[libc++abi] Implement __cxa_init_primary_exception and use it to optimize std::make_exception_ptr (#65534)

This patch implements __cxa_init_primary_exception, an extension to the 
Itanium C++ ABI. This extension is already present in both libsupc++ and 
libcxxrt. This patch also starts making use of this function in 
std::make_exception_ptr: instead of going through a full throw/catch 
cycle, we are now able to initialize an exception directly, thus making 
std::make_exception_ptr around 30x faster.

2 files changed, 20 insertions, 0 deletions

diff --git a/libcxx/benchmarks/CMakeLists.txt b/libcxx/benchmarks/CMakeLists.txt
index ce4f5fd..2434d82 100644
--- a/libcxx/benchmarks/CMakeLists.txt
+++ b/libcxx/benchmarks/CMakeLists.txt
@@ -202,6 +202,7 @@ set(BENCHMARK_TESTS
     allocation.bench.cpp
     deque.bench.cpp
     deque_iterator.bench.cpp
+    exception_ptr.bench.cpp
     filesystem.bench.cpp
     format_to_n.bench.cpp
     format_to.bench.cpp
diff --git a/libcxx/benchmarks/exception_ptr.bench.cpp b/libcxx/benchmarks/exception_ptr.bench.cpp
new file mode 100644
index 0000000..1292ad7
--- /dev/null
+++ b/libcxx/benchmarks/exception_ptr.bench.cpp
@@ -0,0 +1,19 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include <benchmark/benchmark.h>
+#include <exception>
+
+void bm_make_exception_ptr(benchmark::State& state) {
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(std::make_exception_ptr(42));
+  }
+}
+BENCHMARK(bm_make_exception_ptr)->ThreadRange(1, 8);
+
+BENCHMARK_MAIN();