From 1ffb3747bd30d3d7bad9cfedf056aea6b243a687 Mon Sep 17 00:00:00 2001
From: Karl Wiberg <kwiberg@webrtc.org>
Date: Fri, 4 May 2018 15:04:48 +0200
Subject: [PATCH] RTC_LOG(): Internally, pass logging severity as template
 argument

When the logging severity is statically known, passing it as a
template argument instead of as a function argument saves space at the
call site.

Because this is a constructor, it's not possible to pass template
arguments explicitly---they need to be deduced. So we pass a dummy
function argument whose type encodes the logging severity, and because
the dummy is an empty struct, the ABI generally specifies that this is
a no-op with no runtime cost.

In aggregate, this reduces the size of libjingle_peerconnection_so.so
by 4 kB.

Bug: webrtc:9185
Change-Id: I8118f39dc2aed3be34b2979a239fc0d3dffa969f
Reviewed-on: https://webrtc-review.googlesource.com/74582
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23136}
---
 rtc_base/logging.h | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/rtc_base/logging.h b/rtc_base/logging.h
index 6ff31f3fda..e25d56e935 100644
--- a/rtc_base/logging.h
+++ b/rtc_base/logging.h
@@ -123,6 +123,16 @@ class LogSink {
 class LogMessage {
  public:
   LogMessage(const char* file, int line, LoggingSeverity sev);
+
+  // Same as the above, but using a compile-time constant for the logging
+  // severity. This saves space at the call site, since passing an empty struct
+  // is generally the same as not passing an argument at all.
+  template <LoggingSeverity S>
+  RTC_NO_INLINE LogMessage(const char* file,
+                           int line,
+                           std::integral_constant<LoggingSeverity, S>)
+      : LogMessage(file, line, S) {}
+
   LogMessage(const char* file,
              int line,
              LoggingSeverity sev,
@@ -281,9 +291,11 @@ class LogMessageVoidify {
 
 #define RTC_LOG_SEVERITY_PRECONDITION_C(sev) \
   !(rtc::LogMessage::Loggable<rtc::sev>()) ? (void)0 : rtc::LogMessageVoidify()&
-#define RTC_LOG(sev)                   \
-  RTC_LOG_SEVERITY_PRECONDITION_C(sev) \
-  rtc::LogMessage(__FILE__, __LINE__, rtc::sev).stream()
+#define RTC_LOG(sev)                                                        \
+  RTC_LOG_SEVERITY_PRECONDITION_C(sev)                                      \
+  rtc::LogMessage(__FILE__, __LINE__,                                       \
+                  std::integral_constant<rtc::LoggingSeverity, rtc::sev>()) \
+      .stream()
 
 // The _V version is for when a variable is passed in.  It doesn't do the
 // namespace concatenation.