Added Google benchmarks (and gtests) (#6920)

* Added Google benchmarks (and gtests)

* Default building benchmarks to OFF as it requires c++11

* Separate benchmark CMakeLists.txt to its own file

* Move output directory to target just flatbenchmark

benchmarks/cpp/flatbuffers/fb_bench.cpp

@@ -0,0 +1,81 @@
+#include "benchmarks/cpp/flatbuffers/fb_bench.h"
+
+#include <cstdint>
+#include <memory>
+
+#include "benchmarks/cpp/bench.h"
+#include "benchmarks/cpp/flatbuffers/bench_generated.h"
+#include "flatbuffers/flatbuffers.h"
+
+using namespace flatbuffers;
+using namespace benchmarks_flatbuffers;
+
+namespace {
+
+struct FlatBufferBench : Bench {
+  explicit FlatBufferBench(int64_t initial_size,
+                           Allocator *allocator)
+      : fbb(initial_size, allocator, false) {}
+
+  uint8_t *Encode(void *, int64_t &len) override {
+    fbb.Clear();
+
+    const int kVectorLength = 3;
+    Offset<FooBar> vec[kVectorLength];
+
+    for(int i = 0; i < kVectorLength; ++i) {
+      Foo foo(0xABADCAFEABADCAFE + i, 10000 + i, '@' + i, 1000000 + i);
+      Bar bar(foo, 123456 + i, 3.14159f + i, 10000 + i);
+      auto name = fbb.CreateString("Hello, World!");
+      auto foobar = CreateFooBar(fbb, &bar, name, 3.1415432432445543543 + i,
+                                 '!' + i);
+      vec[i] = foobar;
+    }
+    auto location = fbb.CreateString("http://google.com/flatbuffers/");
+    auto foobarvec = fbb.CreateVector(vec, kVectorLength);
+    auto foobarcontainer = CreateFooBarContainer(fbb, foobarvec, true,
+                                                 Enum_Bananas, location);
+    fbb.Finish(foobarcontainer);
+
+    len = fbb.GetSize();
+    return fbb.GetBufferPointer();
+  }
+
+  int64_t Use(void *decoded) override {
+    sum = 0;
+    auto foobarcontainer = GetFooBarContainer(decoded);
+    sum = 0;
+    Add(foobarcontainer->initialized());
+    Add(foobarcontainer->location()->Length());
+    Add(foobarcontainer->fruit());
+    for (unsigned int i = 0; i < foobarcontainer->list()->Length(); i++) {
+      auto foobar = foobarcontainer->list()->Get(i);
+      Add(foobar->name()->Length());
+      Add(foobar->postfix());
+      Add(static_cast<int64_t>(foobar->rating()));
+      auto bar = foobar->sibling();
+      Add(static_cast<int64_t>(bar->ratio()));
+      Add(bar->size());
+      Add(bar->time());
+      auto &foo = bar->parent();
+      Add(foo.count());
+      Add(foo.id());
+      Add(foo.length());
+      Add(foo.prefix());
+    }
+    return sum;
+  }
+
+  void *Decode(void *buffer, int64_t) override { return buffer; }
+  void Dealloc(void *) override{};
+
+  FlatBufferBuilder fbb;
+};
+
+}  // namespace
+
+std::unique_ptr<Bench> NewFlatBuffersBench(int64_t initial_size,
+                                           Allocator *allocator) {
+  return std::unique_ptr<FlatBufferBench>(
+      new FlatBufferBench(initial_size, allocator));
+}