[llvm-exegesis] InstructionBenchmarkClustering::rangeQuery(): use llvm::SmallVector<size_t, 0> for storage.

Summary:
Old: (D54383)
```
 Performance counter stats for './bin/llvm-exegesis -mode=analysis -analysis-epsilon=100000 -benchmarks-file=/tmp/benchmarks.yaml -analysis-inconsistencies-output-file=/tmp/clusters.html' (10 runs):

       9098.781978      task-clock (msec)         #    1.000 CPUs utilized            ( +-  0.16% )
...
            9.1015 +- 0.0148 seconds time elapsed  ( +-  0.16% )
```
New:
```
 Performance counter stats for './bin/llvm-exegesis -mode=analysis -analysis-epsilon=100000 -benchmarks-file=/tmp/benchmarks.yaml -analysis-inconsistencies-output-file=/tmp/clusters.html' (10 runs):

       8553.352480      task-clock (msec)         #    1.000 CPUs utilized            ( +-  0.12% )
...
            8.5539 +- 0.0105 seconds time elapsed  ( +-  0.12% )
```
So another -6%.
That is because the `SmallVector` **doubles** it size when reallocating, which is great here,
since we can't `reserve()` since we can't know how many `Neighbors` we will have.

Reviewers: courbet, MaskRay, RKSimon, gchatelet, john.brawn

Subscribers: tschuett, llvm-commits

Differential Revision: https://reviews.llvm.org/D54388

llvm-svn: 347200

llvm/tools/llvm-exegesis/lib/Clustering.cpp

@@ -9,6 +9,7 @@
 
 #include "Clustering.h"
 #include "llvm/ADT/SetVector.h"
+#include "llvm/ADT/SmallVector.h"
 #include <string>
 
 namespace llvm {
@@ -32,9 +33,9 @@ namespace exegesis {
 
 // Finds the points at distance less than sqrt(EpsilonSquared) of Q (not
 // including Q).
-std::vector<size_t>
+llvm::SmallVector<size_t, 0>
 InstructionBenchmarkClustering::rangeQuery(const size_t Q) const {
-  std::vector<size_t> Neighbors;
+  llvm::SmallVector<size_t, 0> Neighbors;
   const auto &QMeasurements = Points_[Q].Measurements;
   for (size_t P = 0, NumPoints = Points_.size(); P < NumPoints; ++P) {
     if (P == Q)

llvm/tools/llvm-exegesis/lib/Clustering.h

@@ -97,7 +97,7 @@ private:
       const std::vector<InstructionBenchmark> &Points, double EpsilonSquared);
   llvm::Error validateAndSetup();
   void dbScan(size_t MinPts);
-  std::vector<size_t> rangeQuery(size_t Q) const;
+  llvm::SmallVector<size_t, 0> rangeQuery(size_t Q) const;
 
   const std::vector<InstructionBenchmark> &Points_;
   const double EpsilonSquared_;