Make code generated for histogram macros more compact.

In builds where DCHECKs are compiled in (e.g. developer
builds and Chrome Canaries), the DCHECK_EQ() in the
histogram macro was expanding to a lot of generated
machine code.

This CL moves that DCHECK() to a function instead, so that the
code is not duplicated in every call site. The function call is
wrapped behind a DCHECK_IS_ON() conditional, so that it will
still be omitted if DCHECKs are not compiled into the build.

Makes my Chromium release build smaller by over 1MB,
measuring size of Chromium Framework.

Before size: 146547380
After size:  145400692

BUG=343946
TEST=No functional changes.

Review URL: https://codereview.chromium.org/167343002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251486 0039d316-1c4b-4281-b951-d872f2087c98

base/metrics/histogram.h

@@ -155,8 +155,8 @@ class Lock;
       base::subtle::Release_Store(&atomic_histogram_pointer, \
           reinterpret_cast<base::subtle::AtomicWord>(histogram_pointer)); \
     } \
-    DCHECK_EQ(histogram_pointer->histogram_name(), \
-              std::string(constant_histogram_name)); \
+    if (DCHECK_IS_ON()) \
+      histogram_pointer->CheckName(constant_histogram_name); \
     histogram_pointer->histogram_add_method_invocation; \
   } while (0)
 

base/metrics/histogram_base.cc

@@ -20,7 +20,7 @@
 namespace base {
 
 std::string HistogramTypeToString(HistogramType type) {
-  switch(type) {
+  switch (type) {
     case HISTOGRAM:
       return "HISTOGRAM";
     case LINEAR_HISTOGRAM:
@@ -66,6 +66,10 @@ HistogramBase::HistogramBase(const std::string& name)
 
 HistogramBase::~HistogramBase() {}
 
+void HistogramBase::CheckName(const StringPiece& name) const {
+  DCHECK_EQ(histogram_name(), name);
+}
+
 void HistogramBase::SetFlags(int32 flags) {
   flags_ |= flags;
 }

base/metrics/histogram_base.h

@@ -12,6 +12,7 @@
 #include "base/base_export.h"
 #include "base/basictypes.h"
 #include "base/memory/scoped_ptr.h"
+#include "base/strings/string_piece.h"
 #include "base/time/time.h"
 
 class Pickle;
@@ -48,8 +49,8 @@ BASE_EXPORT_PRIVATE HistogramBase* DeserializeHistogramInfo(
 
 class BASE_EXPORT HistogramBase {
  public:
-  typedef int Sample;                   // Used for samples.
-  typedef subtle::Atomic32 AtomicCount; // Used to count samples.
+  typedef int Sample;                    // Used for samples.
+  typedef subtle::Atomic32 AtomicCount;  // Used to count samples.
   typedef int32 Count;  // Used to manipulate counts in temporaries.
 
   static const Sample kSampleType_MAX;  // INT_MAX
@@ -92,6 +93,11 @@ class BASE_EXPORT HistogramBase {
 
   std::string histogram_name() const { return histogram_name_; }
 
+  // Comapres |name| to the histogram name and triggers a DCHECK if they do not
+  // match. This is a helper function used by histogram macros, which results in
+  // in more compact machine code being generated by the macros.
+  void CheckName(const StringPiece& name) const;
+
   // Operations with Flags enum.
   int32 flags() const { return flags_; }
   void SetFlags(int32 flags);
@@ -137,7 +143,7 @@ class BASE_EXPORT HistogramBase {
   // customize the output.
   void WriteJSON(std::string* output) const;
 
-protected:
+ protected:
   // Subclasses should implement this function to make SerializeInfo work.
   virtual bool SerializeInfoImpl(Pickle* pickle) const = 0;