From 71e08c702b0497b350830febdbfee05e2445ea9d Mon Sep 17 00:00:00 2001
From: Rasmus Munk Larsen <rmlarsen@google.com>
Date: Sat, 12 Sep 2020 01:23:29 +0000
Subject: Make blueNorm threadsafe if C++11 atomics are available.

---
 Eigen/src/Core/StableNorm.h | 32 +++++++++++++++++++++++++++++---
 1 file changed, 29 insertions(+), 3 deletions(-)

(limited to 'Eigen/src/Core/StableNorm.h')

diff --git a/Eigen/src/Core/StableNorm.h b/Eigen/src/Core/StableNorm.h
index 77ea3c261..8000f9d33 100644
--- a/Eigen/src/Core/StableNorm.h
+++ b/Eigen/src/Core/StableNorm.h
@@ -10,6 +10,10 @@
 #ifndef EIGEN_STABLENORM_H
 #define EIGEN_STABLENORM_H
 
+#if EIGEN_HAS_CXX11_ATOMIC
+#include <atomic>
+#endif
+
 namespace Eigen { 
 
 namespace internal {
@@ -124,10 +128,29 @@ blueNorm_impl(const EigenBase<Derived>& _vec)
   using std::sqrt;
   using std::abs;
   const Derived& vec(_vec.derived());
-  static bool initialized = false;
+
+#if EIGEN_HAS_CXX11_ATOMIC
+  enum {
+    kNotInitialized = 0,
+    kInitializing = 1,
+    kInitialized = 2
+  };
+  static std::atomic<uint8_t> initialized{kNotInitialized};
+#else
+  // Note: This is not theadsafe without C++11.
+  const bool kNotInitialized = false;
+  const bool kInitialized = true;
+  static bool initialized = kNotInitialized;
+#endif
   static RealScalar b1, b2, s1m, s2m, rbig, relerr;
-  if(!initialized)
+
+  while (initialized != kInitialized)
   {
+#if EIGEN_HAS_CXX11_ATOMIC
+    // Checking again to see if another thread has already initialized the constants.
+    uint8_t val = kNotInitialized;
+    if (initialized.compare_exchange_strong(val, kInitializing) && val != kInitialized) {
+#endif
     int ibeta, it, iemin, iemax, iexp;
     RealScalar eps;
     // This program calculates the machine-dependent constants
@@ -156,7 +179,10 @@ blueNorm_impl(const EigenBase<Derived>& _vec)
 
     eps     = RealScalar(pow(double(ibeta), 1-it));
     relerr  = sqrt(eps);                                            // tolerance for neglecting asml
-    initialized = true;
+    initialized = kInitialized;
+#if EIGEN_HAS_CXX11_ATOMIC
+    }
+#endif
   }
   Index n = vec.size();
   RealScalar ab2 = b2 / RealScalar(n);
-- 
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