; ++I) {
         Threads.emplace_back([=] { work(S, I); });
         if (Stop)