[libc++] Add benchmark for std::exception_ptr

This commit adds benchmarks for `std::exception_ptr` to set a baseline
in preparation for follow-up optimizations.

Author: vogelsgesang

libcxx/test/benchmarks/exception_ptr.bench.cpp

@@ -18,4 +18,49 @@ void bm_make_exception_ptr(benchmark::State& state) {
 }
 BENCHMARK(bm_make_exception_ptr)->ThreadRange(1, 8);
 
+static bool exception_ptr_moves_copies_swap(std::exception_ptr p1) {
+  // Taken from https://github.com/llvm/llvm-project/issues/44892
+  std::exception_ptr p2(p1);            // Copy constructor
+  std::exception_ptr p3(std::move(p2)); // Move constructor
+  p2 = std::move(p1);                   // Move assignment
+  p1 = p2;                              // Copy assignment
+  swap(p1, p2);                         // Swap
+  // Comparisons against nullptr. The overhead from creating temporary `exception_ptr`
+  // instances should be optimized out.
+  bool is_null  = p1 == nullptr && nullptr == p2;
+  bool is_equal = p1 == p2; // Comparison
+  return is_null && is_equal;
+}
+
+void bm_nonnull_exception_ptr(benchmark::State& state) {
+  std::exception_ptr excptr = std::make_exception_ptr(42);
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(excptr);
+    benchmark::DoNotOptimize(exception_ptr_moves_copies_swap(excptr));
+  }
+}
+BENCHMARK(bm_nonnull_exception_ptr);
+
+void bm_null_exception_ptr(benchmark::State& state) {
+  std::exception_ptr excptr;
+  for (auto _ : state) {
+    // All of the `exception_ptr_noops` are no-ops but the optimizer
+    // cannot optimize them away, because the `DoNotOptimize` calls
+    // prevent the optimizer from doing so.
+    benchmark::DoNotOptimize(excptr);
+    benchmark::DoNotOptimize(exception_ptr_moves_copies_swap(excptr));
+  }
+}
+BENCHMARK(bm_null_exception_ptr);
+
+void bm_optimized_null_exception_ptr(benchmark::State& state) {
+  for (auto _ : state) {
+    // All of the `exception_ptr_noops` are no-ops because
+    // the exception_ptr is empty. Hence, the compiler should
+    // be able to optimize them very aggressively.
+    benchmark::DoNotOptimize(exception_ptr_moves_copies_swap(std::exception_ptr{nullptr}));
+  }
+}
+BENCHMARK(bm_optimized_null_exception_ptr);
+
 BENCHMARK_MAIN();