[llvm] [Support] Restrict ManagedStatic ThreadPoolExecutor to Windows (PR #102989)

[Fangrui Song via llvm-commits]

https://github.com/MaskRay created https://github.com/llvm/llvm-project/pull/102989

https://reviews.llvm.org/D70447 switched to `ManagedStatic` to work
around race conditions in MSVC runtimes and the MinGW runtime.
However, `ManagedStatic` is not suitable for other platforms.

However, this workaround is not suitable for other platforms (#66974).
lld::fatal calls exitLld(1), which calls `llvm_shutdown` to destroy
`ManagedStatic` objects. The worker threads will finish and invoke TLS
destructors (glibc `__call_tls_dtors`). This can lead to race conditions
if other threads attempt to access TLS objects that have already been
destroyed.

While lld's early exit mechanism needs more work, I believe Parallel.cpp
should avoid this pitfall as well.


>From 096f5ca2e30af10ae2dfeab31c1fd2c6c872f1d7 Mon Sep 17 00:00:00 2001
From: Fangrui Song <i at maskray.me>
Date: Mon, 12 Aug 2024 17:31:56 -0700
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Created using spr 1.3.5-bogner
---
 llvm/lib/Support/Parallel.cpp | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/llvm/lib/Support/Parallel.cpp b/llvm/lib/Support/Parallel.cpp
index af35947192c0db..849041f0e21505 100644
--- a/llvm/lib/Support/Parallel.cpp
+++ b/llvm/lib/Support/Parallel.cpp
@@ -156,6 +156,7 @@ class ThreadPoolExecutor : public Executor {
 };
 
 Executor *Executor::getDefaultExecutor() {
+#ifdef _WIN32
   // The ManagedStatic enables the ThreadPoolExecutor to be stopped via
   // llvm_shutdown() which allows a "clean" fast exit, e.g. via _exit(). This
   // stops the thread pool and waits for any worker thread creation to complete
@@ -179,6 +180,14 @@ Executor *Executor::getDefaultExecutor() {
       ManagedExec;
   static std::unique_ptr<ThreadPoolExecutor> Exec(&(*ManagedExec));
   return Exec.get();
+#else
+  // ManagedStatic is not desired on other platforms. When `Exec` is terminated
+  // by llvm_shutdown(), worker threads will clean up and invoke TLS
+  // destructors. This can lead to race conditions if other threads attempt to
+  // access TLS objects that have already been destroyed.
+  static ThreadPoolExecutor Exec(strategy);
+  return &Exec;
+#endif
 }
 } // namespace
 } // namespace detail