10 files changed, 44 insertions, 56 deletions

diff --git a/absl/FAQ.md b/FAQ.md
index af721307c2ec..af721307c2ec 100644
--- a/absl/FAQ.md
+++ b/FAQ.md
diff --git a/absl/abseil.podspec.gen.py b/absl/abseil.podspec.gen.py
index 2bf153c002ab..6aefb794df35 100755
--- a/absl/abseil.podspec.gen.py
+++ b/absl/abseil.podspec.gen.py
@@ -46,18 +46,6 @@ Pod::Spec.new do |s|
   s.watchos.deployment_target = '2.0'
 """
 
-# Limited platforms that abseil supports.
-# This is mainly because of sigaltstack unavailable on watchOS.
-LIMITED_SUPPORT_PLATFORMS = [
-    "ios.deployment_target = '7.0'",
-    "osx.deployment_target = '10.9'",
-]
-
-# Custom specification per rule.
-CUSTOM_SPEC_MAP = {
-    "//absl/debugging:failure_signal_handler": LIMITED_SUPPORT_PLATFORMS,
-}
-
 # Rule object representing the rule of Bazel BUILD.
 Rule = collections.namedtuple(
     "Rule", "type name package srcs hdrs textual_hdrs deps visibility testonly")
@@ -200,12 +188,6 @@ def write_podspec_rule(f, rule, depth):
     name = get_spec_name(dep.replace(":", "/"))
     f.write("{indent}{var}.dependency '{dep}'\n".format(
         indent=indent, var=spec_var, dep=name))
-  # Writes custom specification.
-  custom_spec = CUSTOM_SPEC_MAP.get(rule.package + ":" + rule.name)
-  if custom_spec:
-    for spec in custom_spec:
-      f.write("{indent}{var}.{spec}\n".format(
-          indent=indent, var=spec_var, spec=spec))
 
 
 def write_indented_list(f, leading, values):
diff --git a/absl/base/config.h b/absl/base/config.h
index eac5d268e10a..ee99f94629a2 100644
--- a/absl/base/config.h
+++ b/absl/base/config.h
@@ -316,13 +316,19 @@ static_assert(ABSL_INTERNAL_INLINE_NAMESPACE_STR[0] != 'h' ||
 #error ABSL_HAVE_EXCEPTIONS cannot be directly set.
 
 #elif defined(__clang__)
-// TODO(calabrese)
-// Switch to using __cpp_exceptions when we no longer support versions < 3.6.
-// For details on this check, see:
-//   http://releases.llvm.org/3.6.0/tools/clang/docs/ReleaseNotes.html#the-exceptions-macro
+
+#if __clang_major__ > 3 || (__clang_major__ == 3 && __clang_minor__ >= 6)
+// Clang >= 3.6
+#if __has_feature(cxx_exceptions)
+#define ABSL_HAVE_EXCEPTIONS 1
+#endif  // __has_feature(cxx_exceptions)
+#else
+// Clang < 3.6
+// http://releases.llvm.org/3.6.0/tools/clang/docs/ReleaseNotes.html#the-exceptions-macro
 #if defined(__EXCEPTIONS) && __has_feature(cxx_exceptions)
 #define ABSL_HAVE_EXCEPTIONS 1
 #endif  // defined(__EXCEPTIONS) && __has_feature(cxx_exceptions)
+#endif  // __clang_major__ > 3 || (__clang_major__ == 3 && __clang_minor__ >= 6)
 
 // Handle remaining special cases and default to exceptions being supported.
 #elif !(defined(__GNUC__) && (__GNUC__ < 5) && !defined(__EXCEPTIONS)) &&    \
diff --git a/absl/container/node_hash_set.h b/absl/container/node_hash_set.h
index 0e2dee54aa19..ad54b6dccb50 100644
--- a/absl/container/node_hash_set.h
+++ b/absl/container/node_hash_set.h
@@ -77,7 +77,7 @@ struct NodeHashSetPolicy;
 //
 //   // Create a node hash set of three strings
 //   absl::node_hash_map<std::string, std::string> ducks =
-//     {"huey", "dewey"}, "louie"};
+//     {"huey", "dewey", "louie"};
 //
 //  // Insert a new element into the node hash map
 //  ducks.insert("donald"};
@@ -111,7 +111,7 @@ class node_hash_set
   // * Initializer List constructor
   //
   //   absl::node_hash_set<std::string> set2 =
-  //       {{"huey"}, {"dewey"}, {"louie"},};
+  //       {{"huey"}, {"dewey"}, {"louie"}};
   //
   // * Copy constructor
   //
diff --git a/absl/debugging/failure_signal_handler.cc b/absl/debugging/failure_signal_handler.cc
index 470d676812c2..cd141ecf5d32 100644
--- a/absl/debugging/failure_signal_handler.cc
+++ b/absl/debugging/failure_signal_handler.cc
@@ -24,6 +24,10 @@
 #include <unistd.h>
 #endif
 
+#ifdef __APPLE__
+#include <TargetConditionals.h>
+#endif
+
 #ifdef ABSL_HAVE_MMAP
 #include <sys/mman.h>
 #endif
@@ -44,6 +48,11 @@
 
 #ifndef _WIN32
 #define ABSL_HAVE_SIGACTION
+// Apple WatchOS and TVOS don't allow sigaltstack
+#if !(defined(TARGET_OS_WATCH) && TARGET_OS_WATCH) && \
+    !(defined(TARGET_OS_TV) && TARGET_OS_TV)
+#define ABSL_HAVE_SIGALTSTACK
+#endif
 #endif
 
 namespace absl {
@@ -117,7 +126,7 @@ const char* FailureSignalToString(int signo) {
 
 }  // namespace debugging_internal
 
-#ifndef _WIN32
+#ifdef ABSL_HAVE_SIGALTSTACK
 
 static bool SetupAlternateStackOnce() {
 #if defined(__wasm__) || defined (__asjms__)
@@ -169,7 +178,7 @@ static bool SetupAlternateStackOnce() {
 // Returns the appropriate flag for sig_action.sa_flags
 // if the system supports using an alternate stack.
 static int MaybeSetupAlternateStack() {
-#ifndef _WIN32
+#ifdef ABSL_HAVE_SIGALTSTACK
   ABSL_ATTRIBUTE_UNUSED static const bool kOnce = SetupAlternateStackOnce();
   return SA_ONSTACK;
 #else
diff --git a/absl/flags/BUILD.bazel b/absl/flags/BUILD.bazel
index d2ca5c6f8317..cdb4e7e8fe78 100644
--- a/absl/flags/BUILD.bazel
+++ b/absl/flags/BUILD.bazel
@@ -41,6 +41,7 @@ cc_library(
         ":config",
         ":handle",
         ":registry",
+        "//absl/base",
         "//absl/base:config",
         "//absl/base:core_headers",
         "//absl/memory",
diff --git a/absl/flags/CMakeLists.txt b/absl/flags/CMakeLists.txt
index 20e66825a0aa..1d25f0ded8b1 100644
--- a/absl/flags/CMakeLists.txt
+++ b/absl/flags/CMakeLists.txt
@@ -27,6 +27,7 @@ absl_cc_library(
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::base
     absl::config
     absl::flags_config
     absl::flags_handle
diff --git a/absl/flags/internal/flag.cc b/absl/flags/internal/flag.cc
index ba70da91af3b..721e411e0875 100644
--- a/absl/flags/internal/flag.cc
+++ b/absl/flags/internal/flag.cc
@@ -80,41 +80,22 @@ class MutexRelock {
   absl::Mutex* mu_;
 };
 
-// This global lock guards the initialization and destruction of data_guard_,
-// which is used to guard the other Flag data.
-ABSL_CONST_INIT static absl::Mutex flag_mutex_lifetime_guard(absl::kConstInit);
-
 }  // namespace
 
 void FlagImpl::Init() {
-  {
-    absl::MutexLock lock(&flag_mutex_lifetime_guard);
-
-    // Must initialize data guard for this flag.
-    if (!is_data_guard_inited_) {
-      new (&data_guard_) absl::Mutex;
-      is_data_guard_inited_ = true;
-    }
-  }
+  new (&data_guard_) absl::Mutex;
 
   absl::MutexLock lock(reinterpret_cast<absl::Mutex*>(&data_guard_));
 
-  if (value_.dynamic != nullptr) {
-    inited_.store(true, std::memory_order_release);
-  } else {
-    // Need to initialize cur field.
-    value_.dynamic = MakeInitValue().release();
-    StoreAtomic();
-    inited_.store(true, std::memory_order_release);
-  }
+  value_.dynamic = MakeInitValue().release();
+  StoreAtomic();
 }
 
 // Ensures that the lazily initialized data is initialized,
 // and returns pointer to the mutex guarding flags data.
 absl::Mutex* FlagImpl::DataGuard() const {
-  if (ABSL_PREDICT_FALSE(!inited_.load(std::memory_order_acquire))) {
-    const_cast<FlagImpl*>(this)->Init();
-  }
+  absl::call_once(const_cast<FlagImpl*>(this)->init_control_, &FlagImpl::Init,
+                  const_cast<FlagImpl*>(this));
 
   // data_guard_ is initialized.
   return reinterpret_cast<absl::Mutex*>(&data_guard_);
diff --git a/absl/flags/internal/flag.h b/absl/flags/internal/flag.h
index ef30a22f53b8..b426ccb571a6 100644
--- a/absl/flags/internal/flag.h
+++ b/absl/flags/internal/flag.h
@@ -24,6 +24,7 @@
 #include <string>
 #include <type_traits>
 
+#include "absl/base/call_once.h"
 #include "absl/base/config.h"
 #include "absl/base/thread_annotations.h"
 #include "absl/flags/config.h"
@@ -281,10 +282,8 @@ class FlagImpl {
         help_(help.source),
         help_source_kind_(static_cast<uint8_t>(help.kind)),
         def_kind_(static_cast<uint8_t>(FlagDefaultKind::kGenFunc)),
-        is_data_guard_inited_(false),
         modified_(false),
         on_command_line_(false),
-        inited_(false),
         counter_(0),
         callback_(nullptr),
         default_src_(default_value_gen),
@@ -406,20 +405,28 @@ class FlagImpl {
   // Indicates if help message was supplied as literal or generator func.
   const uint8_t help_source_kind_ : 1;
 
+  // ------------------------------------------------------------------------
+  // The bytes containing the const bitfields must not be shared with bytes
+  // containing the mutable bitfields.
+  // ------------------------------------------------------------------------
+
+  // Unique tag for absl::call_once call to initialize this flag.
+  //
+  // The placement of this variable between the immutable and mutable bitfields
+  // is important as prevents them from occupying the same byte. If you remove
+  // this variable, make sure to maintain this property.
+  absl::once_flag init_control_;
+
   // Mutable flag's state (guarded by `data_guard_`).
 
   // If def_kind_ == kDynamicValue, default_src_ holds a dynamically allocated
   // value.
   uint8_t def_kind_ : 1 ABSL_GUARDED_BY(*DataGuard());
-  // Protects against multiple concurrent constructions of `data_guard_`.
-  bool is_data_guard_inited_ : 1;
   // Has this flag's value been modified?
   bool modified_ : 1 ABSL_GUARDED_BY(*DataGuard());
   // Has this flag been specified on command line.
   bool on_command_line_ : 1 ABSL_GUARDED_BY(*DataGuard());
 
-  // Indicates that the flag state is initialized.
-  std::atomic<bool> inited_;
   // Mutation counter
   int64_t counter_ ABSL_GUARDED_BY(*DataGuard());
   // Optional flag's callback and absl::Mutex to guard the invocations.
diff --git a/ci/macos_xcode_cmake.sh b/ci/macos_xcode_cmake.sh
index 75b0f2d1909e..a1f4a857be3f 100755
--- a/ci/macos_xcode_cmake.sh
+++ b/ci/macos_xcode_cmake.sh
@@ -36,6 +36,7 @@ for compilation_mode in ${ABSL_CMAKE_BUILD_TYPES}; do
   time cmake ${ABSEIL_ROOT} \
     -GXcode \
     -DCMAKE_BUILD_TYPE=${compilation_mode} \
+    -DCMAKE_CXX_STANDARD=11 \
     -DABSL_USE_GOOGLETEST_HEAD=ON \
     -DABSL_RUN_TESTS=ON
   time cmake --build .