De-duplicate ranges information in persistent memory.

base/metrics/bucket_ranges.h

Index: base/metrics/bucket_ranges.h
diff --git a/base/metrics/bucket_ranges.h b/base/metrics/bucket_ranges.h
index c356195ba78ca2716897463b576f73a26bbe9b70..db82e552b3ac5c193ffbed00e7c195008dd15681 100644
--- a/base/metrics/bucket_ranges.h
+++ b/base/metrics/bucket_ranges.h
@@ -24,6 +24,7 @@
 
 #include <limits.h>
 
+#include "base/atomicops.h"
 #include "base/base_export.h"
 #include "base/macros.h"
 #include "base/metrics/histogram_base.h"
@@ -58,6 +59,17 @@ class BASE_EXPORT BucketRanges {
   // Return true iff |other| object has same ranges_ as |this| object's ranges_.
   bool Equals(const BucketRanges* other) const;
 
+  // Set and get a reference into persistent memory where this bucket data
+  // can be found (and re-used). These calls are internally atomic with no
+  // safety against overwriting an existing value since though it is wasteful
+  // to have multiple identical persistent records, it is still safe.
+  void set_persistent_reference(uint32_t ref) const {
+    subtle::NoBarrier_Store(&persistent_reference_, ref);
+  }
+  uint32_t persistent_reference() const {
+    return subtle::NoBarrier_Load(&persistent_reference_);
+  }
+
  private:
   // A monotonically increasing list of values which determine which bucket to
   // put a sample into.  For each index, show the smallest sample that can be
@@ -71,6 +83,12 @@ class BASE_EXPORT BucketRanges {
   // noise on UMA dashboard.
   uint32_t checksum_;
 
+  // A reference into a global PersistentMemoryAllocator where the ranges
+  // information is stored. This allows for the record to be created once and
+  // re-used simply by having all histograms with the same ranges use the
+  // same reference.
+  mutable subtle::Atomic32 persistent_reference_ = 0;
+
   DISALLOW_COPY_AND_ASSIGN(BucketRanges);
 };