Shrink most LOG callsites by one instruction (SysV x86_64 ABI) by dispatching to…

Shrink most LOG callsites by one instruction (SysV x86_64 ABI) by dispatching to per-severity constructors and omitting the severity argument.

In that particular ABI, the empty tag structs are NO_CLASS and don't appear in the generated code.

https://godbolt.org/z/crqaPh8Kv

PiperOrigin-RevId: 527989179
Change-Id: I365f11f6ca1de2623690d0a04e6f02f7de82b1fc

absl/log/internal/log_message.cc

@@ -234,6 +234,13 @@ LogMessage::LogMessage(const char* file, int line, absl::LogSeverity severity)
   LogBacktraceIfNeeded();
 }
 
+LogMessage::LogMessage(const char* file, int line, InfoTag)
+    : LogMessage(file, line, absl::LogSeverity::kInfo) {}
+LogMessage::LogMessage(const char* file, int line, WarningTag)
+    : LogMessage(file, line, absl::LogSeverity::kWarning) {}
+LogMessage::LogMessage(const char* file, int line, ErrorTag)
+    : LogMessage(file, line, absl::LogSeverity::kError) {}
+
 LogMessage::~LogMessage() {
 #ifdef ABSL_MIN_LOG_LEVEL
   if (data_->entry.log_severity() <
@@ -383,8 +390,7 @@ template LogMessage& LogMessage::operator<<(const double& v);
 template LogMessage& LogMessage::operator<<(const bool& v);
 
 void LogMessage::Flush() {
-  if (data_->entry.log_severity() < absl::MinLogLevel())
-    return;
+  if (data_->entry.log_severity() < absl::MinLogLevel()) return;
 
   if (data_->is_perror) {
     InternalStream() << ": " << absl::base_internal::StrError(errno_saver_())

absl/log/internal/log_message.h

@@ -51,9 +51,21 @@ constexpr int kLogMessageBufferSize = 15000;
 
 class LogMessage {
  public:
+  struct InfoTag {};
+  struct WarningTag {};
+  struct ErrorTag {};
+
   // Used for `LOG`.
   LogMessage(const char* file, int line,
              absl::LogSeverity severity) ABSL_ATTRIBUTE_COLD;
+  // These constructors are slightly smaller/faster to call; the severity is
+  // curried into the function pointer.
+  LogMessage(const char* file, int line,
+             InfoTag) ABSL_ATTRIBUTE_COLD ABSL_ATTRIBUTE_NOINLINE;
+  LogMessage(const char* file, int line,
+             WarningTag) ABSL_ATTRIBUTE_COLD ABSL_ATTRIBUTE_NOINLINE;
+  LogMessage(const char* file, int line,
+             ErrorTag) ABSL_ATTRIBUTE_COLD ABSL_ATTRIBUTE_NOINLINE;
   LogMessage(const LogMessage&) = delete;
   LogMessage& operator=(const LogMessage&) = delete;
   ~LogMessage() ABSL_ATTRIBUTE_COLD;

absl/log/internal/strip.h

@@ -43,14 +43,14 @@
   ABSL_LOGGING_INTERNAL_LOG_QFATAL
 #else  // !defined(STRIP_LOG) || !STRIP_LOG
 #define ABSL_LOGGING_INTERNAL_LOG_INFO \
-  ::absl::log_internal::LogMessage(__FILE__, __LINE__, \
-                                   ::absl::LogSeverity::kInfo)
+  ::absl::log_internal::LogMessage(    \
+      __FILE__, __LINE__, ::absl::log_internal::LogMessage::InfoTag{})
 #define ABSL_LOGGING_INTERNAL_LOG_WARNING \
-  ::absl::log_internal::LogMessage(__FILE__, __LINE__, \
-                                   ::absl::LogSeverity::kWarning)
+  ::absl::log_internal::LogMessage(       \
+      __FILE__, __LINE__, ::absl::log_internal::LogMessage::WarningTag{})
 #define ABSL_LOGGING_INTERNAL_LOG_ERROR \
-  ::absl::log_internal::LogMessage(__FILE__, __LINE__, \
-                                   ::absl::LogSeverity::kError)
+  ::absl::log_internal::LogMessage(     \
+      __FILE__, __LINE__, ::absl::log_internal::LogMessage::ErrorTag{})
 #define ABSL_LOGGING_INTERNAL_LOG_FATAL \
   ::absl::log_internal::LogMessageFatal(__FILE__, __LINE__)
 #define ABSL_LOGGING_INTERNAL_LOG_QFATAL \