Move vlog_config to the internal directory and change the namespace of VLogSite

PiperOrigin-RevId: 588403935
Change-Id: I6a3af3c044b887ec65b19390d316cfb3ccdcc853

1 files changed, 163 insertions, 0 deletions

diff --git a/absl/log/internal/vlog_config.h b/absl/log/internal/vlog_config.h
new file mode 100644
index 00000000..b6e322c4
--- /dev/null
+++ b/absl/log/internal/vlog_config.h
@@ -0,0 +1,163 @@
+// Copyright 2022 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.
+//
+// -----------------------------------------------------------------------------
+// vlog_config.h
+// -----------------------------------------------------------------------------
+//
+// This header file defines `VLogSite`, a public primitive that represents
+// a callsite for the `VLOG` family of macros and related libraries.
+// It also declares and defines multiple internal utilities used to implement
+// `VLOG`, such as `VLogSiteManager`.
+
+#ifndef ABSL_LOG_INTERNAL_VLOG_CONFIG_H_
+#define ABSL_LOG_INTERNAL_VLOG_CONFIG_H_
+
+// IWYU pragma: private, include "absl/log/log.h"
+
+#include <atomic>
+#include <cstdint>
+#include <functional>
+#include <limits>
+#include <type_traits>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/optimization.h"
+#include "absl/base/thread_annotations.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+
+class SyntheticBinary;
+class VLogSite;
+
+int RegisterAndInitialize(VLogSite* v);
+void UpdateVLogSites();
+constexpr int kUseFlag = (std::numeric_limits<int16_t>::min)();
+
+// Represents a unique callsite for a `VLOG()` or `VLOG_IS_ON()` call.
+//
+// Libraries that provide `VLOG`-like functionality should use this to
+// efficiently handle --vmodule.
+//
+// VLogSite objects must not be destroyed until the program exits. Doing so will
+// probably yield nasty segfaults in VLogSiteManager::UpdateLogSites(). The
+// recommendation is to make all such objects function-local statics.
+class VLogSite final {
+ public:
+  // `f` must not be destroyed until the program exits.
+  explicit constexpr VLogSite(const char* f)
+      : file_(f), v_(kUninitialized), next_(nullptr) {}
+  VLogSite(const VLogSite&) = delete;
+  VLogSite& operator=(const VLogSite&) = delete;
+
+  // Inlining the function yields a ~3x performance improvement at the cost of a
+  // 1.5x code size increase at the call site.
+  // Takes locks but does not allocate memory.
+  ABSL_ATTRIBUTE_ALWAYS_INLINE
+  bool IsEnabled(int level) {
+    int stale_v = v_.load(std::memory_order_relaxed);
+    if (ABSL_PREDICT_TRUE(level > stale_v)) {
+      return false;
+    }
+
+    // We put everything other than the fast path, i.e. vlogging is initialized
+    // but not on, behind an out-of-line function to reduce code size.
+    // "level" is almost always a call-site constant, so we can save a bit
+    // of code space by special-casing for a few common levels.
+#if ABSL_HAVE_BUILTIN(__builtin_constant_p) || defined(__GNUC__)
+    if (__builtin_constant_p(level)) {
+      if (level == 0) return SlowIsEnabled0(stale_v);
+      if (level == 1) return SlowIsEnabled1(stale_v);
+      if (level == 2) return SlowIsEnabled2(stale_v);
+      if (level == 3) return SlowIsEnabled3(stale_v);
+      if (level == 4) return SlowIsEnabled4(stale_v);
+      if (level == 5) return SlowIsEnabled5(stale_v);
+    }
+#endif
+    return SlowIsEnabled(stale_v, level);
+  }
+
+ private:
+  friend int log_internal::RegisterAndInitialize(VLogSite* v);
+  friend void log_internal::UpdateVLogSites();
+  friend class log_internal::SyntheticBinary;
+  static constexpr int kUninitialized = (std::numeric_limits<int>::max)();
+
+  // SlowIsEnabled performs slower checks to determine whether a log site is
+  // enabled. Because it is expected to be called somewhat rarely
+  // (comparatively), it is not inlined to save on code size.
+  //
+  // Prerequisites to calling SlowIsEnabled:
+  //   1) stale_v is uninitialized OR
+  //   2) stale_v is initialized and >= level (meaning we must log).
+  // Takes locks but does not allocate memory.
+  ABSL_ATTRIBUTE_NOINLINE
+  bool SlowIsEnabled(int stale_v, int level);
+  ABSL_ATTRIBUTE_NOINLINE bool SlowIsEnabled0(int stale_v);
+  ABSL_ATTRIBUTE_NOINLINE bool SlowIsEnabled1(int stale_v);
+  ABSL_ATTRIBUTE_NOINLINE bool SlowIsEnabled2(int stale_v);
+  ABSL_ATTRIBUTE_NOINLINE bool SlowIsEnabled3(int stale_v);
+  ABSL_ATTRIBUTE_NOINLINE bool SlowIsEnabled4(int stale_v);
+  ABSL_ATTRIBUTE_NOINLINE bool SlowIsEnabled5(int stale_v);
+
+  // This object is too size-sensitive to use absl::string_view.
+  const char* const file_;
+  std::atomic<int> v_;
+  std::atomic<VLogSite*> next_;
+};
+static_assert(std::is_trivially_destructible<VLogSite>::value,
+              "VLogSite must be trivially destructible");
+
+// Returns the current verbose log level of `file`.
+// Does not allocate memory.
+int VLogLevel(absl::string_view file);
+
+// Registers a site `v` to get updated as `vmodule` and `v` change.  Also
+// initializes the site based on their current values, and returns that result.
+// Does not allocate memory.
+int RegisterAndInitialize(VLogSite* v);
+
+// Allocates memory.
+void UpdateVLogSites();
+
+// Completely overwrites the saved value of `vmodule`.
+// Allocates memory.
+void UpdateVModule(absl::string_view vmodule);
+
+// Updates the global verbosity level to `v` and returns the prior value.
+// Allocates memory.
+int UpdateGlobalVLogLevel(int v);
+
+// Atomically prepends `module_pattern=log_level` to the start of vmodule.
+// Returns the prior value for `module_pattern` if there was an exact match and
+// `global_v` otherwise.
+// Allocates memory.
+int PrependVModule(absl::string_view module_pattern, int log_level);
+
+// Registers `on_update` to be called whenever `v` or `vmodule` change.
+// Allocates memory.
+void OnVLogVerbosityUpdate(std::function<void()> cb);
+
+// Does not allocate memory.
+VLogSite* SetVModuleListHeadForTestOnly(VLogSite* v);
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_LOG_INTERNAL_VLOG_CONFIG_H_