[base] Reduce string copies in HistogramTester

This change reduces string copies in base::HistogramTester by changing
its APIs to take base::StringPieces instead of const std::string&. The
majority of callsites pass string literals to the HistogramTester APIs,
resulting in a deep copy with the old implementation.

Furthermore, this change updates the underlying std::maps to use the
heterogeneous std::less<> comparator, allowing the direct comparison of
base::StringPiece with std::string.

Bug: 384011
Change-Id: I6a8dd3e76280162b6046eac63a3d1ac84ed2ee6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1887612Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710454}

base/test/metrics/histogram_tester.cc

@@ -30,7 +30,7 @@ HistogramTester::HistogramTester() {
 HistogramTester::~HistogramTester() = default;
 
 void HistogramTester::ExpectUniqueSample(
-    const std::string& name,
+    StringPiece name,
     HistogramBase::Sample sample,
     HistogramBase::Count expected_count) const {
   HistogramBase* histogram = StatisticsRecorder::FindHistogram(name);
@@ -45,7 +45,7 @@ void HistogramTester::ExpectUniqueSample(
 }
 
 void HistogramTester::ExpectBucketCount(
-    const std::string& name,
+    StringPiece name,
     HistogramBase::Sample sample,
     HistogramBase::Count expected_count) const {
   HistogramBase* histogram = StatisticsRecorder::FindHistogram(name);
@@ -58,7 +58,7 @@ void HistogramTester::ExpectBucketCount(
   }
 }
 
-void HistogramTester::ExpectTotalCount(const std::string& name,
+void HistogramTester::ExpectTotalCount(StringPiece name,
                                        HistogramBase::Count count) const {
   HistogramBase* histogram = StatisticsRecorder::FindHistogram(name);
   if (histogram) {
@@ -70,14 +70,13 @@ void HistogramTester::ExpectTotalCount(const std::string& name,
   }
 }
 
-void HistogramTester::ExpectTimeBucketCount(const std::string& name,
+void HistogramTester::ExpectTimeBucketCount(StringPiece name,
                                             TimeDelta sample,
                                             HistogramBase::Count count) const {
   ExpectBucketCount(name, sample.InMilliseconds(), count);
 }
 
-std::vector<Bucket> HistogramTester::GetAllSamples(
-    const std::string& name) const {
+std::vector<Bucket> HistogramTester::GetAllSamples(StringPiece name) const {
   std::vector<Bucket> samples;
   std::unique_ptr<HistogramSamples> snapshot =
       GetHistogramSamplesSinceCreation(name);
@@ -93,7 +92,7 @@ std::vector<Bucket> HistogramTester::GetAllSamples(
 }
 
 HistogramBase::Count HistogramTester::GetBucketCount(
-    const std::string& name,
+    StringPiece name,
     HistogramBase::Sample sample) const {
   HistogramBase* histogram = StatisticsRecorder::FindHistogram(name);
   EXPECT_NE(nullptr, histogram)
@@ -107,7 +106,7 @@ HistogramBase::Count HistogramTester::GetBucketCount(
 }
 
 void HistogramTester::GetBucketCountForSamples(
-    const std::string& name,
+    StringPiece name,
     HistogramBase::Sample sample,
     const HistogramSamples& samples,
     HistogramBase::Count* count) const {
@@ -118,8 +117,8 @@ void HistogramTester::GetBucketCountForSamples(
 }
 
 HistogramTester::CountsMap HistogramTester::GetTotalCountsForPrefix(
-    const std::string& prefix) const {
-  EXPECT_TRUE(prefix.find('.') != std::string::npos)
+    StringPiece prefix) const {
+  EXPECT_TRUE(prefix.find('.') != StringPiece::npos)
       << "|prefix| ought to contain at least one period, to avoid matching too"
       << " many histograms.";
 
@@ -143,7 +142,7 @@ HistogramTester::CountsMap HistogramTester::GetTotalCountsForPrefix(
 
 std::unique_ptr<HistogramSamples>
 HistogramTester::GetHistogramSamplesSinceCreation(
-    const std::string& histogram_name) const {
+    StringPiece histogram_name) const {
   HistogramBase* histogram = StatisticsRecorder::FindHistogram(histogram_name);
   // Whether the histogram exists or not may not depend on the current test
   // calling this method, but rather on which tests ran before and possibly
@@ -192,7 +191,7 @@ std::string HistogramTester::GetAllHistogramsRecorded() const {
   return output;
 }
 
-void HistogramTester::CheckBucketCount(const std::string& name,
+void HistogramTester::CheckBucketCount(StringPiece name,
                                        HistogramBase::Sample sample,
                                        HistogramBase::Count expected_count,
                                        const HistogramSamples& samples) const {
@@ -206,7 +205,7 @@ void HistogramTester::CheckBucketCount(const std::string& name,
       << ").";
 }
 
-void HistogramTester::CheckTotalCount(const std::string& name,
+void HistogramTester::CheckTotalCount(StringPiece name,
                                       HistogramBase::Count expected_count,
                                       const HistogramSamples& samples) const {
   int actual_count = samples.TotalCount();

base/test/metrics/histogram_tester.h

@@ -5,6 +5,7 @@
 #ifndef BASE_TEST_METRICS_HISTOGRAM_TESTER_H_
 #define BASE_TEST_METRICS_HISTOGRAM_TESTER_H_
 
+#include <functional>
 #include <map>
 #include <memory>
 #include <ostream>
@@ -15,6 +16,7 @@
 #include "base/macros.h"
 #include "base/metrics/histogram.h"
 #include "base/metrics/histogram_base.h"
+#include "base/strings/string_piece.h"
 #include "base/time/time.h"
 
 namespace base {
@@ -33,7 +35,7 @@ class HistogramSamples;
 // should be used to achieve that.
 class HistogramTester {
  public:
-  using CountsMap = std::map<std::string, HistogramBase::Count>;
+  using CountsMap = std::map<std::string, HistogramBase::Count, std::less<>>;
 
   // Takes a snapshot of all current histograms counts.
   HistogramTester();
@@ -42,11 +44,11 @@ class HistogramTester {
   // We know the exact number of samples in a bucket, and that no other bucket
   // should have samples. Measures the diff from the snapshot taken when this
   // object was constructed.
-  void ExpectUniqueSample(const std::string& name,
+  void ExpectUniqueSample(StringPiece name,
                           HistogramBase::Sample sample,
                           HistogramBase::Count expected_count) const;
   template <typename T>
-  void ExpectUniqueSample(const std::string& name,
+  void ExpectUniqueSample(StringPiece name,
                           T sample,
                           HistogramBase::Count expected_count) const {
     ExpectUniqueSample(name, static_cast<HistogramBase::Sample>(sample),
@@ -56,11 +58,11 @@ class HistogramTester {
   // We know the exact number of samples in a bucket, but other buckets may
   // have samples as well. Measures the diff from the snapshot taken when this
   // object was constructed.
-  void ExpectBucketCount(const std::string& name,
+  void ExpectBucketCount(StringPiece name,
                          HistogramBase::Sample sample,
                          HistogramBase::Count expected_count) const;
   template <typename T>
-  void ExpectBucketCount(const std::string& name,
+  void ExpectBucketCount(StringPiece name,
                          T sample,
                          HistogramBase::Count expected_count) const {
     ExpectBucketCount(name, static_cast<HistogramBase::Sample>(sample),
@@ -70,12 +72,11 @@ class HistogramTester {
   // We don't know the values of the samples, but we know how many there are.
   // This measures the diff from the snapshot taken when this object was
   // constructed.
-  void ExpectTotalCount(const std::string& name,
-                        HistogramBase::Count count) const;
+  void ExpectTotalCount(StringPiece name, HistogramBase::Count count) const;
 
   // We know exact number of samples for buckets corresponding to a time
   // interval. Other intervals may have samples too.
-  void ExpectTimeBucketCount(const std::string& name,
+  void ExpectTimeBucketCount(StringPiece name,
                              TimeDelta sample,
                              HistogramBase::Count count) const;
 
@@ -95,10 +96,10 @@ class HistogramTester {
   // slightly less helpful failure message:
   //   EXPECT_EQ(expected_buckets,
   //             histogram_tester.GetAllSamples("HistogramName"));
-  std::vector<Bucket> GetAllSamples(const std::string& name) const;
+  std::vector<Bucket> GetAllSamples(StringPiece name) const;
 
   // Returns the value of the |sample| bucket for ths histogram |name|.
-  HistogramBase::Count GetBucketCount(const std::string& name,
+  HistogramBase::Count GetBucketCount(StringPiece name,
                                       HistogramBase::Sample sample) const;
 
   // Finds histograms whose names start with |prefix|, and returns them along
@@ -119,12 +120,12 @@ class HistogramTester {
   //   expected_counts["MyMetric.B"] = 1;
   //   EXPECT_THAT(histogram_tester.GetTotalCountsForPrefix("MyMetric."),
   //               testing::ContainerEq(expected_counts));
-  CountsMap GetTotalCountsForPrefix(const std::string& prefix) const;
+  CountsMap GetTotalCountsForPrefix(StringPiece prefix) const;
 
   // Access a modified HistogramSamples containing only what has been logged
   // to the histogram since the creation of this object.
   std::unique_ptr<HistogramSamples> GetHistogramSamplesSinceCreation(
-      const std::string& histogram_name) const;
+      StringPiece histogram_name) const;
 
   // Dumps all histograms that have had new samples added to them into a string,
   // for debugging purposes. Note: this will dump the entire contents of any
@@ -135,7 +136,7 @@ class HistogramTester {
   // Verifies and asserts that value in the |sample| bucket matches the
   // |expected_count|. The bucket's current value is determined from |samples|
   // and is modified based on the snapshot stored for histogram |name|.
-  void CheckBucketCount(const std::string& name,
+  void CheckBucketCount(StringPiece name,
                         HistogramBase::Sample sample,
                         Histogram::Count expected_count,
                         const HistogramSamples& samples) const;
@@ -143,21 +144,22 @@ class HistogramTester {
   // Verifies that the total number of values recorded for the histogram |name|
   // is |expected_count|. This is checked against |samples| minus the snapshot
   // that was taken for |name|.
-  void CheckTotalCount(const std::string& name,
+  void CheckTotalCount(StringPiece name,
                        Histogram::Count expected_count,
                        const HistogramSamples& samples) const;
 
   // Sets the value for |count| to be the value in the |sample| bucket. The
   // bucket's current value is determined from |samples| and is modified based
   // on the snapshot stored for histogram |name|.
-  void GetBucketCountForSamples(const std::string& name,
+  void GetBucketCountForSamples(StringPiece name,
                                 HistogramBase::Sample sample,
                                 const HistogramSamples& samples,
                                 HistogramBase::Count* count) const;
 
   // Used to determine the histogram changes made during this instance's
   // lifecycle.
-  std::map<std::string, std::unique_ptr<HistogramSamples>> histograms_snapshot_;
+  std::map<std::string, std::unique_ptr<HistogramSamples>, std::less<>>
+      histograms_snapshot_;
 
   DISALLOW_COPY_AND_ASSIGN(HistogramTester);
 };