[libcxx-commits] [libcxx] [libc++] Add benchmark for `std::exception_ptr` (PR #164278)

[Adrian Vogelsgesang via libcxx-commits]

https://github.com/vogelsgesang created https://github.com/llvm/llvm-project/pull/164278

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

>From e4db02bd230727b7d553b5b1c64aaf6b9242b96e Mon Sep 17 00:00:00 2001
From: Adrian Vogelsgesang <avogelsgesang at salesforce.com>
Date: Mon, 20 Oct 2025 13:47:44 +0000
Subject: [PATCH] [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.
---
 .../test/benchmarks/exception_ptr.bench.cpp   | 33 +++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/libcxx/test/benchmarks/exception_ptr.bench.cpp b/libcxx/test/benchmarks/exception_ptr.bench.cpp
index 7791c510b1eb6..a0c8e9b1d5fba 100644
--- a/libcxx/test/benchmarks/exception_ptr.bench.cpp
+++ b/libcxx/test/benchmarks/exception_ptr.bench.cpp
@@ -18,4 +18,37 @@ 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_empty_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_empty_exception_ptr);
+
+void bm_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_exception_ptr);
+
 BENCHMARK_MAIN();