Improve EAT_STREAM_PARAMETERS for Windows x86

base/logging.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_LOGGING_H_
 #define BASE_LOGGING_H_
 
 #include <stddef.h>
 
 #include <cassert>
@@ skipping 408 matching lines @@
   COMPACT_GOOGLE_LOG_EX_ ## severity(ErrnoLogMessage, \
       ::logging::GetLastSystemErrorCode()).stream()
 #endif
 
 #define PLOG(severity)                                          \
   LAZY_STREAM(PLOG_STREAM(severity), LOG_IS_ON(severity))
 
 #define PLOG_IF(severity, condition) \
   LAZY_STREAM(PLOG_STREAM(severity), LOG_IS_ON(severity) && (condition))
 
-// The actual stream used isn't important.
-#define EAT_STREAM_PARAMETERS                                           \
-  true ? (void) 0 : ::logging::LogMessageVoidify() & LOG_STREAM(FATAL)
+// Note that the null ostream is used instead of an arbitrary LOG() stream to
+// avoid the creation of an object with a non-trivial destructor (LogMessage).
+// On MSVC x86 (checked on 2015 Update 3), doing so causes a few additional
+// pointless instructions to be emitted even at full optimization level, and

[dcheng, 2016/12/10 03:44:03]

Nit: remove 'and'

[scottmg, 2016/12/10 05:02:01]

Oops, done.

+// even though the : arm of the ternary operator is clearly never executed.
+// Using a simpler POD object with a templated operator<< also works to avoid
+// these instructions. However, this causes warnings on statically defined
+// implementations of operator<<(std::ostream, ...) in some .cc files, because
+// they become defined-but-unreferenced functions.
+#define EAT_STREAM_PARAMETERS \
+  true                        \
+      ? (void)0               \
+      : ::logging::LogMessageVoidify() & (*reinterpret_cast<std::ostream*>(0))
 
 // Captures the result of a CHECK_EQ (for example) and facilitates testing as a
 // boolean.
 class CheckOpResult {
  public:
   // |message| must be non-null if and only if the check failed.
   CheckOpResult(std::string* message) : message_(message) {}
   // Returns true if the check succeeded.
   operator bool() const { return !message_; }
   // Returns the message.
@@ skipping 573 matching lines @@
 #elif NOTIMPLEMENTED_POLICY == 5
 #define NOTIMPLEMENTED() do {\
   static bool logged_once = false;\
   LOG_IF(ERROR, !logged_once) << NOTIMPLEMENTED_MSG;\
   logged_once = true;\
 } while(0);\
 EAT_STREAM_PARAMETERS
 #endif
 
 #endif  // BASE_LOGGING_H_