Export of internal Abseil changes

--
9beb68204986a015c9cb065b9fae4f9a8879a788 by Abseil Team <absl-team@google.com>:

Move Base64EscapeInternal and CalculateBase64EscapedLenInternal to an internal header.

PiperOrigin-RevId: 288917378

--
90acfbe03b3f9f6de3ffa49c39343dfaa2c5d38c by Greg Falcon <gfalcon@google.com>:

Update macos CI script to support the ALTERNATE_OPTIONS environment variable.

PiperOrigin-RevId: 288913564

--
f1572e870678cdcda6b48cb39780d1ad984e4c1b by Derek Mauro <dmauro@google.com>:

Makes absl::NullSafeStringView constexpr
Fixes https://github.com/abseil/abseil-cpp/issues/583

PiperOrigin-RevId: 288906940

--
d28a8471e32c10caa64bfffe6d6d4d0a8d144013 by Abseil Team <absl-team@google.com>:

absl::GetFlag is lock free for small trivially copyable types.

PiperOrigin-RevId: 288768172

--
2643b8ed1a1dc836b38ab9e46538a1af129ffd67 by Gennadiy Rozental <rogeeff@google.com>:

Eliminate call to callback from flag initialization.

We do not need to have this invocation inside FlagImpl::Init since SetCallback performs invocation anyways. Calling InitCallback from inside of Init complicates separation of value initialization from data guard initialization, which is about to happen.

PiperOrigin-RevId: 288732526

--
22caa880b7a4cb6da34e16a2e064a473c99e880b by Abseil Team <absl-team@google.com>:

Fix the documentation on how to create a null string_view.

PiperOrigin-RevId: 288727968

--
10727c5cadc561837141176f4c9b9717cec9233a by Greg Falcon <gfalcon@google.com>:

Change CI scripts for gcc to use the ALTERNATE_OPTIONS file as well.

PiperOrigin-RevId: 288718855

--
5d1e2dd6c7fb12af8aa4337a0f61872f5f0c5992 by Greg Falcon <gfalcon@google.com>:

Add an option for using inline namespaces in Abseil.

PiperOrigin-RevId: 288614491
GitOrigin-RevId: 9beb68204986a015c9cb065b9fae4f9a8879a788
Change-Id: If9acd46301e3df8cb231b4c16f7ed651bf4fb3c3

19 files changed, 527 insertions, 255 deletions

diff --git a/absl/base/config.h b/absl/base/config.h
index 87f5b4a..edbf224 100644
--- a/absl/base/config.h
+++ b/absl/base/config.h
@@ -90,8 +90,42 @@
 // not support forward declarations of its own types, nor does it support
 // user-provided specialization of Abseil templates.  Code that violates these
 // rules may be broken without warning.)
+#if !defined(ABSL_OPTION_USE_INLINE_NAMESPACE) || \
+    !defined(ABSL_OPTION_INLINE_NAMESPACE_NAME)
+#error options.h is misconfigured.
+#endif
+
+// Check that ABSL_OPTION_INLINE_NAMESPACE_NAME is neither "head" nor ""
+#if defined(__cplusplus) && ABSL_OPTION_USE_INLINE_NAMESPACE == 1
+
+#define ABSL_INTERNAL_DO_TOKEN_STR(x) #x
+#define ABSL_INTERNAL_TOKEN_STR(x) ABSL_INTERNAL_DO_TOKEN_STR(x)
+#define ABSL_INTERNAL_INLINE_NAMESPACE_STR \
+  ABSL_INTERNAL_TOKEN_STR(ABSL_OPTION_INLINE_NAMESPACE_NAME)
+
+static_assert(ABSL_INTERNAL_INLINE_NAMESPACE_STR[0] != '\0',
+              "options.h misconfigured: ABSL_OPTION_INLINE_NAMESPACE_NAME must "
+              "not be empty.");
+static_assert(ABSL_INTERNAL_INLINE_NAMESPACE_STR[0] != 'h' ||
+                  ABSL_INTERNAL_INLINE_NAMESPACE_STR[1] != 'e' ||
+                  ABSL_INTERNAL_INLINE_NAMESPACE_STR[2] != 'a' ||
+                  ABSL_INTERNAL_INLINE_NAMESPACE_STR[3] != 'd' ||
+                  ABSL_INTERNAL_INLINE_NAMESPACE_STR[4] != '\0',
+              "options.h misconfigured: ABSL_OPTION_INLINE_NAMESPACE_NAME must "
+              "be changed to a new, unique identifier name.");
+
+#endif
+
+#if ABSL_OPTION_USE_INLINE_NAMESPACE == 0
 #define ABSL_NAMESPACE_BEGIN
 #define ABSL_NAMESPACE_END
+#elif ABSL_OPTION_USE_INLINE_NAMESPACE == 1
+#define ABSL_NAMESPACE_BEGIN \
+  inline namespace ABSL_OPTION_INLINE_NAMESPACE_NAME {
+#define ABSL_NAMESPACE_END }
+#else
+#error options.h is misconfigured.
+#endif
 
 // -----------------------------------------------------------------------------
 // Compiler Feature Checks
diff --git a/absl/base/options.h b/absl/base/options.h
index 3961e63..592b33b 100644
--- a/absl/base/options.h
+++ b/absl/base/options.h
@@ -185,4 +185,30 @@
 
 #define ABSL_OPTION_USE_STD_VARIANT 2
 
+
+// ABSL_OPTION_USE_INLINE_NAMESPACE
+// ABSL_OPTION_INLINE_NAMESPACE_NAME
+//
+// These options controls whether all entities in the absl namespace are
+// contained within an inner inline namespace.  This does not affect the
+// user-visible API of Abseil, but it changes the mangled names of all symbols.
+//
+// This can be useful as a version tag if you are distributing Abseil in
+// precompiled form.  This will prevent a binary library build of Abseil with
+// one inline namespace being used with headers configured with a different
+// inline namespace name.  Binary packagers are reminded that Abseil does not
+// guarantee any ABI stability in Abseil, so any update of Abseil or
+// configuration change in such a binary package should be combined with a
+// new, unique value for the inline namespace name.
+//
+// A value of 0 means not to use inline namespaces.
+//
+// A value of 1 means to use an inline namespace with the given name inside
+// namespace absl.  If this is set, ABSL_OPTION_INLINE_NAMESPACE_NAME must also
+// be changed to a new, unique identifier name.  In particular "head" is not
+// allowed.
+
+#define ABSL_OPTION_USE_INLINE_NAMESPACE 0
+#define ABSL_OPTION_INLINE_NAMESPACE_NAME head
+
 #endif  // ABSL_BASE_OPTIONS_H_
diff --git a/absl/flags/config.h b/absl/flags/config.h
index a9fd97a..fbe3496 100644
--- a/absl/flags/config.h
+++ b/absl/flags/config.h
@@ -45,4 +45,15 @@
 #define ABSL_FLAGS_STRIP_HELP ABSL_FLAGS_STRIP_NAMES
 #endif
 
+// ABSL_FLAGS_INTERNAL_ATOMIC_DOUBLE_WORD macro is used for using atomics with
+// double words, e.g. absl::Duration.
+// For reasons in bug https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80878, modern
+// versions of GCC do not support cmpxchg16b instruction in standard atomics.
+#ifdef ABSL_FLAGS_INTERNAL_ATOMIC_DOUBLE_WORD
+#error "ABSL_FLAGS_INTERNAL_ATOMIC_DOUBLE_WORD should not be defined."
+#elif defined(__clang__) && defined(__x86_64__) && \
+    defined(__GCC_HAVE_SYNC_COMPARE_AND_SWAP_16)
+#define ABSL_FLAGS_INTERNAL_ATOMIC_DOUBLE_WORD 1
+#endif
+
 #endif  // ABSL_FLAGS_CONFIG_H_
diff --git a/absl/flags/flag.cc b/absl/flags/flag.cc
index 7faa7ad..491a66b 100644
--- a/absl/flags/flag.cc
+++ b/absl/flags/flag.cc
@@ -20,28 +20,6 @@
 namespace absl {
 ABSL_NAMESPACE_BEGIN
 
-// We want to validate the type mismatch between type definition and
-// declaration. The lock-free implementation does not allow us to do it,
-// so in debug builds we always use the slower implementation, which always
-// validates the type.
-#ifndef NDEBUG
-#define ABSL_FLAGS_ATOMIC_GET(T) \
-  T GetFlag(const absl::Flag<T>& flag) { return flag.Get(); }
-#else
-#define ABSL_FLAGS_ATOMIC_GET(T)         \
-  T GetFlag(const absl::Flag<T>& flag) { \
-    T result;                            \
-    if (flag.AtomicGet(&result)) {       \
-      return result;                     \
-    }                                    \
-    return flag.Get();                   \
-  }
-#endif
-
-ABSL_FLAGS_INTERNAL_FOR_EACH_LOCK_FREE(ABSL_FLAGS_ATOMIC_GET)
-
-#undef ABSL_FLAGS_ATOMIC_GET
-
 // This global nutex protects on-demand construction of flag objects in MSVC
 // builds.
 #if defined(_MSC_VER) && !defined(__clang__)
diff --git a/absl/flags/flag.h b/absl/flags/flag.h
index 326fb8e..62e73f8 100644
--- a/absl/flags/flag.h
+++ b/absl/flags/flag.h
@@ -29,6 +29,8 @@
 #ifndef ABSL_FLAGS_FLAG_H_
 #define ABSL_FLAGS_FLAG_H_
 
+#include <type_traits>
+
 #include "absl/base/attributes.h"
 #include "absl/base/casts.h"
 #include "absl/flags/config.h"
@@ -181,23 +183,42 @@ class Flag {
 //
 //   // FLAGS_firstname is a Flag of type `std::string`
 //   std::string first_name = absl::GetFlag(FLAGS_firstname);
-template <typename T>
+template <typename T,
+          typename std::enable_if<
+              !flags_internal::IsAtomicFlagTypeTrait<T>::value, int>::type = 0>
 ABSL_MUST_USE_RESULT T GetFlag(const absl::Flag<T>& flag) {
-#define ABSL_FLAGS_INTERNAL_LOCK_FREE_VALIDATE(BIT) \
-  static_assert(                                    \
-      !std::is_same<T, BIT>::value,                 \
-      "Do not specify explicit template parameters to absl::GetFlag");
-  ABSL_FLAGS_INTERNAL_FOR_EACH_LOCK_FREE(ABSL_FLAGS_INTERNAL_LOCK_FREE_VALIDATE)
-#undef ABSL_FLAGS_INTERNAL_LOCK_FREE_VALIDATE
-
   return flag.Get();
 }
 
+// We want to validate the type mismatch between type definition and
+// declaration. The lock-free implementation does not allow us to do it,
+// so in debug builds we always use the slower implementation, which always
+// validates the type.
+#ifndef NDEBUG
+template <typename T,
+          typename std::enable_if<
+              flags_internal::IsAtomicFlagTypeTrait<T>::value, int>::type = 0>
+ABSL_MUST_USE_RESULT T GetFlag(const absl::Flag<T>& flag) {
+  return flag.Get();
+}
+#else
 // Overload for `GetFlag()` for types that support lock-free reads.
-#define ABSL_FLAGS_INTERNAL_LOCK_FREE_EXPORT(T) \
-  ABSL_MUST_USE_RESULT T GetFlag(const absl::Flag<T>& flag);
-ABSL_FLAGS_INTERNAL_FOR_EACH_LOCK_FREE(ABSL_FLAGS_INTERNAL_LOCK_FREE_EXPORT)
-#undef ABSL_FLAGS_INTERNAL_LOCK_FREE_EXPORT
+template <typename T,
+          typename std::enable_if<
+              flags_internal::IsAtomicFlagTypeTrait<T>::value, int>::type = 0>
+ABSL_MUST_USE_RESULT T GetFlag(const absl::Flag<T>& flag) {
+  // T might not be default constructible.
+  union U {
+    T value;
+    U() {}
+  };
+  U result;
+  if (flag.AtomicGet(&result.value)) {
+    return result.value;
+  }
+  return flag.Get();
+}
+#endif
 
 // SetFlag()
 //
diff --git a/absl/flags/internal/commandlineflag.h b/absl/flags/internal/commandlineflag.h
index a0c18e8..1862306 100644
--- a/absl/flags/internal/commandlineflag.h
+++ b/absl/flags/internal/commandlineflag.h
@@ -259,22 +259,22 @@ class CommandLineFlag {
   virtual void Read(void* dst) const = 0;
 };
 
-// This macro is the "source of truth" for the list of supported flag types we
-// expect to perform lock free operations on. Specifically it generates code,
-// a one argument macro operating on a type, supplied as a macro argument, for
-// each type in the list.
-#define ABSL_FLAGS_INTERNAL_FOR_EACH_LOCK_FREE(A) \
-  A(bool)                                         \
-  A(short)                                        \
-  A(unsigned short)                               \
-  A(int)                                          \
-  A(unsigned int)                                 \
-  A(long)                                         \
-  A(unsigned long)                                \
-  A(long long)                                    \
-  A(unsigned long long)                           \
-  A(double)                                       \
-  A(float)
+// This macro is the "source of truth" for the list of supported flag built-in
+// types.
+#define ABSL_FLAGS_INTERNAL_BUILTIN_TYPES(A) \
+  A(bool)                                    \
+  A(short)                                   \
+  A(unsigned short)                          \
+  A(int)                                     \
+  A(unsigned int)                            \
+  A(long)                                    \
+  A(unsigned long)                           \
+  A(long long)                               \
+  A(unsigned long long)                      \
+  A(double)                                  \
+  A(float)                                   \
+  A(std::string)                             \
+  A(std::vector<std::string>)
 
 }  // namespace flags_internal
 ABSL_NAMESPACE_END
diff --git a/absl/flags/internal/flag.cc b/absl/flags/internal/flag.cc
index bb9a98f..6979dc4 100644
--- a/absl/flags/internal/flag.cc
+++ b/absl/flags/internal/flag.cc
@@ -16,6 +16,7 @@
 #include "absl/flags/internal/flag.h"
 
 #include "absl/base/optimization.h"
+#include "absl/flags/config.h"
 #include "absl/flags/usage_config.h"
 #include "absl/synchronization/mutex.h"
 
@@ -35,9 +36,7 @@ namespace {
 bool ShouldValidateFlagValue(FlagOpFn flag_type_id) {
 #define DONT_VALIDATE(T) \
   if (flag_type_id == &flags_internal::FlagOps<T>) return false;
-  ABSL_FLAGS_INTERNAL_FOR_EACH_LOCK_FREE(DONT_VALIDATE)
-  DONT_VALIDATE(std::string)
-  DONT_VALIDATE(std::vector<std::string>)
+  ABSL_FLAGS_INTERNAL_BUILTIN_TYPES(DONT_VALIDATE)
 #undef DONT_VALIDATE
 
   return true;
@@ -85,7 +84,6 @@ void FlagImpl::Init() {
     cur_ = MakeInitValue().release();
     StoreAtomic();
     inited_.store(true, std::memory_order_release);
-    InvokeCallback();
   }
 }
 
@@ -264,8 +262,15 @@ void FlagImpl::StoreAtomic() {
   if (data_size <= sizeof(int64_t)) {
     int64_t t = 0;
     std::memcpy(&t, cur_, data_size);
-    atomic_.store(t, std::memory_order_release);
+    atomics_.small_atomic.store(t, std::memory_order_release);
   }
+#if defined(ABSL_FLAGS_INTERNAL_ATOMIC_DOUBLE_WORD)
+  else if (data_size <= sizeof(FlagsInternalTwoWordsType)) {
+    FlagsInternalTwoWordsType t{0, 0};
+    std::memcpy(&t, cur_, data_size);
+    atomics_.big_atomic.store(t, std::memory_order_release);
+  }
+#endif
 }
 
 void FlagImpl::Write(const void* src, const flags_internal::FlagOpFn src_op) {
diff --git a/absl/flags/internal/flag.h b/absl/flags/internal/flag.h
index 7d5271c..a5edfd1 100644
--- a/absl/flags/internal/flag.h
+++ b/absl/flags/internal/flag.h
@@ -20,6 +20,7 @@
 #include <cstring>
 
 #include "absl/base/thread_annotations.h"
+#include "absl/flags/config.h"
 #include "absl/flags/internal/commandlineflag.h"
 #include "absl/flags/internal/registry.h"
 #include "absl/memory/memory.h"
@@ -30,7 +31,61 @@ namespace absl {
 ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 
-constexpr int64_t AtomicInit() { return 0xababababababababll; }
+// The minimum atomic size we believe to generate lock free code, i.e. all
+// trivially copyable types not bigger this size generate lock free code.
+static constexpr int kMinLockFreeAtomicSize = 8;
+
+// The same as kMinLockFreeAtomicSize but maximum atomic size. As double words
+// might use two registers, we want to dispatch the logic for them.
+#if defined(ABSL_FLAGS_INTERNAL_ATOMIC_DOUBLE_WORD)
+static constexpr int kMaxLockFreeAtomicSize = 16;
+#else
+static constexpr int kMaxLockFreeAtomicSize = 8;
+#endif
+
+// We can use atomic in cases when it fits in the register, trivially copyable
+// in order to make memcpy operations.
+template <typename T>
+struct IsAtomicFlagTypeTrait {
+  static constexpr bool value =
+      (sizeof(T) <= kMaxLockFreeAtomicSize &&
+       type_traits_internal::is_trivially_copyable<T>::value);
+};
+
+// Clang does not always produce cmpxchg16b instruction when alignment of a 16
+// bytes type is not 16.
+struct alignas(16) FlagsInternalTwoWordsType {
+  int64_t first;
+  int64_t second;
+};
+
+constexpr bool operator==(const FlagsInternalTwoWordsType& that,
+                          const FlagsInternalTwoWordsType& other) {
+  return that.first == other.first && that.second == other.second;
+}
+constexpr bool operator!=(const FlagsInternalTwoWordsType& that,
+                          const FlagsInternalTwoWordsType& other) {
+  return !(that == other);
+}
+
+constexpr int64_t SmallAtomicInit() { return 0xababababababababll; }
+
+template <typename T, typename S = void>
+struct BestAtomicType {
+  using type = int64_t;
+  static constexpr int64_t AtomicInit() { return SmallAtomicInit(); }
+};
+
+template <typename T>
+struct BestAtomicType<
+    T, typename std::enable_if<(kMinLockFreeAtomicSize < sizeof(T) &&
+                                sizeof(T) <= kMaxLockFreeAtomicSize),
+                               void>::type> {
+  using type = FlagsInternalTwoWordsType;
+  static constexpr FlagsInternalTwoWordsType AtomicInit() {
+    return {SmallAtomicInit(), SmallAtomicInit()};
+  }
+};
 
 template <typename T>
 class Flag;
@@ -182,14 +237,15 @@ class FlagImpl {
   // it replaces `dst` with the new value.
   bool TryParse(void** dst, absl::string_view value, std::string* err) const
       ABSL_EXCLUSIVE_LOCKS_REQUIRED(*DataGuard());
+
   template <typename T>
   bool AtomicGet(T* v) const {
-    const int64_t r = atomic_.load(std::memory_order_acquire);
-    if (r != flags_internal::AtomicInit()) {
-      std::memcpy(v, &r, sizeof(T));
+    using U = flags_internal::BestAtomicType<T>;
+    const typename U::type r = atomics_.template load<T>();
+    if (r != U::AtomicInit()) {
+      std::memcpy(static_cast<void*>(v), &r, sizeof(T));
       return true;
     }
-
     return false;
   }
 
@@ -271,7 +327,34 @@ class FlagImpl {
   int64_t counter_ ABSL_GUARDED_BY(*DataGuard()) = 0;
   // For some types, a copy of the current value is kept in an atomically
   // accessible field.
-  std::atomic<int64_t> atomic_{flags_internal::AtomicInit()};
+  union Atomics {
+    // Using small atomic for small types.
+    std::atomic<int64_t> small_atomic;
+    template <typename T,
+              typename K = typename std::enable_if<
+                  (sizeof(T) <= kMinLockFreeAtomicSize), void>::type>
+    int64_t load() const {
+      return small_atomic.load(std::memory_order_acquire);
+    }
+
+#if defined(ABSL_FLAGS_INTERNAL_ATOMIC_DOUBLE_WORD)
+    // Using big atomics for big types.
+    std::atomic<FlagsInternalTwoWordsType> big_atomic;
+    template <typename T, typename K = typename std::enable_if<
+                              (kMinLockFreeAtomicSize < sizeof(T) &&
+                               sizeof(T) <= kMaxLockFreeAtomicSize),
+                              void>::type>
+    FlagsInternalTwoWordsType load() const {
+      return big_atomic.load(std::memory_order_acquire);
+    }
+    constexpr Atomics()
+        : big_atomic{FlagsInternalTwoWordsType{SmallAtomicInit(),
+                                               SmallAtomicInit()}} {}
+#else
+    constexpr Atomics() : small_atomic{SmallAtomicInit()} {}
+#endif
+  };
+  Atomics atomics_{};
 
   struct CallbackData {
     FlagCallback func;
diff --git a/absl/flags/parse.cc b/absl/flags/parse.cc
index 2f07725..a288ace 100644
--- a/absl/flags/parse.cc
+++ b/absl/flags/parse.cc
@@ -280,9 +280,7 @@ void CheckDefaultValuesParsingRoundtrip() {
 #define IGNORE_TYPE(T) \
   if (flag->IsOfType<T>()) return;
 
-    ABSL_FLAGS_INTERNAL_FOR_EACH_LOCK_FREE(IGNORE_TYPE)
-    IGNORE_TYPE(std::string)
-    IGNORE_TYPE(std::vector<std::string>)
+    ABSL_FLAGS_INTERNAL_BUILTIN_TYPES(IGNORE_TYPE)
 #undef IGNORE_TYPE
 
     flag->CheckDefaultValueParsingRoundtrip();
diff --git a/absl/strings/BUILD.bazel b/absl/strings/BUILD.bazel
index 8d0a6b6..dc7e1bf 100644
--- a/absl/strings/BUILD.bazel
+++ b/absl/strings/BUILD.bazel
@@ -37,6 +37,7 @@ cc_library(
         "internal/charconv_bigint.h",
         "internal/charconv_parse.cc",
         "internal/charconv_parse.h",
+        "internal/escaping.cc",
         "internal/memutil.cc",
         "internal/memutil.h",
         "internal/stl_type_traits.h",
@@ -54,6 +55,7 @@ cc_library(
         "ascii.h",
         "charconv.h",
         "escaping.h",
+        "internal/escaping.h",
         "match.h",
         "numbers.h",
         "str_cat.h",
diff --git a/absl/strings/CMakeLists.txt b/absl/strings/CMakeLists.txt
index 9810157..36702f7 100644
--- a/absl/strings/CMakeLists.txt
+++ b/absl/strings/CMakeLists.txt
@@ -38,6 +38,8 @@ absl_cc_library(
     "internal/charconv_bigint.h"
     "internal/charconv_parse.cc"
     "internal/charconv_parse.h"
+    "internal/escaping.cc"
+    "internal/escaping.h"
     "internal/memutil.cc"
     "internal/memutil.h"
     "internal/stl_type_traits.h"
diff --git a/absl/strings/escaping.cc b/absl/strings/escaping.cc
index d2fcd9c..7adc1b6 100644
--- a/absl/strings/escaping.cc
+++ b/absl/strings/escaping.cc
@@ -26,6 +26,7 @@
 #include "absl/base/internal/raw_logging.h"
 #include "absl/base/internal/unaligned_access.h"
 #include "absl/strings/internal/char_map.h"
+#include "absl/strings/internal/escaping.h"
 #include "absl/strings/internal/resize_uninitialized.h"
 #include "absl/strings/internal/utf8.h"
 #include "absl/strings/str_cat.h"
@@ -764,177 +765,10 @@ constexpr signed char kUnWebSafeBase64[] = {
 };
 /* clang-format on */
 
-size_t CalculateBase64EscapedLenInternal(size_t input_len, bool do_padding) {
-  // Base64 encodes three bytes of input at a time. If the input is not
-  // divisible by three, we pad as appropriate.
-  //
-  // (from https://tools.ietf.org/html/rfc3548)
-  // Special processing is performed if fewer than 24 bits are available
-  // at the end of the data being encoded.  A full encoding quantum is
-  // always completed at the end of a quantity.  When fewer than 24 input
-  // bits are available in an input group, zero bits are added (on the
-  // right) to form an integral number of 6-bit groups.  Padding at the
-  // end of the data is performed using the '=' character.  Since all base
-  // 64 input is an integral number of octets, only the following cases
-  // can arise:
-
-  // Base64 encodes each three bytes of input into four bytes of output.
-  size_t len = (input_len / 3) * 4;
-
-  if (input_len % 3 == 0) {
-    // (from https://tools.ietf.org/html/rfc3548)
-    // (1) the final quantum of encoding input is an integral multiple of 24
-    // bits; here, the final unit of encoded output will be an integral
-    // multiple of 4 characters with no "=" padding,
-  } else if (input_len % 3 == 1) {
-    // (from https://tools.ietf.org/html/rfc3548)
-    // (2) the final quantum of encoding input is exactly 8 bits; here, the
-    // final unit of encoded output will be two characters followed by two
-    // "=" padding characters, or
-    len += 2;
-    if (do_padding) {
-      len += 2;
-    }
-  } else {  // (input_len % 3 == 2)
-    // (from https://tools.ietf.org/html/rfc3548)
-    // (3) the final quantum of encoding input is exactly 16 bits; here, the
-    // final unit of encoded output will be three characters followed by one
-    // "=" padding character.
-    len += 3;
-    if (do_padding) {
-      len += 1;
-    }
-  }
-
-  assert(len >= input_len);  // make sure we didn't overflow
-  return len;
-}
-
-size_t Base64EscapeInternal(const unsigned char* src, size_t szsrc, char* dest,
-                            size_t szdest, const char* base64,
-                            bool do_padding) {
-  static const char kPad64 = '=';
-
-  if (szsrc * 4 > szdest * 3) return 0;
-
-  char* cur_dest = dest;
-  const unsigned char* cur_src = src;
-
-  char* const limit_dest = dest + szdest;
-  const unsigned char* const limit_src = src + szsrc;
-
-  // Three bytes of data encodes to four characters of cyphertext.
-  // So we can pump through three-byte chunks atomically.
-  if (szsrc >= 3) {  // "limit_src - 3" is UB if szsrc < 3.
-    while (cur_src < limit_src - 3) {  // While we have >= 32 bits.
-      uint32_t in = absl::big_endian::Load32(cur_src) >> 8;
-
-      cur_dest[0] = base64[in >> 18];
-      in &= 0x3FFFF;
-      cur_dest[1] = base64[in >> 12];
-      in &= 0xFFF;
-      cur_dest[2] = base64[in >> 6];
-      in &= 0x3F;
-      cur_dest[3] = base64[in];
-
-      cur_dest += 4;
-      cur_src += 3;
-    }
-  }
-  // To save time, we didn't update szdest or szsrc in the loop.  So do it now.
-  szdest = limit_dest - cur_dest;
-  szsrc = limit_src - cur_src;
-
-  /* now deal with the tail (<=3 bytes) */
-  switch (szsrc) {
-    case 0:
-      // Nothing left; nothing more to do.
-      break;
-    case 1: {
-      // One byte left: this encodes to two characters, and (optionally)
-      // two pad characters to round out the four-character cypherblock.
-      if (szdest < 2) return 0;
-      uint32_t in = cur_src[0];
-      cur_dest[0] = base64[in >> 2];
-      in &= 0x3;
-      cur_dest[1] = base64[in << 4];
-      cur_dest += 2;
-      szdest -= 2;
-      if (do_padding) {
-        if (szdest < 2) return 0;
-        cur_dest[0] = kPad64;
-        cur_dest[1] = kPad64;
-        cur_dest += 2;
-        szdest -= 2;
-      }
-      break;
-    }
-    case 2: {
-      // Two bytes left: this encodes to three characters, and (optionally)
-      // one pad character to round out the four-character cypherblock.
-      if (szdest < 3) return 0;
-      uint32_t in = absl::big_endian::Load16(cur_src);
-      cur_dest[0] = base64[in >> 10];
-      in &= 0x3FF;
-      cur_dest[1] = base64[in >> 4];
-      in &= 0x00F;
-      cur_dest[2] = base64[in << 2];
-      cur_dest += 3;
-      szdest -= 3;
-      if (do_padding) {
-        if (szdest < 1) return 0;
-        cur_dest[0] = kPad64;
-        cur_dest += 1;
-        szdest -= 1;
-      }
-      break;
-    }
-    case 3: {
-      // Three bytes left: same as in the big loop above.  We can't do this in
-      // the loop because the loop above always reads 4 bytes, and the fourth
-      // byte is past the end of the input.
-      if (szdest < 4) return 0;
-      uint32_t in = (cur_src[0] << 16) + absl::big_endian::Load16(cur_src + 1);
-      cur_dest[0] = base64[in >> 18];
-      in &= 0x3FFFF;
-      cur_dest[1] = base64[in >> 12];
-      in &= 0xFFF;
-      cur_dest[2] = base64[in >> 6];
-      in &= 0x3F;
-      cur_dest[3] = base64[in];
-      cur_dest += 4;
-      szdest -= 4;
-      break;
-    }
-    default:
-      // Should not be reached: blocks of 4 bytes are handled
-      // in the while loop before this switch statement.
-      ABSL_RAW_LOG(FATAL, "Logic problem? szsrc = %zu", szsrc);
-      break;
-  }
-  return (cur_dest - dest);
-}
-
-constexpr char kBase64Chars[] =
-    "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
-
 constexpr char kWebSafeBase64Chars[] =
     "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_";
 
 template <typename String>
-void Base64EscapeInternal(const unsigned char* src, size_t szsrc, String* dest,
-                          bool do_padding, const char* base64_chars) {
-  const size_t calc_escaped_size =
-      CalculateBase64EscapedLenInternal(szsrc, do_padding);
-  strings_internal::STLStringResizeUninitialized(dest, calc_escaped_size);
-
-  const size_t escaped_len = Base64EscapeInternal(
-      src, szsrc, &(*dest)[0], dest->size(), base64_chars, do_padding);
-  assert(calc_escaped_size == escaped_len);
-  dest->erase(escaped_len);
-}
-
-template <typename String>
 bool Base64UnescapeInternal(const char* src, size_t slen, String* dest,
                             const signed char* unbase64) {
   // Determine the size of the output std::string.  Base64 encodes every 3 bytes into
@@ -1068,26 +902,30 @@ bool WebSafeBase64Unescape(absl::string_view src, std::string* dest) {
 }
 
 void Base64Escape(absl::string_view src, std::string* dest) {
-  Base64EscapeInternal(reinterpret_cast<const unsigned char*>(src.data()),
-                       src.size(), dest, true, kBase64Chars);
+  strings_internal::Base64EscapeInternal(
+      reinterpret_cast<const unsigned char*>(src.data()), src.size(), dest,
+      true, strings_internal::kBase64Chars);
 }
 
 void WebSafeBase64Escape(absl::string_view src, std::string* dest) {
-  Base64EscapeInternal(reinterpret_cast<const unsigned char*>(src.data()),
-                       src.size(), dest, false, kWebSafeBase64Chars);
+  strings_internal::Base64EscapeInternal(
+      reinterpret_cast<const unsigned char*>(src.data()), src.size(), dest,
+      false, kWebSafeBase64Chars);
 }
 
 std::string Base64Escape(absl::string_view src) {
   std::string dest;
-  Base64EscapeInternal(reinterpret_cast<const unsigned char*>(src.data()),
-                       src.size(), &dest, true, kBase64Chars);
+  strings_internal::Base64EscapeInternal(
+      reinterpret_cast<const unsigned char*>(src.data()), src.size(), &dest,
+      true, strings_internal::kBase64Chars);
   return dest;
 }
 
 std::string WebSafeBase64Escape(absl::string_view src) {
   std::string dest;
-  Base64EscapeInternal(reinterpret_cast<const unsigned char*>(src.data()),
-                       src.size(), &dest, false, kWebSafeBase64Chars);
+  strings_internal::Base64EscapeInternal(
+      reinterpret_cast<const unsigned char*>(src.data()), src.size(), &dest,
+      false, kWebSafeBase64Chars);
   return dest;
 }
 
diff --git a/absl/strings/internal/escaping.cc b/absl/strings/internal/escaping.cc
new file mode 100644
index 0000000..c527128
--- /dev/null
+++ b/absl/strings/internal/escaping.cc
@@ -0,0 +1,180 @@
+// Copyright 2020 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/strings/internal/escaping.h"
+
+#include "absl/base/internal/endian.h"
+#include "absl/base/internal/raw_logging.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace strings_internal {
+
+const char kBase64Chars[] =
+    "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
+
+size_t CalculateBase64EscapedLenInternal(size_t input_len, bool do_padding) {
+  // Base64 encodes three bytes of input at a time. If the input is not
+  // divisible by three, we pad as appropriate.
+  //
+  // (from https://tools.ietf.org/html/rfc3548)
+  // Special processing is performed if fewer than 24 bits are available
+  // at the end of the data being encoded.  A full encoding quantum is
+  // always completed at the end of a quantity.  When fewer than 24 input
+  // bits are available in an input group, zero bits are added (on the
+  // right) to form an integral number of 6-bit groups.  Padding at the
+  // end of the data is performed using the '=' character.  Since all base
+  // 64 input is an integral number of octets, only the following cases
+  // can arise:
+
+  // Base64 encodes each three bytes of input into four bytes of output.
+  size_t len = (input_len / 3) * 4;
+
+  if (input_len % 3 == 0) {
+    // (from https://tools.ietf.org/html/rfc3548)
+    // (1) the final quantum of encoding input is an integral multiple of 24
+    // bits; here, the final unit of encoded output will be an integral
+    // multiple of 4 characters with no "=" padding,
+  } else if (input_len % 3 == 1) {
+    // (from https://tools.ietf.org/html/rfc3548)
+    // (2) the final quantum of encoding input is exactly 8 bits; here, the
+    // final unit of encoded output will be two characters followed by two
+    // "=" padding characters, or
+    len += 2;
+    if (do_padding) {
+      len += 2;
+    }
+  } else {  // (input_len % 3 == 2)
+    // (from https://tools.ietf.org/html/rfc3548)
+    // (3) the final quantum of encoding input is exactly 16 bits; here, the
+    // final unit of encoded output will be three characters followed by one
+    // "=" padding character.
+    len += 3;
+    if (do_padding) {
+      len += 1;
+    }
+  }
+
+  assert(len >= input_len);  // make sure we didn't overflow
+  return len;
+}
+
+size_t Base64EscapeInternal(const unsigned char* src, size_t szsrc, char* dest,
+                            size_t szdest, const char* base64,
+                            bool do_padding) {
+  static const char kPad64 = '=';
+
+  if (szsrc * 4 > szdest * 3) return 0;
+
+  char* cur_dest = dest;
+  const unsigned char* cur_src = src;
+
+  char* const limit_dest = dest + szdest;
+  const unsigned char* const limit_src = src + szsrc;
+
+  // Three bytes of data encodes to four characters of cyphertext.
+  // So we can pump through three-byte chunks atomically.
+  if (szsrc >= 3) {                    // "limit_src - 3" is UB if szsrc < 3.
+    while (cur_src < limit_src - 3) {  // While we have >= 32 bits.
+      uint32_t in = absl::big_endian::Load32(cur_src) >> 8;
+
+      cur_dest[0] = base64[in >> 18];
+      in &= 0x3FFFF;
+      cur_dest[1] = base64[in >> 12];
+      in &= 0xFFF;
+      cur_dest[2] = base64[in >> 6];
+      in &= 0x3F;
+      cur_dest[3] = base64[in];
+
+      cur_dest += 4;
+      cur_src += 3;
+    }
+  }
+  // To save time, we didn't update szdest or szsrc in the loop.  So do it now.
+  szdest = limit_dest - cur_dest;
+  szsrc = limit_src - cur_src;
+
+  /* now deal with the tail (<=3 bytes) */
+  switch (szsrc) {
+    case 0:
+      // Nothing left; nothing more to do.
+      break;
+    case 1: {
+      // One byte left: this encodes to two characters, and (optionally)
+      // two pad characters to round out the four-character cypherblock.
+      if (szdest < 2) return 0;
+      uint32_t in = cur_src[0];
+      cur_dest[0] = base64[in >> 2];
+      in &= 0x3;
+      cur_dest[1] = base64[in << 4];
+      cur_dest += 2;
+      szdest -= 2;
+      if (do_padding) {
+        if (szdest < 2) return 0;
+        cur_dest[0] = kPad64;
+        cur_dest[1] = kPad64;
+        cur_dest += 2;
+        szdest -= 2;
+      }
+      break;
+    }
+    case 2: {
+      // Two bytes left: this encodes to three characters, and (optionally)
+      // one pad character to round out the four-character cypherblock.
+      if (szdest < 3) return 0;
+      uint32_t in = absl::big_endian::Load16(cur_src);
+      cur_dest[0] = base64[in >> 10];
+      in &= 0x3FF;
+      cur_dest[1] = base64[in >> 4];
+      in &= 0x00F;
+      cur_dest[2] = base64[in << 2];
+      cur_dest += 3;
+      szdest -= 3;
+      if (do_padding) {
+        if (szdest < 1) return 0;
+        cur_dest[0] = kPad64;
+        cur_dest += 1;
+        szdest -= 1;
+      }
+      break;
+    }
+    case 3: {
+      // Three bytes left: same as in the big loop above.  We can't do this in
+      // the loop because the loop above always reads 4 bytes, and the fourth
+      // byte is past the end of the input.
+      if (szdest < 4) return 0;
+      uint32_t in = (cur_src[0] << 16) + absl::big_endian::Load16(cur_src + 1);
+      cur_dest[0] = base64[in >> 18];
+      in &= 0x3FFFF;
+      cur_dest[1] = base64[in >> 12];
+      in &= 0xFFF;
+      cur_dest[2] = base64[in >> 6];
+      in &= 0x3F;
+      cur_dest[3] = base64[in];
+      cur_dest += 4;
+      szdest -= 4;
+      break;
+    }
+    default:
+      // Should not be reached: blocks of 4 bytes are handled
+      // in the while loop before this switch statement.
+      ABSL_RAW_LOG(FATAL, "Logic problem? szsrc = %zu", szsrc);
+      break;
+  }
+  return (cur_dest - dest);
+}
+
+}  // namespace strings_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/escaping.h b/absl/strings/internal/escaping.h
new file mode 100644
index 0000000..6a9ce60
--- /dev/null
+++ b/absl/strings/internal/escaping.h
@@ -0,0 +1,58 @@
+// Copyright 2020 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_STRINGS_INTERNAL_ESCAPING_H_
+#define ABSL_STRINGS_INTERNAL_ESCAPING_H_
+
+#include <cassert>
+
+#include "absl/strings/internal/resize_uninitialized.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace strings_internal {
+
+ABSL_CONST_INIT extern const char kBase64Chars[];
+
+// Calculates how long a string will be when it is base64 encoded given its
+// length and whether or not the result should be padded.
+size_t CalculateBase64EscapedLenInternal(size_t input_len, bool do_padding);
+
+// Base64-encodes `src` using the alphabet provided in `base64` and writes the
+// result to `dest`. If `do_padding` is true, `dest` is padded with '=' chars
+// until its length is a multiple of 3. Returns the length of `dest`.
+size_t Base64EscapeInternal(const unsigned char* src, size_t szsrc, char* dest,
+                            size_t szdest, const char* base64, bool do_padding);
+
+// Base64-encodes `src` using the alphabet provided in `base64` and writes the
+// result to `dest`. If `do_padding` is true, `dest` is padded with '=' chars
+// until its length is a multiple of 3.
+template <typename String>
+void Base64EscapeInternal(const unsigned char* src, size_t szsrc, String* dest,
+                          bool do_padding, const char* base64_chars) {
+  const size_t calc_escaped_size =
+      CalculateBase64EscapedLenInternal(szsrc, do_padding);
+  STLStringResizeUninitialized(dest, calc_escaped_size);
+
+  const size_t escaped_len = Base64EscapeInternal(
+      src, szsrc, &(*dest)[0], dest->size(), base64_chars, do_padding);
+  assert(calc_escaped_size == escaped_len);
+  dest->erase(escaped_len);
+}
+
+}  // namespace strings_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_ESCAPING_H_
diff --git a/absl/strings/string_view.h b/absl/strings/string_view.h
index 4b34e56..4f7dd6b 100644
--- a/absl/strings/string_view.h
+++ b/absl/strings/string_view.h
@@ -122,7 +122,7 @@ ABSL_NAMESPACE_BEGIN
 //
 // You may create a null `string_view` in two ways:
 //
-//   absl::string_view sv();
+//   absl::string_view sv;
 //   absl::string_view sv(nullptr, 0);
 //
 // For the above, `sv.data() == nullptr`, `sv.length() == 0`, and
@@ -605,7 +605,7 @@ inline string_view ClippedSubstr(string_view s, size_t pos,
 // Creates an `absl::string_view` from a pointer `p` even if it's null-valued.
 // This function should be used where an `absl::string_view` can be created from
 // a possibly-null pointer.
-inline string_view NullSafeStringView(const char* p) {
+constexpr string_view NullSafeStringView(const char* p) {
   return p ? string_view(p) : string_view();
 }
 
diff --git a/absl/strings/string_view_test.cc b/absl/strings/string_view_test.cc
index 96dacdf..c4fbd16 100644
--- a/absl/strings/string_view_test.cc
+++ b/absl/strings/string_view_test.cc
@@ -931,6 +931,31 @@ TEST(StringViewTest, NullSafeStringView) {
   }
 }
 
+TEST(StringViewTest, ConstexprNullSafeStringView) {
+  {
+    constexpr absl::string_view s = absl::NullSafeStringView(nullptr);
+    EXPECT_EQ(nullptr, s.data());
+    EXPECT_EQ(0, s.size());
+    EXPECT_EQ(absl::string_view(), s);
+  }
+#if !defined(_MSC_VER) || _MSC_VER >= 1910
+  // MSVC 2017+ is required for good constexpr string_view support.
+  // See the implementation of `absl::string_view::StrlenInternal()`.
+  {
+    static constexpr char kHi[] = "hi";
+    absl::string_view s = absl::NullSafeStringView(kHi);
+    EXPECT_EQ(kHi, s.data());
+    EXPECT_EQ(strlen(kHi), s.size());
+    EXPECT_EQ(absl::string_view("hi"), s);
+  }
+  {
+    constexpr absl::string_view s = absl::NullSafeStringView("hello");
+    EXPECT_EQ(s.size(), 5);
+    EXPECT_EQ("hello", s);
+  }
+#endif
+}
+
 TEST(StringViewTest, ConstexprCompiles) {
   constexpr absl::string_view sp;
 #ifdef ABSL_HAVE_STRING_VIEW_FROM_NULLPTR
diff --git a/ci/absl_types_options.h b/ci/absl_types_options.h
index 8f8ed34..307fde6 100644
--- a/ci/absl_types_options.h
+++ b/ci/absl_types_options.h
@@ -22,6 +22,7 @@
 #define ABSL_OPTION_USE_STD_OPTIONAL 0
 #define ABSL_OPTION_USE_STD_STRING_VIEW 0
 #define ABSL_OPTION_USE_STD_VARIANT 0
-
+#define ABSL_OPTION_USE_INLINE_NAMESPACE 1
+#define ABSL_OPTION_INLINE_NAMESPACE_NAME ns
 
 #endif  // ABSL_BASE_OPTIONS_H_
diff --git a/ci/linux_gcc-latest_libstdcxx_bazel.sh b/ci/linux_gcc-latest_libstdcxx_bazel.sh
index 2d41d51..5964703 100755
--- a/ci/linux_gcc-latest_libstdcxx_bazel.sh
+++ b/ci/linux_gcc-latest_libstdcxx_bazel.sh
@@ -55,7 +55,8 @@ for std in ${STD}; do
     for exceptions_mode in ${EXCEPTIONS_MODE}; do
       echo "--------------------------------------------------------------------"
       time docker run \
-        --volume="${ABSEIL_ROOT}:/abseil-cpp:ro" \
+        --volume="${ABSEIL_ROOT}:/abseil-cpp-ro:ro" \
+        --tmpfs=/abseil-cpp \
         --workdir=/abseil-cpp \
         --cap-add=SYS_PTRACE \
         --rm \
@@ -63,18 +64,23 @@ for std in ${STD}; do
         -e BAZEL_CXXOPTS="-std=${std}" \
         ${DOCKER_EXTRA_ARGS:-} \
         ${DOCKER_CONTAINER} \
-        /usr/local/bin/bazel test ... \
-          --compilation_mode="${compilation_mode}" \
-          --copt="${exceptions_mode}" \
-          --copt=-Werror \
-          --define="absl=1" \
-          --keep_going \
-          --show_timestamps \
-          --test_env="GTEST_INSTALL_FAILURE_SIGNAL_HANDLER=1" \
-          --test_env="TZDIR=/abseil-cpp/absl/time/internal/cctz/testdata/zoneinfo" \
-          --test_output=errors \
-          --test_tag_filters=-benchmark \
-          ${BAZEL_EXTRA_ARGS:-}
+        /bin/sh -c "
+          cp -r /abseil-cpp-ro/* /abseil-cpp/
+          if [ -n \"${ALTERNATE_OPTIONS:-}\" ]; then
+            cp ${ALTERNATE_OPTIONS:-} absl/base/options.h || exit 1
+          fi
+          /usr/local/bin/bazel test ... \
+            --compilation_mode=\"${compilation_mode}\" \
+            --copt=\"${exceptions_mode}\" \
+            --copt=-Werror \
+            --define=\"absl=1\" \
+            --keep_going \
+            --show_timestamps \
+            --test_env=\"GTEST_INSTALL_FAILURE_SIGNAL_HANDLER=1\" \
+            --test_env=\"TZDIR=/abseil-cpp/absl/time/internal/cctz/testdata/zoneinfo\" \
+            --test_output=errors \
+            --test_tag_filters=-benchmark \
+            ${BAZEL_EXTRA_ARGS:-}"
     done
   done
 done
diff --git a/ci/macos_xcode_bazel.sh b/ci/macos_xcode_bazel.sh
index 3cf1be4..f5f2d75 100755
--- a/ci/macos_xcode_bazel.sh
+++ b/ci/macos_xcode_bazel.sh
@@ -41,6 +41,10 @@ echo "---------------"
 
 cd ${ABSEIL_ROOT}
 
+if [ -n "${ALTERNATE_OPTIONS:-}" ]; then
+  cp ${ALTERNATE_OPTIONS:-} absl/base/options.h || exit 1
+fi
+
 ${BAZEL_BIN} test ... \
   --copt=-Werror \
   --keep_going \