Export of internal Abseil changes

--
8bdb2020150ed0fd4a4e520e454dc5f54e33f776 by Eric Fiselier <ericwf@google.com>:

Workaround bug in GCC 9.2 and after.

PiperOrigin-RevId: 291982551

--
47ff4820e595f96c082a90d733725f6882d83e3b by Abseil Team <absl-team@google.com>:

Improve ABSL_ATTRIBUTE_PACKED documentation

Recommend to apply ABSL_ATTRIBUTE_PACKED to structure members instead of to an entire structure because applying this attribute to an entire structure may cause the compiler to generate suboptimal code. It reduces the alignment of the data structure from a value larger than one to one. When applied to a structure, ABSL_ATTRIBUTE_PACKED reduces the alignment of a structure (alignof()) to 1. As a result, the compiler can no longer assume that e.g. uint32 members are aligned on a four byte boundary and hence is forced to use single-byte load and store instructions on CPU architectures that do not support non-aligned loads or stores.

PiperOrigin-RevId: 291977920

--
902b7a86f860da699d3a2e5c738be5ef73ede3b4 by Mark Barolak <mbar@google.com>:

Internal change

PiperOrigin-RevId: 291963048

--
bb3bd3247e376d53a3080b105f13ec7566d3ae50 by Abseil Team <absl-team@google.com>:

Support the C++17 insert_or_assign() API in btree_map.

PiperOrigin-RevId: 291945474

--
ff3b3cfcbbc64f086f95501f48d49426bcde356f by Gennadiy Rozental <rogeeff@google.com>:

Import of CCTZ from GitHub.

PiperOrigin-RevId: 291861110

--
fd465cd9cbbacd3962f67a7346d6462edaddd809 by Derek Mauro <dmauro@google.com>:

Add flaky=1 to beta_distribution_test.

PiperOrigin-RevId: 291757364

--
3603adfb59c4128c542b670952cce250d59e1f67 by Derek Mauro <dmauro@google.com>:

Separate the initialization of NumCPUs() and NominalCPUFrequency()

The OSS version of Abseil never needs to call NominalCPUFrequency().
In some configurations, initializing NominalCPUFrequency() requires
spending at least 3ms measuring the CPU frequency. By separating the
initialization from NumCPUs(), which is called in most configurations,
we can save at least 3ms of program startup time.

PiperOrigin-RevId: 291737273

--
bea9e4a6bff5a0351d340deab966641867e08c4d by Abseil Team <absl-team@google.com>:

Change the cmake library names not to have a redundant `absl_` prefix.

PiperOrigin-RevId: 291640501

--
501b602ef260cd7c8c527342581ceffb3c5b6d4c by Gennadiy Rozental <rogeeff@google.com>:

Introducing benchmark for absl::GetFlag.

PiperOrigin-RevId: 291433394

--
4eeaddc788da4b91c272a8adca77ca6dbbbc1d44 by Xiaoyi Zhang <zhangxy@google.com>:

fix: Add support for more ARM processors detection

Import of https://github.com/abseil/abseil-cpp/pull/608

PiperOrigin-RevId: 291420397

--
a3087a8e883c5d71de7d9bd4ec8f4db5142dfcf5 by Derek Mauro <dmauro@google.com>:

Removes the flaky raw_hash_set prefetch test

PiperOrigin-RevId: 291197079

--
aad6c2121c102ac36216e771c83227cf3e3bfd66 by Andy Soffer <asoffer@google.com>:

Enable building Abseil as a DLL.
This is currently experimental and unsupported.

This CL does a few things:
1. Adds the ABSL_DLL macro to any class holding a static data member, or to global constants in headers.
2. Adds a whitelist of all files in the DLL and all the build targets that are conglomerated into the DLL.
3. When BUILD_SHARED_LIBS is specified, any build target that would be in the DLL still exists, but we swap out all of it's dependencies so it just depends on abseil_dll

PiperOrigin-RevId: 291192055

--
5e888cd6f2a7722805d41f872108a03a84e421c7 by Mark Barolak <mbar@google.com>:

Move absl/strings/internal/escaping.{cc,h} into internal build targets.

This puts absl/strings/internal/escaping.h behind a whitelist and it also resolves https://github.com/abseil/abseil-cpp/issues/604.

PiperOrigin-RevId: 291173320

--
166836d24970da87587c1728036f53f05a28f0af by Eric Fiselier <ericwf@google.com>:

Internal Change.

PiperOrigin-RevId: 291012718

--
996ddb3dffda02440fa93f30ca5d71b14b688875 by Abseil Team <absl-team@google.com>:

Fix shared libraries log spam for built-in types in absl::GetFlag

PiperOrigin-RevId: 290772743
GitOrigin-RevId: 8bdb2020150ed0fd4a4e520e454dc5f54e33f776
Change-Id: I8bf2265dd14ebbace220a1b6b982bb5040ad2a26

8 files changed, 82 insertions, 19 deletions

diff --git a/absl/base/attributes.h b/absl/base/attributes.h
index acd1c5269829..8f77db77d23a 100644
--- a/absl/base/attributes.h
+++ b/absl/base/attributes.h
@@ -563,7 +563,19 @@
 
 // ABSL_ATTRIBUTE_PACKED
 //
-// Prevents the compiler from padding a structure to natural alignment
+// Instructs the compiler not to use natural alignment for a tagged data
+// structure, but instead to reduce its alignment to 1. This attribute can
+// either be applied to members of a structure or to a structure in its
+// entirety. Applying this attribute (judiciously) to a structure in its
+// entirety to optimize the memory footprint of very commonly-used structs is
+// fine. Do not apply this attribute to a structure in its entirety if the
+// purpose is to control the offsets of the members in the structure. Instead,
+// apply this attribute only to structure members that need it.
+//
+// When applying ABSL_ATTRIBUTE_PACKED only to specific structure members the
+// natural alignment of structure members not annotated is preserved. Aligned
+// member accesses are faster than non-aligned member accesses even if the
+// targeted microprosessor supports non-aligned accesses.
 #if ABSL_HAVE_ATTRIBUTE(packed) || (defined(__GNUC__) && !defined(__clang__))
 #define ABSL_ATTRIBUTE_PACKED __attribute__((__packed__))
 #else
diff --git a/absl/base/config.h b/absl/base/config.h
index c4e8dce403bb..eac5d268e10a 100644
--- a/absl/base/config.h
+++ b/absl/base/config.h
@@ -643,4 +643,23 @@ static_assert(ABSL_INTERNAL_INLINE_NAMESPACE_STR[0] != 'h' ||
 
 #undef ABSL_INTERNAL_HAS_KEYWORD
 
+// ABSL_DLL
+//
+// When building Abseil as a DLL, this macro expands to `__declspec(dllexport)`
+// so we can annotate symbols appropriately as being exported. When used in
+// headers consuming a DLL, this macro expands to `__declspec(dllimport)` so
+// that consumers know the symbol is defined inside the DLL. In all other cases,
+// the macro expands to nothing.
+#if defined(_MSC_VER)
+#if defined(ABSL_BUILD_DLL)
+#define ABSL_DLL __declspec(dllexport)
+#elif defined(ABSL_CONSUME_DLL)
+#define ABSL_DLL __declspec(dllimport)
+#else
+#define ABSL_DLL
+#endif
+#else
+#define ABSL_DLL
+#endif  // defined(_MSC_VER)
+
 #endif  // ABSL_BASE_CONFIG_H_
diff --git a/absl/base/internal/raw_logging.cc b/absl/base/internal/raw_logging.cc
index d79c5486658e..e36eb29abf97 100644
--- a/absl/base/internal/raw_logging.cc
+++ b/absl/base/internal/raw_logging.cc
@@ -225,8 +225,9 @@ bool RawLoggingFullySupported() {
 #endif  // !ABSL_LOW_LEVEL_WRITE_SUPPORTED
 }
 
-ABSL_CONST_INIT absl::base_internal::AtomicHook<InternalLogFunction>
-    internal_log_function(DefaultInternalLog);
+ABSL_CONST_INIT ABSL_DLL
+    absl::base_internal::AtomicHook<InternalLogFunction>
+        internal_log_function(DefaultInternalLog);
 
 void RegisterInternalLogFunction(InternalLogFunction func) {
   internal_log_function.Store(func);
diff --git a/absl/base/internal/raw_logging.h b/absl/base/internal/raw_logging.h
index cff45058b3d5..ac74f97dc3ae 100644
--- a/absl/base/internal/raw_logging.h
+++ b/absl/base/internal/raw_logging.h
@@ -22,9 +22,11 @@
 #include <string>
 
 #include "absl/base/attributes.h"
+#include "absl/base/config.h"
 #include "absl/base/internal/atomic_hook.h"
 #include "absl/base/log_severity.h"
 #include "absl/base/macros.h"
+#include "absl/base/optimization.h"
 #include "absl/base/port.h"
 
 // This is similar to LOG(severity) << format..., but
@@ -168,7 +170,8 @@ using InternalLogFunction = void (*)(absl::LogSeverity severity,
                                      const char* file, int line,
                                      const std::string& message);
 
-extern base_internal::AtomicHook<InternalLogFunction> internal_log_function;
+ABSL_DLL extern base_internal::AtomicHook<InternalLogFunction>
+    internal_log_function;
 
 void RegisterInternalLogFunction(InternalLogFunction func);
 
diff --git a/absl/base/internal/sysinfo.cc b/absl/base/internal/sysinfo.cc
index 7945322f582e..a0930e978183 100644
--- a/absl/base/internal/sysinfo.cc
+++ b/absl/base/internal/sysinfo.cc
@@ -58,10 +58,6 @@ namespace absl {
 ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
-static once_flag init_system_info_once;
-static int num_cpus = 0;
-static double nominal_cpu_frequency = 1.0;  // 0.0 might be dangerous.
-
 static int GetNumCPUs() {
 #if defined(__myriad2__)
   return 1;
@@ -265,21 +261,27 @@ static double GetNominalCPUFrequency() {
 
 #endif
 
-// InitializeSystemInfo() may be called before main() and before
-// malloc is properly initialized, therefore this must not allocate
-// memory.
-static void InitializeSystemInfo() {
-  num_cpus = GetNumCPUs();
-  nominal_cpu_frequency = GetNominalCPUFrequency();
-}
+ABSL_CONST_INIT static once_flag init_num_cpus_once;
+ABSL_CONST_INIT static int num_cpus = 0;
 
+// NumCPUs() may be called before main() and before malloc is properly
+// initialized, therefore this must not allocate memory.
 int NumCPUs() {
-  base_internal::LowLevelCallOnce(&init_system_info_once, InitializeSystemInfo);
+  base_internal::LowLevelCallOnce(
+      &init_num_cpus_once, []() { num_cpus = GetNumCPUs(); });
   return num_cpus;
 }
 
+// A default frequency of 0.0 might be dangerous if it is used in division.
+ABSL_CONST_INIT static once_flag init_nominal_cpu_frequency_once;
+ABSL_CONST_INIT static double nominal_cpu_frequency = 1.0;
+
+// NominalCPUFrequency() may be called before main() and before malloc is
+// properly initialized, therefore this must not allocate memory.
 double NominalCPUFrequency() {
-  base_internal::LowLevelCallOnce(&init_system_info_once, InitializeSystemInfo);
+  base_internal::LowLevelCallOnce(
+      &init_nominal_cpu_frequency_once,
+      []() { nominal_cpu_frequency = GetNominalCPUFrequency(); });
   return nominal_cpu_frequency;
 }
 
diff --git a/absl/base/internal/thread_identity.cc b/absl/base/internal/thread_identity.cc
index 6a28f246dbdb..d63a04ae91d5 100644
--- a/absl/base/internal/thread_identity.cc
+++ b/absl/base/internal/thread_identity.cc
@@ -113,6 +113,18 @@ void SetCurrentThreadIdentity(
 #endif
 }
 
+#if ABSL_THREAD_IDENTITY_MODE == ABSL_THREAD_IDENTITY_MODE_USE_TLS || \
+    ABSL_THREAD_IDENTITY_MODE == ABSL_THREAD_IDENTITY_MODE_USE_CPP11
+
+// Please see the comment on `CurrentThreadIdentityIfPresent` in
+// thread_identity.h. Because DLLs cannot expose thread_local variables in
+// headers, we opt for the correct-but-slower option of placing the definition
+// of this function only in a translation unit inside DLL.
+#if defined(ABSL_BUILD_DLL) || defined(ABSL_CONSUME_DLL)
+ThreadIdentity* CurrentThreadIdentityIfPresent() { return thread_identity_ptr; }
+#endif
+#endif
+
 void ClearCurrentThreadIdentity() {
 #if ABSL_THREAD_IDENTITY_MODE == ABSL_THREAD_IDENTITY_MODE_USE_TLS || \
     ABSL_THREAD_IDENTITY_MODE == ABSL_THREAD_IDENTITY_MODE_USE_CPP11
diff --git a/absl/base/internal/thread_identity.h b/absl/base/internal/thread_identity.h
index 5dfd0715dc5d..ceb109b41c6a 100644
--- a/absl/base/internal/thread_identity.h
+++ b/absl/base/internal/thread_identity.h
@@ -30,6 +30,7 @@
 #include <atomic>
 #include <cstdint>
 
+#include "absl/base/config.h"
 #include "absl/base/internal/per_thread_tls.h"
 
 namespace absl {
@@ -234,9 +235,17 @@ ABSL_CONST_INIT extern thread_local ThreadIdentity* thread_identity_ptr;
 #error Thread-local storage not detected on this platform
 #endif
 
+// thread_local variables cannot be in headers exposed by DLLs. However, it is
+// important for performance reasons in general that
+// `CurrentThreadIdentityIfPresent` be inlined. This is not possible across a
+// DLL boundary so, with DLLs, we opt to have the function not be inlined. Note
+// that `CurrentThreadIdentityIfPresent` is declared above so we can exclude
+// this entire inline definition when compiling as a DLL.
+#if !defined(ABSL_BUILD_DLL) && !defined(ABSL_CONSUME_DLL)
 inline ThreadIdentity* CurrentThreadIdentityIfPresent() {
   return thread_identity_ptr;
 }
+#endif
 
 #elif ABSL_THREAD_IDENTITY_MODE != \
     ABSL_THREAD_IDENTITY_MODE_USE_POSIX_SETSPECIFIC
diff --git a/absl/base/options.h b/absl/base/options.h
index 592b33b734d7..6868c77b103a 100644
--- a/absl/base/options.h
+++ b/absl/base/options.h
@@ -66,7 +66,13 @@
 // NOTE: the defaults within this file all assume that Abseil can select the
 // proper Abseil implementation at compile-time, which will not be sufficient
 // to guarantee ABI stability to package managers.
-//
+
+// Include a standard library header to allow configuration based on the
+// standard library in use.
+#ifdef __cplusplus
+#include <ciso646>
+#endif
+
 // -----------------------------------------------------------------------------
 // Type Compatibility Options
 // -----------------------------------------------------------------------------
@@ -158,7 +164,6 @@
 
 #define ABSL_OPTION_USE_STD_STRING_VIEW 2
 
-
 // ABSL_OPTION_USE_STD_VARIANT
 //
 // This option controls whether absl::variant is implemented as an alias to