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+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <cstdint>
+
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/no_destructor.h"
+#include "benchmark/benchmark.h"
+
+namespace {
+
+// Number of static-NoDestructor-in-a-function to exercise.
+// This must be low enough not to hit template instantiation limits
+// (happens around 1000).
+constexpr int kNumObjects = 1; // set to 512 when doing benchmarks
+ // 1 is faster to compile: just one templated
+ // function instantiation
+
+// Size of individual objects to benchmark static-NoDestructor-in-a-function
+// usage with.
+constexpr int kObjSize = sizeof(void*)*1;
+
+// Simple object of kObjSize bytes (rounded to int).
+// We benchmark complete reading of its state via Verify().
+class BM_Blob {
+ public:
+ BM_Blob(int val) { for (auto& d : data_) d = val; }
+ BM_Blob() : BM_Blob(-1) {}
+ void Verify(int val) const { // val must be the c-tor argument
+ for (auto& d : data_) ABSL_INTERNAL_CHECK(d == val, "");
+ }
+ private:
+ int data_[kObjSize / sizeof(int) > 0 ? kObjSize / sizeof(int) : 1];
+};
+
+// static-NoDestructor-in-a-function pattern instances.
+// We'll instantiate kNumObjects of them.
+template<int i>
+const BM_Blob& NoDestrBlobFunc() {
+ static absl::NoDestructor<BM_Blob> x(i);
+ return *x;
+}
+
+// static-heap-ptr-in-a-function pattern instances
+// We'll instantiate kNumObjects of them.
+template<int i>
+const BM_Blob& OnHeapBlobFunc() {
+ static BM_Blob* x = new BM_Blob(i);
+ return *x;
+}
+
+// Type for NoDestrBlobFunc or OnHeapBlobFunc.
+typedef const BM_Blob& (*FuncType)();
+
+// ========================================================================= //
+// Simple benchmarks that read a single BM_Blob over and over, hence
+// all they touch fits into L1 CPU cache:
+
+// Direct non-POD global variable (style guide violation) as a baseline.
+static BM_Blob direct_blob(0);
+
+void BM_Direct(benchmark::State& state) {
+ for (auto s : state) {
+ direct_blob.Verify(0);
+ }
+}
+BENCHMARK(BM_Direct);
+
+void BM_NoDestr(benchmark::State& state) {
+ for (auto s : state) {
+ NoDestrBlobFunc<0>().Verify(0);
+ }
+}
+BENCHMARK(BM_NoDestr);
+
+void BM_OnHeap(benchmark::State& state) {
+ for (auto s : state) {
+ OnHeapBlobFunc<0>().Verify(0);
+ }
+}
+BENCHMARK(BM_OnHeap);
+
+// ========================================================================= //
+// Benchmarks that read kNumObjects of BM_Blob over and over, hence with
+// appropriate values of sizeof(BM_Blob) and kNumObjects their working set
+// can exceed a given layer of CPU cache.
+
+// Type of benchmark to select between NoDestrBlobFunc and OnHeapBlobFunc.
+enum BM_Type { kNoDestr, kOnHeap, kDirect };
+
+// BlobFunc<n>(t, i) returns the i-th function of type t.
+// n must be larger than i (we'll use kNumObjects for n).
+template<int n>
+FuncType BlobFunc(BM_Type t, int i) {
+ if (i == n) {
+ switch (t) {
+ case kNoDestr: return &NoDestrBlobFunc<n>;
+ case kOnHeap: return &OnHeapBlobFunc<n>;
+ case kDirect: return nullptr;
+ }
+ }
+ return BlobFunc<n-1>(t, i);
+}
+
+template<>
+FuncType BlobFunc<0>(BM_Type t, int i) {
+ ABSL_INTERNAL_CHECK(i == 0, "");
+ switch (t) {
+ case kNoDestr: return &NoDestrBlobFunc<0>;
+ case kOnHeap: return &OnHeapBlobFunc<0>;
+ case kDirect: return nullptr;
+ }
+ return nullptr;
+}
+
+// Direct non-POD global variables (style guide violation) as a baseline.
+static BM_Blob direct_blobs[kNumObjects];
+
+// Helper that cheaply maps benchmark iteration to randomish index in
+// [0, kNumObjects).
+int RandIdx(int i) {
+ // int64 is to avoid overflow and generating negative return values:
+ return (static_cast<int64_t>(i) * 13) % kNumObjects;
+}
+
+// Generic benchmark working with kNumObjects for any of the possible BM_Type.
+template <BM_Type t>
+void BM_Many(benchmark::State& state) {
+ FuncType funcs[kNumObjects];
+ for (int i = 0; i < kNumObjects; ++i) {
+ funcs[i] = BlobFunc<kNumObjects-1>(t, i);
+ }
+ if (t == kDirect) {
+ for (auto s : state) {
+ int idx = RandIdx(state.iterations());
+ direct_blobs[idx].Verify(-1);
+ }
+ } else {
+ for (auto s : state) {
+ int idx = RandIdx(state.iterations());
+ funcs[idx]().Verify(idx);
+ }
+ }
+}
+
+void BM_DirectMany(benchmark::State& state) { BM_Many<kDirect>(state); }
+void BM_NoDestrMany(benchmark::State& state) { BM_Many<kNoDestr>(state); }
+void BM_OnHeapMany(benchmark::State& state) { BM_Many<kOnHeap>(state); }
+
+BENCHMARK(BM_DirectMany);
+BENCHMARK(BM_NoDestrMany);
+BENCHMARK(BM_OnHeapMany);
+
+} // namespace