Use results in concurrent hash map bench (#2393)

Summary:
This PR adds `folly::doNotOptimizeAway` to all concurrent hash map benchmarks.
Without this, gcc and clang (with larger inlining threshold) may remove parts of the benchmark.

Pull Request resolved: #2393

Reviewed By: yfeldblum

Differential Revision: D70711095

Pulled By: YifanYuan3

fbshipit-source-id: 97440b5f823ff732efd5763545c2fd603a714b1c

Author: krenzland

folly/concurrency/test/ConcurrentHashMapBench.cpp

@@ -20,6 +20,7 @@
 #include <iostream>
 #include <thread>
 
+#include <folly/BenchmarkUtil.h>
 #include <folly/portability/GFlags.h>
 #include <folly/synchronization/test/Barrier.h>
 
@@ -93,7 +94,7 @@ uint64_t bench_ctor_dtor(
       for (int i = 0; i < ops; ++i) {
         folly::ConcurrentHashMap<int, int> m;
         for (int j = 0; j < size; ++j) {
-          m.insert(j, j);
+          folly::doNotOptimizeAway(m.insert(j, j));
         }
       }
     };
@@ -115,11 +116,11 @@ uint64_t bench_find(
     auto fn = [&](int) {
       if (sameItem) {
         for (int i = 0; i < ops; ++i) {
-          m.find(key);
+          folly::doNotOptimizeAway(m.find(key));
         }
       } else {
         for (int i = 0; i < ops; ++i) {
-          m.find(i);
+          folly::doNotOptimizeAway(m.find(i));
         }
       }
     };
@@ -139,7 +140,7 @@ uint64_t bench_iter(const int nthr, int size, const std::string& name) {
   auto repFn = [&] {
     auto fn = [&](int) {
       for (int i = 0; i < reps; ++i) {
-        for (auto it = m.begin(); it != m.end(); ++it) {
+        for (auto it = m.begin(); it != m.end(); doNotOptimizeAway(++it)) {
         }
       }
     };
@@ -158,7 +159,7 @@ uint64_t bench_begin(const int nthr, int size, const std::string& name) {
   auto repFn = [&] {
     auto fn = [&](int) {
       for (int i = 0; i < ops; ++i) {
-        auto it = m.begin();
+        folly::doNotOptimizeAway(m.begin());
       }
     };
     auto endfn = [&] {};
@@ -176,7 +177,7 @@ uint64_t bench_empty(const int nthr, int size, const std::string& name) {
   auto repFn = [&] {
     auto fn = [&](int) {
       for (int i = 0; i < ops; ++i) {
-        m.empty();
+        folly::doNotOptimizeAway(m.empty());
       }
     };
     auto endfn = [&] {};
@@ -194,7 +195,7 @@ uint64_t bench_size(const int nthr, int size, const std::string& name) {
   auto repFn = [&] {
     auto fn = [&](int) {
       for (int i = 0; i < ops; ++i) {
-        m.size();
+        folly::doNotOptimizeAway(m.size());
       }
     };
     auto endfn = [&] {};