Refactor histogram_macros.h. This improves documentation for the macros in the file, moves LOCAL_* …

base/metrics/histogram_macros.h

Index: base/metrics/histogram_macros.h
diff --git a/base/metrics/histogram_macros.h b/base/metrics/histogram_macros.h
index 1307257441f39ccbcafaf22ce41253e8cb045cbb..1284b4ec17599d4e6847890b192590fc175db04d 100644
--- a/base/metrics/histogram_macros.h
+++ b/base/metrics/histogram_macros.h
@@ -5,317 +5,248 @@
 #ifndef BASE_METRICS_HISTOGRAM_MACROS_H_
 #define BASE_METRICS_HISTOGRAM_MACROS_H_
 
-#include "base/atomicops.h"
-#include "base/logging.h"
 #include "base/metrics/histogram.h"
+#include "base/metrics/histogram_macros_internal.h"
+#include "base/metrics/histogram_macros_local.h"
 #include "base/time/time.h"
 
-// Macros for efficient use of histograms. See documentation in histogram.h.
-//
+// TODO(nikunjb): Move sparse macros to this file.
 // UMA_HISTOGRAM_SPARSE_SLOWLY is defined in sparse_histogram.h as it has
 // different #include dependencies.
 
+// TODO(rkaplow): Link to proper documentation on metric creation once we have
+// it in a good state.
+
+// All of these macros must be called with |name| as a runtime constant - it
+// doesn't have to literally be a constant, but it must be the same string on
+// all calls from a particular call site. If this rule is violated, it is
+// possible the data will be written to the wrong histogram.
+
 //------------------------------------------------------------------------------
-// Histograms are often put in areas where they are called many many times, and
-// performance is critical.  As a result, they are designed to have a very low
-// recurring cost of executing (adding additional samples).  Toward that end,
-// the macros declare a static pointer to the histogram in question, and only
-// take a "slow path" to construct (or find) the histogram on the first run
-// through the macro.  We leak the histograms at shutdown time so that we don't
-// have to validate using the pointers at any time during the running of the
-// process.
-
-// The following code is generally what a thread-safe static pointer
-// initialization looks like for a histogram (after a macro is expanded).  This
-// sample is an expansion (with comments) of the code for
-// LOCAL_HISTOGRAM_CUSTOM_COUNTS().
-
-/*
-  do {
-    // The pointer's presence indicates the initialization is complete.
-    // Initialization is idempotent, so it can safely be atomically repeated.
-    static base::subtle::AtomicWord atomic_histogram_pointer = 0;
-
-    // Acquire_Load() ensures that we acquire visibility to the pointed-to data
-    // in the histogram.
-    base::Histogram* histogram_pointer(reinterpret_cast<base::Histogram*>(
-        base::subtle::Acquire_Load(&atomic_histogram_pointer)));
-
-    if (!histogram_pointer) {
-      // This is the slow path, which will construct OR find the matching
-      // histogram.  FactoryGet includes locks on a global histogram name map
-      // and is completely thread safe.
-      histogram_pointer = base::Histogram::FactoryGet(
-          name, min, max, bucket_count, base::HistogramBase::kNoFlags);
-
-      // Use Release_Store to ensure that the histogram data is made available
-      // globally before we make the pointer visible.
-      // Several threads may perform this store, but the same value will be
-      // stored in all cases (for a given named/spec'ed histogram).
-      // We could do this without any barrier, since FactoryGet entered and
-      // exited a lock after construction, but this barrier makes things clear.
-      base::subtle::Release_Store(&atomic_histogram_pointer,
-          reinterpret_cast<base::subtle::AtomicWord>(histogram_pointer));
-    }
-
-    // Ensure calling contract is upheld, and the name does NOT vary.
-    DCHECK(histogram_pointer->histogram_name() == constant_histogram_name);
-
-    histogram_pointer->Add(sample);
-  } while (0);
-*/
-
-// The above pattern is repeated in several macros.  The only elements that
-// vary are the invocation of the Add(sample) vs AddTime(sample), and the choice
-// of which FactoryGet method to use.  The different FactoryGet methods have
-// various argument lists, so the function with its argument list is provided as
-// a macro argument here.  The name is only used in a DCHECK, to assure that
-// callers don't try to vary the name of the histogram (which would tend to be
-// ignored by the one-time initialization of the histogtram_pointer).
-
-// In some cases (integration into 3rd party code), it's useful to seperate the
-// definition of |atomic_histogram_poiner| from its use. To achieve this we
-// define HISTOGRAM_POINTER_USE, which uses an |atomic_histogram_pointer|, and
-// STATIC_HISTOGRAM_POINTER_BLOCK, which defines an |atomic_histogram_pointer|
-// and forwards to HISTOGRAM_POINTER_USE.
-#define HISTOGRAM_POINTER_USE(atomic_histogram_pointer,                   \
-                              constant_histogram_name,                    \
-                              histogram_add_method_invocation,            \
-                              histogram_factory_get_invocation)           \
-  do {                                                                    \
-    base::HistogramBase* histogram_pointer(                               \
-        reinterpret_cast<base::HistogramBase*>(                           \
-            base::subtle::Acquire_Load(atomic_histogram_pointer)));       \
-    if (!histogram_pointer) {                                             \
-      histogram_pointer = histogram_factory_get_invocation;               \
-      base::subtle::Release_Store(                                        \
-          atomic_histogram_pointer,                                       \
-          reinterpret_cast<base::subtle::AtomicWord>(histogram_pointer)); \
-    }                                                                     \
-    if (DCHECK_IS_ON())                                                   \
-      histogram_pointer->CheckName(constant_histogram_name);              \
-    histogram_pointer->histogram_add_method_invocation;                   \
-  } while (0)
-
-// Defines the static |atomic_histogram_pointer| and forwards to
-// HISTOGRAM_POINTER_USE.
-#define STATIC_HISTOGRAM_POINTER_BLOCK(constant_histogram_name,               \
-                                       histogram_add_method_invocation,       \
-                                       histogram_factory_get_invocation)      \
-  do {                                                                        \
-    static base::subtle::AtomicWord atomic_histogram_pointer = 0;             \
-    HISTOGRAM_POINTER_USE(&atomic_histogram_pointer, constant_histogram_name, \
-                          histogram_add_method_invocation,                    \
-                          histogram_factory_get_invocation);                  \
-  } while (0)
+// Enumeration histograms.
+
+// These macros create histograms for enumerated data. Ideally, the data should
+// be of the form of "event occurs, log the result". We recommended not putting
+// related but not directly connected data as enums within the same histogram.
+// You should be defining an associated Enum, and the input sample should be
+// an element of the Enum.
+// All of these macros must be called with |name| as a runtime constant.
+
+// Sample usage:
+//   UMA_HISTOGRAM_ENUMERATION("My.Enumeration", Enum::VALUE, Enum::EVENT_MAX);

[Alexei Svitkine (slow), 2016/09/26 21:25:17]

Nit: Enums actually don't require prefixing with "Enum::" foo unless you use
enum classes, which require casting. So this example isn't very real.

[rkaplow, 2016/09/26 21:37:48]

Done.

+// Enum values can be appended, but existing enums must never be renumbered or

[Alexei Svitkine (slow), 2016/09/26 21:25:16]

Nit: "Enum values can be appended" sounds confusing to me. I would rephrase
("new enum values can be added later on, but").

[rkaplow, 2016/09/26 21:37:48]

Done.

+// delete and reused. The value in |sample| must be strictly less than
+// |enum_max|.
+
+#define UMA_HISTOGRAM_ENUMERATION(name, sample, enum_max)                      \
+    INTERNAL_HISTOGRAM_ENUMERATION_WITH_FLAG(                                  \
+        name, sample, enum_max,                                                \
+        base::HistogramBase::kUmaTargetedHistogramFlag)
+
+// Histogram for boolean values.
+
+// Sample usage:
+//   UMA_HISTOGRAM_BOOLEAN("Histogram.Boolean", bool);
+#define UMA_HISTOGRAM_BOOLEAN(name, sample)                                    \
+    STATIC_HISTOGRAM_POINTER_BLOCK(name, AddBoolean(sample),                   \
+        base::BooleanHistogram::FactoryGet(name,                               \
+            base::HistogramBase::kUmaTargetedHistogramFlag))
 
 //------------------------------------------------------------------------------
-// Provide easy general purpose histogram in a macro, just like stats counters.
-// Most of these macros use 50 buckets, but check the definition for details.
-//
-// All of these macros must be called with |name| as a runtime constant --- it
-// doesn't have to literally be a constant, but it must be the same string on
-// all calls from a particular call site. If this rule is violated,
-// STATIC_HISTOGRAM_POINTER_BLOCK will DCHECK, and if DCHECKS are disabled, the
-// data will be written to the wrong histogram.
+// Percentage histograms.
 
-#define LOCAL_HISTOGRAM_TIMES(name, sample) LOCAL_HISTOGRAM_CUSTOM_TIMES( \
-    name, sample, base::TimeDelta::FromMilliseconds(1), \
-    base::TimeDelta::FromSeconds(10), 50)
+// Used for capturing basic percentages. This will be 100 buckets of size 1.
+
+// Sample usage:
+//   UMA_HISTOGRAM_PERCENTAGE("Histogram.Percent", percent_as_int);
+#define UMA_HISTOGRAM_PERCENTAGE(name, under_one_hundred)                      \
+    UMA_HISTOGRAM_ENUMERATION(name, under_one_hundred, 101)
+
+
+//------------------------------------------------------------------------------
+// Count histograms. These are used for collecting numeric data. Note that we
+// have macros for more specialized use cases below (memory, time, percentages).
+
+// The number suffixes here refer to the max size of the sample, i.e. COUNT_1000
+// will be able to collect samples of counts up to 1000. The default number of
+// buckets in all default macros is 50. We recommend erring on the side of too
+// large a range versus too short a range.
+// These macros default to exponential histograms - i.e. the lengths of the
+// bucket ranges exponentially increase as the sample range increases.
+// These should *not* be used if you are interested in exact counts, i.e. a
+// bucket range of 1. In these cases, you should use the ENUMERATION macros
+// defined later. These should also not be used to capture the number of some
+// event, i.e. "button X was clicked N times". In this cases, an enum should be
+// used, ideally with an appropriate baseline enum entry included.
+
+// Sample usage:
+//   UMA_HISTOGRAM_COUNTS_1M("My.Histogram", sample);
+
+#define UMA_HISTOGRAM_COUNTS_100(name, sample) UMA_HISTOGRAM_CUSTOM_COUNTS(    \
+    name, sample, 1, 100, 50)
 
-// For folks that need real specific times, use this to select a precise range
-// of times you want plotted, and the number of buckets you want used.
-#define LOCAL_HISTOGRAM_CUSTOM_TIMES(name, sample, min, max, bucket_count) \
-    STATIC_HISTOGRAM_POINTER_BLOCK(name, AddTime(sample), \
-        base::Histogram::FactoryTimeGet(name, min, max, bucket_count, \
-                                        base::HistogramBase::kNoFlags))
+#define UMA_HISTOGRAM_COUNTS_1000(name, sample) UMA_HISTOGRAM_CUSTOM_COUNTS(   \
+    name, sample, 1, 1000, 50)
+
+#define UMA_HISTOGRAM_COUNTS_10000(name, sample) UMA_HISTOGRAM_CUSTOM_COUNTS(  \
+    name, sample, 1, 10000, 50)
+
+#define UMA_HISTOGRAM_COUNTS_100000(name, sample) UMA_HISTOGRAM_CUSTOM_COUNTS( \
+    name, sample, 1, 100000, 50)
 
-#define LOCAL_HISTOGRAM_COUNTS(name, sample) LOCAL_HISTOGRAM_CUSTOM_COUNTS( \
+#define UMA_HISTOGRAM_COUNTS_1M(name, sample) UMA_HISTOGRAM_CUSTOM_COUNTS(     \
     name, sample, 1, 1000000, 50)
 
-#define LOCAL_HISTOGRAM_COUNTS_100(name, sample) \
-    LOCAL_HISTOGRAM_CUSTOM_COUNTS(name, sample, 1, 100, 50)
-
-#define LOCAL_HISTOGRAM_COUNTS_10000(name, sample) \
-    LOCAL_HISTOGRAM_CUSTOM_COUNTS(name, sample, 1, 10000, 50)
-
-#define LOCAL_HISTOGRAM_CUSTOM_COUNTS(name, sample, min, max, bucket_count) \
-    INTERNAL_HISTOGRAM_CUSTOM_COUNTS_WITH_FLAG(                             \
-        name, sample, min, max, bucket_count, base::HistogramBase::kNoFlags)
-
-// This is a helper macro used by other macros and shouldn't be used directly.
-#define INTERNAL_HISTOGRAM_CUSTOM_COUNTS_WITH_FLAG(name, sample, min, max, \
-                                                   bucket_count, flag)     \
-    STATIC_HISTOGRAM_POINTER_BLOCK(                                        \
-        name, Add(sample),                                                 \
-        base::Histogram::FactoryGet(name, min, max, bucket_count, flag))
-
-// This is a helper macro used by other macros and shouldn't be used directly.
-// One additional bucket is created in the LinearHistogram for the illegal
-// values >= boundary_value so that mistakes in calling the UMA enumeration
-// macros can be detected.
-#define INTERNAL_HISTOGRAM_ENUMERATION_WITH_FLAG(name, sample, boundary, flag) \
-    STATIC_HISTOGRAM_POINTER_BLOCK(                                            \
-        name, Add(sample),                                                     \
-        base::LinearHistogram::FactoryGet(                                     \
-            name, 1, boundary, boundary + 1, flag))
-
-#define LOCAL_HISTOGRAM_PERCENTAGE(name, under_one_hundred) \
-    LOCAL_HISTOGRAM_ENUMERATION(name, under_one_hundred, 101)
-
-#define LOCAL_HISTOGRAM_BOOLEAN(name, sample) \
-    STATIC_HISTOGRAM_POINTER_BLOCK(name, AddBoolean(sample), \
-        base::BooleanHistogram::FactoryGet(name, base::Histogram::kNoFlags))
-
-// Support histograming of an enumerated value.  The samples should always be
-// strictly less than |boundary_value| -- this prevents you from running into
-// problems down the line if you add additional buckets to the histogram.  Note
-// also that, despite explicitly setting the minimum bucket value to |1| below,
-// it is fine for enumerated histograms to be 0-indexed -- this is because
-// enumerated histograms should never have underflow. One additional bucket is
-// created in the LinearHistogram for the illegal values >= boundary_value so
-// that mistakes in calling this macro can be detected.
-#define LOCAL_HISTOGRAM_ENUMERATION(name, sample, boundary_value) \
-    STATIC_HISTOGRAM_POINTER_BLOCK(name, Add(sample), \
-        base::LinearHistogram::FactoryGet(name, 1, boundary_value, \
-            boundary_value + 1, base::HistogramBase::kNoFlags))
-
-// Support histograming of an enumerated value. Samples should be one of the
-// std::vector<int> list provided via |custom_ranges|. See comments above
-// CustomRanges::FactoryGet about the requirement of |custom_ranges|.
-// You can use the helper function CustomHistogram::ArrayToCustomRanges to
-// transform a C-style array of valid sample values to a std::vector<int>.
-#define LOCAL_HISTOGRAM_CUSTOM_ENUMERATION(name, sample, custom_ranges) \
-    STATIC_HISTOGRAM_POINTER_BLOCK(name, Add(sample), \
-        base::CustomHistogram::FactoryGet(name, custom_ranges, \
-                                          base::HistogramBase::kNoFlags))
-
-#define LOCAL_HISTOGRAM_MEMORY_KB(name, sample) LOCAL_HISTOGRAM_CUSTOM_COUNTS( \
-    name, sample, 1000, 500000, 50)
+#define UMA_HISTOGRAM_COUNTS_10M(name, sample) UMA_HISTOGRAM_CUSTOM_COUNTS(    \
+    name, sample, 1, 10000000, 50)
+
+// This can be used when the default ranges are not sufficient. This macro lets
+// the metric developer customize the min and max of the sampled range, as well
+// as the number of buckets recorded.
+// Any data outside the range here will be put in underflow and overflow
+// buckets.
+
+// Sample usage:
+//   UMA_HISTOGRAM_CUSTOM_COUNTS("My.Histogram", 1, 100000000, 100);
+#define UMA_HISTOGRAM_CUSTOM_COUNTS(name, sample, min, max, bucket_count)      \
+    INTERNAL_HISTOGRAM_CUSTOM_COUNTS_WITH_FLAG(                                \
+        name, sample, min, max, bucket_count,                                  \
+        base::HistogramBase::kUmaTargetedHistogramFlag)
+
 
 //------------------------------------------------------------------------------
-// The following macros provide typical usage scenarios for callers that wish
-// to record histogram data, and have the data submitted/uploaded via UMA.
-// Not all systems support such UMA, but if they do, the following macros
-// should work with the service.
+// Timing histograms. These are used for collecting timing data (generally
+// latencies).
 
-#define UMA_HISTOGRAM_TIMES(name, sample) UMA_HISTOGRAM_CUSTOM_TIMES( \
-    name, sample, base::TimeDelta::FromMilliseconds(1), \
+// These macros create exponentially sized histograms (lengths of the bucket
+// ranges exponentially increase as the sample range increases). The input
+// sample is a base::TimeDelta. The output data is measured in ms granularity.
+// All of these macros must be called with |name| as a runtime constant.
+
+// Sample usage:
+//   UMA_HISTOGRAM_TIMES("My.Timing.Histogram", time_delta);
+
+// Short timings - up to 10 seconds.
+#define UMA_HISTOGRAM_TIMES(name, sample) UMA_HISTOGRAM_CUSTOM_TIMES(          \
+    name, sample, base::TimeDelta::FromMilliseconds(1),                        \
     base::TimeDelta::FromSeconds(10), 50)
 
-#define UMA_HISTOGRAM_MEDIUM_TIMES(name, sample) UMA_HISTOGRAM_CUSTOM_TIMES( \
-    name, sample, base::TimeDelta::FromMilliseconds(10), \
+// Medium timings - up to 3 minutes. Note this starts at 10ms (no good reason,
+// but not worth changing).
+#define UMA_HISTOGRAM_MEDIUM_TIMES(name, sample) UMA_HISTOGRAM_CUSTOM_TIMES(   \
+    name, sample, base::TimeDelta::FromMilliseconds(10),                       \
     base::TimeDelta::FromMinutes(3), 50)
 
-// Use this macro when times can routinely be much longer than 10 seconds.
-#define UMA_HISTOGRAM_LONG_TIMES(name, sample) UMA_HISTOGRAM_CUSTOM_TIMES( \
-    name, sample, base::TimeDelta::FromMilliseconds(1), \
+// Long timings - up to an hour.
+#define UMA_HISTOGRAM_LONG_TIMES(name, sample) UMA_HISTOGRAM_CUSTOM_TIMES(     \
+    name, sample, base::TimeDelta::FromMilliseconds(1),                        \
     base::TimeDelta::FromHours(1), 50)
 
-// Use this macro when times can routinely be much longer than 10 seconds and
-// you want 100 buckets.
+// Long timings with higher granularity - up to an hour with 100 buckets.
 #define UMA_HISTOGRAM_LONG_TIMES_100(name, sample) UMA_HISTOGRAM_CUSTOM_TIMES( \
-    name, sample, base::TimeDelta::FromMilliseconds(1), \
+    name, sample, base::TimeDelta::FromMilliseconds(1),                        \
     base::TimeDelta::FromHours(1), 100)
 
-#define UMA_HISTOGRAM_CUSTOM_TIMES(name, sample, min, max, bucket_count) \
-    STATIC_HISTOGRAM_POINTER_BLOCK(name, AddTime(sample), \
-        base::Histogram::FactoryTimeGet(name, min, max, bucket_count, \
-            base::HistogramBase::kUmaTargetedHistogramFlag))
+// This can be used when the default ranges are not sufficient. This macro lets
+// the metric developer customize the min and max of the sampled range, as well
+// as the number of buckets recorded.
 
-#define UMA_HISTOGRAM_COUNTS(name, sample) UMA_HISTOGRAM_CUSTOM_COUNTS( \
-    name, sample, 1, 1000000, 50)
+// Sample usage:
+//   UMA_HISTOGRAM_CUSTOM_TIMES("Very.Long.Timing.Histogram", duration_in_ms,
+//       base::TimeDelta::FromSeconds(1), base::TimeDelta::FromDays(1), 100);
+#define UMA_HISTOGRAM_CUSTOM_TIMES(name, sample, min, max, bucket_count)       \
+    STATIC_HISTOGRAM_POINTER_BLOCK(name, AddTime(sample),                      \
+        base::Histogram::FactoryTimeGet(name, min, max, bucket_count,          \
+            base::HistogramBase::kUmaTargetedHistogramFlag))
 
-#define UMA_HISTOGRAM_COUNTS_100(name, sample) UMA_HISTOGRAM_CUSTOM_COUNTS( \
-    name, sample, 1, 100, 50)
+// Scoped class which logs its time on this earth as a UMA statistic. This is
+// recommended for when you want a histogram which measures the time it takes
+// for a method to execute. This measures up to 10 seconds. This uses
+// UMA_HISTOGRAM_TIMES under the hood.
 
-#define UMA_HISTOGRAM_COUNTS_1000(name, sample) UMA_HISTOGRAM_CUSTOM_COUNTS( \
-    name, sample, 1, 1000, 50)
+// Sample usage:
+//   void Function() {
+//     SCOPED_UMA_HISTOGRAM_TIMER("Component.FunctionTime");
+//     ...
+//   }
+#define SCOPED_UMA_HISTOGRAM_TIMER(name)                                       \
+  INTERNAL_SCOPED_UMA_HISTOGRAM_TIMER_EXPANDER(name, false, __COUNTER__)
 
-#define UMA_HISTOGRAM_COUNTS_10000(name, sample) UMA_HISTOGRAM_CUSTOM_COUNTS( \
-    name, sample, 1, 10000, 50)
+// Similar scoped histogram timer, but this uses UMA_HISTOGRAM_LONG_TIMES_100,
+// which measures up to an hour, and uses 100 buckets. This is more expensive
+// to store, so only use if this often takes >10 seconds.
+#define SCOPED_UMA_HISTOGRAM_LONG_TIMER(name)                                  \
+  INTERNAL_SCOPED_UMA_HISTOGRAM_TIMER_EXPANDER(name, true, __COUNTER__)
 
-#define UMA_HISTOGRAM_CUSTOM_COUNTS(name, sample, min, max, bucket_count) \
-    INTERNAL_HISTOGRAM_CUSTOM_COUNTS_WITH_FLAG(                           \
-        name, sample, min, max, bucket_count,                             \
-        base::HistogramBase::kUmaTargetedHistogramFlag)
 
-#define UMA_STABILITY_HISTOGRAM_COUNTS_100(name, sample) \
-    UMA_STABILITY_HISTOGRAM_CUSTOM_COUNTS(name, sample, 1, 100, 50)
+//------------------------------------------------------------------------------
+// Memory histograms.
 
-#define UMA_STABILITY_HISTOGRAM_CUSTOM_COUNTS(name, sample, min, max, \
-                                              bucket_count)           \
-    INTERNAL_HISTOGRAM_CUSTOM_COUNTS_WITH_FLAG(                       \
-        name, sample, min, max, bucket_count,                         \
-        base::HistogramBase::kUmaStabilityHistogramFlag)
+// These macros create exponentially sized histograms (lengths of the bucket
+// ranges exponentially increase as the sample range increases). The input
+// sample must be a number measured in kilobytes.
+// All of these macros must be called with |name| as a runtime constant.
 
-#define UMA_HISTOGRAM_MEMORY_KB(name, sample) UMA_HISTOGRAM_CUSTOM_COUNTS( \
-    name, sample, 1000, 500000, 50)
+// Sample usage:
+//   UMA_HISTOGRAM_MEMORY_KB("My.Memory.Histogram", memory_in_kb);
 
-#define UMA_HISTOGRAM_MEMORY_MB(name, sample) UMA_HISTOGRAM_CUSTOM_COUNTS( \
-    name, sample, 1, 1000, 50)
+// Used to measure common KB-granularity memory stats. Range is up to 500000KB -
+// approximately 500M.
+#define UMA_HISTOGRAM_MEMORY_KB(name, sample)                                  \
+    UMA_HISTOGRAM_CUSTOM_COUNTS(name, sample, 1000, 500000, 50)
 
-#define UMA_HISTOGRAM_MEMORY_LARGE_MB(name, sample) \
+// Used to measure common MB-granularity memory stats. Range is up to ~64G.
+#define UMA_HISTOGRAM_MEMORY_LARGE_MB(name, sample)                            \
     UMA_HISTOGRAM_CUSTOM_COUNTS(name, sample, 1, 64000, 100)
 
-#define UMA_HISTOGRAM_PERCENTAGE(name, under_one_hundred) \
-    UMA_HISTOGRAM_ENUMERATION(name, under_one_hundred, 101)
 
-#define UMA_HISTOGRAM_BOOLEAN(name, sample) \
-    STATIC_HISTOGRAM_POINTER_BLOCK(name, AddBoolean(sample), \
-        base::BooleanHistogram::FactoryGet(name, \
-            base::HistogramBase::kUmaTargetedHistogramFlag))
+//------------------------------------------------------------------------------
+// Stability-specific histograms.
 
-// The samples should always be strictly less than |boundary_value|.  For more
-// details, see the comment for the |LOCAL_HISTOGRAM_ENUMERATION| macro, above.
-#define UMA_HISTOGRAM_ENUMERATION(name, sample, boundary_value) \
-    INTERNAL_HISTOGRAM_ENUMERATION_WITH_FLAG(                   \
-        name, sample, boundary_value,                           \
-        base::HistogramBase::kUmaTargetedHistogramFlag)
+// Histograms logged in as stability histograms will be included in the initial
+// stability log. See comments by declaration of
+// MetricsService::PrepareInitialStabilityLog().
+// All of these macros must be called with |name| as a runtime constant.
 
-// Similar to UMA_HISTOGRAM_ENUMERATION, but used for recording stability
-// histograms.  Use this if recording a histogram that should be part of the
-// initial stability log.
-#define UMA_STABILITY_HISTOGRAM_ENUMERATION(name, sample, boundary_value) \
-    INTERNAL_HISTOGRAM_ENUMERATION_WITH_FLAG(                             \
-        name, sample, boundary_value,                                     \
+// For details on usage, see the documentation on the non-stability equivalents.
+
+#define UMA_STABILITY_HISTOGRAM_COUNTS_100(name, sample)                       \
+    UMA_STABILITY_HISTOGRAM_CUSTOM_COUNTS(name, sample, 1, 100, 50)
+
+#define UMA_STABILITY_HISTOGRAM_CUSTOM_COUNTS(name, sample, min, max,          \
+                                              bucket_count)                    \
+    INTERNAL_HISTOGRAM_CUSTOM_COUNTS_WITH_FLAG(                                \
+        name, sample, min, max, bucket_count,                                  \
         base::HistogramBase::kUmaStabilityHistogramFlag)
 
-#define UMA_HISTOGRAM_CUSTOM_ENUMERATION(name, sample, custom_ranges) \
-    STATIC_HISTOGRAM_POINTER_BLOCK(name, Add(sample), \
-        base::CustomHistogram::FactoryGet(name, custom_ranges, \
-            base::HistogramBase::kUmaTargetedHistogramFlag))
+#define UMA_STABILITY_HISTOGRAM_ENUMERATION(name, sample, enum_max)            \
+    INTERNAL_HISTOGRAM_ENUMERATION_WITH_FLAG(                                  \
+        name, sample, enum_max,                                                \
+        base::HistogramBase::kUmaStabilityHistogramFlag)
 
-// Scoped class which logs its time on this earth as a UMA statistic. This is
-// recommended for when you want a histogram which measures the time it takes
-// for a method to execute. This measures up to 10 seconds.
-#define SCOPED_UMA_HISTOGRAM_TIMER(name) \
-  SCOPED_UMA_HISTOGRAM_TIMER_EXPANDER(name, false, __COUNTER__)
 
-// Similar scoped histogram timer, but this uses UMA_HISTOGRAM_LONG_TIMES_100,
-// which measures up to an hour, and uses 100 buckets. This is more expensive
-// to store, so only use if this often takes >10 seconds.
-#define SCOPED_UMA_HISTOGRAM_LONG_TIMER(name) \
-  SCOPED_UMA_HISTOGRAM_TIMER_EXPANDER(name, true, __COUNTER__)
-
-// This nested macro is necessary to expand __COUNTER__ to an actual value.
-#define SCOPED_UMA_HISTOGRAM_TIMER_EXPANDER(name, is_long, key) \
-  SCOPED_UMA_HISTOGRAM_TIMER_UNIQUE(name, is_long, key)
-
-#define SCOPED_UMA_HISTOGRAM_TIMER_UNIQUE(name, is_long, key) \
-  class ScopedHistogramTimer##key { \
-   public: \
-    ScopedHistogramTimer##key() : constructed_(base::TimeTicks::Now()) {} \
-    ~ScopedHistogramTimer##key() { \
-      base::TimeDelta elapsed = base::TimeTicks::Now() - constructed_; \
-      if (is_long) { \
-        UMA_HISTOGRAM_LONG_TIMES_100(name, elapsed); \
-      } else { \
-        UMA_HISTOGRAM_TIMES(name, elapsed); \
-      } \
-    } \
-   private: \
-    base::TimeTicks constructed_; \
-  } scoped_histogram_timer_##key
+//------------------------------------------------------------------------------
+// Deprecated histograms. Not recommended for current use.

[Alexei Svitkine (slow), 2016/09/26 21:25:17]

Nit: Deprecated histogram macros.

[rkaplow, 2016/09/26 21:37:48]

Done.

+
+// Legacy name for UMA_HISTOGRAM_COUNTS_1M. Suggest using explicit naming
+// and not using this macro going forward.
+#define UMA_HISTOGRAM_COUNTS(name, sample) UMA_HISTOGRAM_CUSTOM_COUNTS(        \
+    name, sample, 1, 1000000, 50)
+
+// MB-granularity memory metric. This has a short max (1G).
+#define UMA_HISTOGRAM_MEMORY_MB(name, sample)                                  \
+    UMA_HISTOGRAM_CUSTOM_COUNTS(name, sample, 1, 1000, 50)
+
+// For an enum with customized range. In general, sparse histograms should be
+// used instead.
+// Samples should be one of the std::vector<int> list provided via
+// |custom_ranges|. See comments above CustomRanges::FactoryGet about the
+// requirement of |custom_ranges|. You can use the helper function
+// CustomHistogram::ArrayToCustomRanges to transform a C-style array of valid
+// sample values to a std::vector<int>.
+#define UMA_HISTOGRAM_CUSTOM_ENUMERATION(name, sample, custom_ranges)          \
+    STATIC_HISTOGRAM_POINTER_BLOCK(name, Add(sample),                          \
+        base::CustomHistogram::FactoryGet(name, custom_ranges,                 \
+            base::HistogramBase::kUmaTargetedHistogramFlag))
 
 #endif  // BASE_METRICS_HISTOGRAM_MACROS_H_