Fix absl cached tid on Android

When compiling for Android with a minimum version less than 29, this emits calls to emulated TLS, which is substantially slower than "proper" TLS. In contrast, bionic's own gettid uses real thread local storage, so it's better to avoid this caching when the caching is more expensive than what it saves a call to.

PiperOrigin-RevId: 947447727
Change-Id: Ic96ed936ba199852e40fed4f8e8e0dea894dfcc2

absl/base/internal/sysinfo.cc

diff --git a/absl/base/internal/sysinfo.cc b/absl/base/internal/sysinfo.cc
index cd08e51..b103f49 100644
--- a/absl/base/internal/sysinfo.cc
+++ b/absl/base/internal/sysinfo.cc
@@ -378,28 +378,36 @@
 // NominalCPUFrequency() may be called before main() and before malloc is
 // properly initialized, therefore this must not allocate memory.
 double NominalCPUFrequency() {
-  base_internal::LowLevelCallOnce(
-      &init_nominal_cpu_frequency_once,
-      []() { nominal_cpu_frequency = GetNominalCPUFrequency(); });
+  base_internal::LowLevelCallOnce(&init_nominal_cpu_frequency_once, []() {
+    nominal_cpu_frequency = GetNominalCPUFrequency();
+  });
   return nominal_cpu_frequency;
 }
 
 #if defined(_WIN32)
 
-pid_t GetTID() {
-  return pid_t{GetCurrentThreadId()};
-}
+pid_t GetTID() { return pid_t{GetCurrentThreadId()}; }
 
 #elif defined(__linux__)
+#ifdef __ANDROID__
+#if __ANDROID_API__ >= 21
+#define ABSL_INTERNAL_HAVE_GETTID 1
+#endif
+#endif
 
+#ifdef ABSL_INTERNAL_HAVE_GETTID
+pid_t GetTID() {
+  return static_cast<pid_t>(gettid());
+}
+#else
 #ifndef SYS_gettid
 #define SYS_gettid __NR_gettid
 #endif
 
-pid_t GetTID() {
-  return static_cast<pid_t>(syscall(SYS_gettid));
-}
+pid_t GetTID() { return static_cast<pid_t>(syscall(SYS_gettid)); }
 
+#endif
+#undef ABSL_INTERNAL_HAVE_GETTID
 #elif defined(__akaros__)
 
 pid_t GetTID() {
@@ -419,9 +427,8 @@
   // TODO(dcross): Akaros anticipates moving the thread ID to the uthread
   // structure at some point. We should modify this code to remove the cast
   // when that happens.
-  if (in_vcore_context())
-    return 0;
-  return reinterpret_cast<struct pthread_tcb *>(current_uthread)->id;
+  if (in_vcore_context()) return 0;
+  return reinterpret_cast<struct pthread_tcb*>(current_uthread)->id;
 }
 
 #elif defined(__myriad2__)
@@ -480,12 +487,23 @@
 // userspace construct) to avoid unnecessary system calls. Without this caching,
 // it can take roughly 98ns, while it takes roughly 1ns with this caching.
 pid_t GetCachedTID() {
-#ifdef ABSL_HAVE_THREAD_LOCAL
+#ifdef __ANDROID__
+// NDK defaults to emulated TLS for API < 29, and native ELF TLS for API >= 29.
+// Emulated TLS is slower than bionic's internal caching.
+#if __ANDROID_API__ < 29
+#define ABSL_INTERNAL_USING_EMULATED_TLS 1
+#endif
+#endif
+
+#if defined(ABSL_HAVE_THREAD_LOCAL) && \
+    !defined(ABSL_INTERNAL_USING_EMULATED_TLS)
   static thread_local pid_t thread_id = GetTID();
   return thread_id;
 #else
   return GetTID();
-#endif  // ABSL_HAVE_THREAD_LOCAL
+#endif  // defined(ABSL_HAVE_THREAD_LOCAL) &&
+        // !defined(ABSL_INTERNAL_USING_EMULATED_TLS)
+#undef ABSL_INTERNAL_USING_EMULATED_TLS
 }
 
 }  // namespace base_internal