12 files changed, 166 insertions, 12 deletions
diff --git a/absl/base/BUILD.bazel b/absl/base/BUILD.bazel
index 57caa0656e59..1e1f0d2de318 100644
--- a/absl/base/BUILD.bazel
+++ b/absl/base/BUILD.bazel
@@ -207,6 +207,19 @@ cc_library(
     ],
 )
 
+cc_library(
+    name = "atomic_hook_test_helper",
+    testonly = 1,
+    srcs = ["internal/atomic_hook_test_helper.cc"],
+    hdrs = ["internal/atomic_hook_test_helper.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":atomic_hook",
+        ":core_headers",
+    ],
+)
+
 cc_test(
     name = "atomic_hook_test",
     size = "small",
@@ -215,6 +228,7 @@ cc_test(
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":atomic_hook",
+        ":atomic_hook_test_helper",
         ":core_headers",
         "@com_google_googletest//:gtest_main",
     ],
diff --git a/absl/base/CMakeLists.txt b/absl/base/CMakeLists.txt
index 2f11ef86556f..51705a5a102c 100644
--- a/absl/base/CMakeLists.txt
+++ b/absl/base/CMakeLists.txt
@@ -254,6 +254,19 @@ absl_cc_test(
     gtest_main
 )
 
+absl_cc_library(
+  NAME
+    atomic_hook_test_helper
+  SRCS
+    "internal/atomic_hook_test_helper.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::atomic_hook
+    absl::core_headers
+  TESTONLY
+)
+
 absl_cc_test(
   NAME
     atomic_hook_test
@@ -262,8 +275,10 @@ absl_cc_test(
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
+    absl::atomic_hook_test_helper
     absl::atomic_hook
     absl::core_headers
+    gmock
     gtest_main
 )
 
diff --git a/absl/base/internal/atomic_hook.h b/absl/base/internal/atomic_hook.h
index 803e9059761c..09f763d0742b 100644
--- a/absl/base/internal/atomic_hook.h
+++ b/absl/base/internal/atomic_hook.h
@@ -11,7 +11,6 @@
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.
-//
 
 #ifndef ABSL_BASE_INTERNAL_ATOMIC_HOOK_H_
 #define ABSL_BASE_INTERNAL_ATOMIC_HOOK_H_
@@ -23,8 +22,10 @@
 
 #ifdef _MSC_FULL_VER
 #define ABSL_HAVE_WORKING_ATOMIC_POINTER 0
+#define ABSL_HAVE_WORKING_CONSTEXPR_STATIC_INIT 0
 #else
 #define ABSL_HAVE_WORKING_ATOMIC_POINTER 1
+#define ABSL_HAVE_WORKING_CONSTEXPR_STATIC_INIT 1
 #endif
 
 namespace absl {
@@ -33,16 +34,17 @@ namespace base_internal {
 template <typename T>
 class AtomicHook;
 
-// AtomicHook is a helper class, templatized on a raw function pointer type, for
-// implementing Abseil customization hooks.  It is a callable object that
-// dispatches to the registered hook.
+// `AtomicHook` is a helper class, templatized on a raw function pointer type,
+// for implementing Abseil customization hooks.  It is a callable object that
+// dispatches to the registered hook.  Objects of type `AtomicHook` must have
+// static or thread storage duration.
 //
 // A default constructed object performs a no-op (and returns a default
 // constructed object) if no hook has been registered.
 //
 // Hooks can be pre-registered via constant initialization, for example,
-// ABSL_CONST_INIT static AtomicHook<void(*)()> my_hook(DefaultAction);
-// and then changed at runtime via a call to Store().
+// `ABSL_CONST_INIT static AtomicHook<void(*)()> my_hook(DefaultAction);`
+// and then changed at runtime via a call to `Store()`.
 //
 // Reads and writes guarantee memory_order_acquire/memory_order_release
 // semantics.
@@ -57,12 +59,19 @@ class AtomicHook<ReturnType (*)(Args...)> {
 
   // Constructs an object that by default dispatches to/returns the
   // pre-registered default_fn when no hook has been registered at runtime.
-#if ABSL_HAVE_WORKING_ATOMIC_POINTER
+#if ABSL_HAVE_WORKING_ATOMIC_POINTER && ABSL_HAVE_WORKING_CONSTEXPR_STATIC_INIT
   explicit constexpr AtomicHook(FnPtr default_fn)
       : hook_(default_fn), default_fn_(default_fn) {}
 #else
+  // On MSVC, this function sometimes executes after dynamic initiazliation =(.
+  // If a non-zero `hook_` has been installed by a dynamic initializer, we want
+  // to preserve it.  If not, `hook_` will be zero initialized and we have no
+  // need to set it to `kUninitialized`.
+  // https://developercommunity.visualstudio.com/content/problem/336946/class-with-constexpr-constructor-not-using-static.html
   explicit constexpr AtomicHook(FnPtr default_fn)
-      : hook_(kUninitialized), default_fn_(default_fn) {}
+      : /* hook_(deliberately omitted), */ default_fn_(default_fn) {
+    static_assert(kUninitialized == 0, "here we rely on zero-initialization");
+  }
 #endif
 
   // Stores the provided function pointer as the value for this hook.
@@ -158,6 +167,7 @@ class AtomicHook<ReturnType (*)(Args...)> {
 };
 
 #undef ABSL_HAVE_WORKING_ATOMIC_POINTER
+#undef ABSL_HAVE_WORKING_CONSTEXPR_STATIC_INIT
 
 }  // namespace base_internal
 }  // namespace absl
diff --git a/absl/base/internal/atomic_hook_test.cc b/absl/base/internal/atomic_hook_test.cc
index ecc80406b003..794072ee2fc2 100644
--- a/absl/base/internal/atomic_hook_test.cc
+++ b/absl/base/internal/atomic_hook_test.cc
@@ -14,11 +14,15 @@
 
 #include "absl/base/internal/atomic_hook.h"
 
+#include "gmock/gmock.h"
 #include "gtest/gtest.h"
 #include "absl/base/attributes.h"
+#include "absl/base/internal/atomic_hook_test_helper.h"
 
 namespace {
 
+using ::testing::Eq;
+
 int value = 0;
 void TestHook(int x) { value = x; }
 
@@ -67,4 +71,24 @@ TEST(AtomicHookTest, WithDefaultFunction) {
   EXPECT_EQ(value, 2);
 }
 
+ABSL_CONST_INIT int override_func_calls = 0;
+void OverrideFunc() { override_func_calls++; }
+static struct OverrideInstaller {
+  OverrideInstaller() { absl::atomic_hook_internal::func.Store(OverrideFunc); }
+} override_installer;
+
+TEST(AtomicHookTest, DynamicInitFromAnotherTU) {
+  // MSVC 14.2 doesn't do constexpr static init correctly; in particular it
+  // tends to sequence static init (i.e. defaults) of `AtomicHook` objects
+  // after their dynamic init (i.e. overrides), overwriting whatever value was
+  // written during dynamic init.  This regression test validates the fix.
+  // https://developercommunity.visualstudio.com/content/problem/336946/class-with-constexpr-constructor-not-using-static.html
+  EXPECT_THAT(absl::atomic_hook_internal::default_func_calls, Eq(0));
+  EXPECT_THAT(override_func_calls, Eq(0));
+  absl::atomic_hook_internal::func();
+  EXPECT_THAT(absl::atomic_hook_internal::default_func_calls, Eq(0));
+  EXPECT_THAT(override_func_calls, Eq(1));
+  EXPECT_THAT(absl::atomic_hook_internal::func.Load(), Eq(OverrideFunc));
+}
+
 }  // namespace
diff --git a/absl/base/internal/atomic_hook_test_helper.cc b/absl/base/internal/atomic_hook_test_helper.cc
new file mode 100644
index 000000000000..fad7a89a6ffa
--- /dev/null
+++ b/absl/base/internal/atomic_hook_test_helper.cc
@@ -0,0 +1,29 @@
+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/internal/atomic_hook_test_helper.h"
+
+#include "absl/base/attributes.h"
+#include "absl/base/internal/atomic_hook.h"
+
+namespace absl {
+namespace atomic_hook_internal {
+
+ABSL_CONST_INIT absl::base_internal::AtomicHook<VoidF> func(DefaultFunc);
+ABSL_CONST_INIT int default_func_calls = 0;
+void DefaultFunc() { default_func_calls++; }
+void RegisterFunc(VoidF f) { func.Store(f); }
+
+}  // namespace atomic_hook_internal
+}  // namespace absl
diff --git a/absl/base/internal/atomic_hook_test_helper.h b/absl/base/internal/atomic_hook_test_helper.h
new file mode 100644
index 000000000000..44ff780d6b93
--- /dev/null
+++ b/absl/base/internal/atomic_hook_test_helper.h
@@ -0,0 +1,32 @@
+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_BASE_ATOMIC_HOOK_TEST_HELPER_H_
+#define ABSL_BASE_ATOMIC_HOOK_TEST_HELPER_H_
+
+#include "absl/base/internal/atomic_hook.h"
+
+namespace absl {
+namespace atomic_hook_internal {
+
+using VoidF = void (*)();
+extern absl::base_internal::AtomicHook<VoidF> func;
+extern int default_func_calls;
+void DefaultFunc();
+void RegisterFunc(VoidF func);
+
+}  // namespace atomic_hook_internal
+}  // namespace absl
+
+#endif  // ABSL_BASE_ATOMIC_HOOK_TEST_HELPER_H_
diff --git a/absl/container/internal/hashtablez_sampler.cc b/absl/container/internal/hashtablez_sampler.cc
index d03dd82e7fd5..054e89817815 100644
--- a/absl/container/internal/hashtablez_sampler.cc
+++ b/absl/container/internal/hashtablez_sampler.cc
@@ -228,8 +228,26 @@ int64_t HashtablezSampler::Iterate(
   return dropped_samples_.load(std::memory_order_relaxed);
 }
 
+static bool ShouldForceSampling() {
+  enum ForceState {
+    kDontForce,
+    kForce,
+    kUninitialized
+  };
+  ABSL_CONST_INIT static std::atomic<ForceState> global_state{
+      kUninitialized};
+  ForceState state = global_state.load(std::memory_order_relaxed);
+  if (ABSL_PREDICT_TRUE(state == kDontForce)) return false;
+
+  if (state == kUninitialized) {
+    state = AbslContainerInternalSampleEverything() ? kForce : kDontForce;
+    global_state.store(state, std::memory_order_relaxed);
+  }
+  return state == kForce;
+}
+
 HashtablezInfo* SampleSlow(int64_t* next_sample) {
-  if (kAbslContainerInternalSampleEverything) {
+  if (ABSL_PREDICT_FALSE(ShouldForceSampling())) {
     *next_sample = 1;
     return HashtablezSampler::Global().Register();
   }
diff --git a/absl/container/internal/hashtablez_sampler.h b/absl/container/internal/hashtablez_sampler.h
index 4112175449ef..53996bb9fa4c 100644
--- a/absl/container/internal/hashtablez_sampler.h
+++ b/absl/container/internal/hashtablez_sampler.h
@@ -280,7 +280,7 @@ void SetHashtablezMaxSamples(int32_t max);
 // initialization of static storage duration objects.
 // The definition of this constant is weak, which allows us to inject a
 // different value for it at link time.
-extern "C" const bool kAbslContainerInternalSampleEverything;
+extern "C" bool AbslContainerInternalSampleEverything();
 
 }  // namespace container_internal
 }  // namespace absl
diff --git a/absl/container/internal/hashtablez_sampler_force_weak_definition.cc b/absl/container/internal/hashtablez_sampler_force_weak_definition.cc
index 4ca6ffdadd7d..984dce5dd992 100644
--- a/absl/container/internal/hashtablez_sampler_force_weak_definition.cc
+++ b/absl/container/internal/hashtablez_sampler_force_weak_definition.cc
@@ -20,8 +20,9 @@ namespace absl {
 namespace container_internal {
 
 // See hashtablez_sampler.h for details.
-extern "C" ABSL_ATTRIBUTE_WEAK const bool
-    kAbslContainerInternalSampleEverything = false;
+extern "C" ABSL_ATTRIBUTE_WEAK bool AbslContainerInternalSampleEverything() {
+  return false;
+}
 
 }  // namespace container_internal
 }  // namespace absl
diff --git a/absl/random/exponential_distribution_test.cc b/absl/random/exponential_distribution_test.cc
index dc49044d3b76..f3cfd76442c4 100644
--- a/absl/random/exponential_distribution_test.cc
+++ b/absl/random/exponential_distribution_test.cc
@@ -46,7 +46,11 @@ using absl::random_internal::kChiSquared;
 template <typename RealType>
 class ExponentialDistributionTypedTest : public ::testing::Test {};
 
+#if defined(__EMSCRIPTEN__)
+using RealTypes = ::testing::Types<float, double>;
+#else
 using RealTypes = ::testing::Types<float, double, long double>;
+#endif  // defined(__EMSCRIPTEN__)
 TYPED_TEST_CASE(ExponentialDistributionTypedTest, RealTypes);
 
 TYPED_TEST(ExponentialDistributionTypedTest, SerializeTest) {
diff --git a/absl/random/internal/iostream_state_saver_test.cc b/absl/random/internal/iostream_state_saver_test.cc
index 2ecbaac143dc..722766d0ff6b 100644
--- a/absl/random/internal/iostream_state_saver_test.cc
+++ b/absl/random/internal/iostream_state_saver_test.cc
@@ -272,6 +272,7 @@ TEST(IOStreamStateSaver, RoundTripDoubles) {
   }
 }
 
+#if !defined(__EMSCRIPTEN__)
 TEST(IOStreamStateSaver, RoundTripLongDoubles) {
   // Technically, C++ only guarantees that long double is at least as large as a
   // double.  Practically it varies from 64-bits to 128-bits.
@@ -349,6 +350,7 @@ TEST(IOStreamStateSaver, RoundTripLongDoubles) {
     }
   }
 }
+#endif  // !defined(__EMSCRIPTEN__)
 
 TEST(StrToDTest, DoubleMin) {
   const char kV[] = "2.22507385850720138e-308";
diff --git a/absl/random/uniform_real_distribution_test.cc b/absl/random/uniform_real_distribution_test.cc
index 597f0ee5efa5..9f14d1c58985 100644
--- a/absl/random/uniform_real_distribution_test.cc
+++ b/absl/random/uniform_real_distribution_test.cc
@@ -54,7 +54,12 @@ namespace {
 template <typename RealType>
 class UniformRealDistributionTest : public ::testing::Test {};
 
+#if defined(__EMSCRIPTEN__)
+using RealTypes = ::testing::Types<float, double>;
+#else
 using RealTypes = ::testing::Types<float, double, long double>;
+#endif  // defined(__EMSCRIPTEN__)
+
 TYPED_TEST_SUITE(UniformRealDistributionTest, RealTypes);
 
 TYPED_TEST(UniformRealDistributionTest, ParamSerializeTest) {