[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.

3 files changed, 26 insertions, 13 deletions

diff --git a/libcxxabi/include/cxxabi.h b/libcxxabi/include/cxxabi.h
index 6f4f823..d070118 100644
--- a/libcxxabi/include/cxxabi.h
+++ b/libcxxabi/include/cxxabi.h
@@ -36,6 +36,9 @@ class type_info; // forward declaration
 
 // runtime routines use C calling conventions, but are in __cxxabiv1 namespace
 namespace __cxxabiv1 {
+
+struct __cxa_exception;
+
 extern "C"  {
 
 // 2.4.2 Allocating the Exception Object
@@ -43,6 +46,9 @@ extern _LIBCXXABI_FUNC_VIS void *
 __cxa_allocate_exception(size_t thrown_size) throw();
 extern _LIBCXXABI_FUNC_VIS void
 __cxa_free_exception(void *thrown_exception) throw();
+// This function is an LLVM extension, which mirrors the same extension in libsupc++ and libcxxrt
+extern _LIBCXXABI_FUNC_VIS __cxa_exception*
+__cxa_init_primary_exception(void* object, std::type_info* tinfo, void(_LIBCXXABI_DTOR_FUNC* dest)(void*)) throw();
 
 // 2.4.3 Throwing the Exception Object
 extern _LIBCXXABI_FUNC_VIS _LIBCXXABI_NORETURN void
diff --git a/libcxxabi/lib/exceptions.exp b/libcxxabi/lib/exceptions.exp
index c3780d2..9dcfbdb 100644
--- a/libcxxabi/lib/exceptions.exp
+++ b/libcxxabi/lib/exceptions.exp
@@ -7,5 +7,6 @@ ___cxa_end_catch
 ___cxa_free_dependent_exception
 ___cxa_free_exception
 ___cxa_get_exception_ptr
+___cxa_init_primary_exception
 ___cxa_rethrow
 ___cxa_throw
diff --git a/libcxxabi/src/cxa_exception.cpp b/libcxxabi/src/cxa_exception.cpp
index f723ece..65e9f45 100644
--- a/libcxxabi/src/cxa_exception.cpp
+++ b/libcxxabi/src/cxa_exception.cpp
@@ -206,6 +206,19 @@ void __cxa_free_exception(void *thrown_object) throw() {
     __aligned_free_with_fallback((void *)raw_buffer);
 }
 
+__cxa_exception* __cxa_init_primary_exception(void* object, std::type_info* tinfo,
+                                              void(_LIBCXXABI_DTOR_FUNC* dest)(void*)) throw() {
+  __cxa_exception* exception_header = cxa_exception_from_thrown_object(object);
+  exception_header->referenceCount = 0;
+  exception_header->unexpectedHandler = std::get_unexpected();
+  exception_header->terminateHandler = std::get_terminate();
+  exception_header->exceptionType = tinfo;
+  exception_header->exceptionDestructor = dest;
+  setOurExceptionClass(&exception_header->unwindHeader);
+  exception_header->unwindHeader.exception_cleanup = exception_cleanup_func;
+
+  return exception_header;
+}
 
 //  This function shall allocate a __cxa_dependent_exception and
 //  return a pointer to it. (Really to the object, not past its' end).
@@ -260,22 +273,15 @@ __cxa_throw(void *thrown_object, std::type_info *tinfo, void *(_LIBCXXABI_DTOR_F
 #else
 __cxa_throw(void *thrown_object, std::type_info *tinfo, void (_LIBCXXABI_DTOR_FUNC *dest)(void *)) {
 #endif
-    __cxa_eh_globals *globals = __cxa_get_globals();
-    __cxa_exception* exception_header = cxa_exception_from_thrown_object(thrown_object);
-
-    exception_header->unexpectedHandler = std::get_unexpected();
-    exception_header->terminateHandler  = std::get_terminate();
-    exception_header->exceptionType = tinfo;
-    exception_header->exceptionDestructor = dest;
-    setOurExceptionClass(&exception_header->unwindHeader);
-    exception_header->referenceCount = 1;  // This is a newly allocated exception, no need for thread safety.
-    globals->uncaughtExceptions += 1;   // Not atomically, since globals are thread-local
+  __cxa_eh_globals* globals = __cxa_get_globals();
+  globals->uncaughtExceptions += 1; // Not atomically, since globals are thread-local
 
-    exception_header->unwindHeader.exception_cleanup = exception_cleanup_func;
+  __cxa_exception* exception_header = __cxa_init_primary_exception(thrown_object, tinfo, dest);
+  exception_header->referenceCount = 1; // This is a newly allocated exception, no need for thread safety.
 
 #if __has_feature(address_sanitizer)
-    // Inform the ASan runtime that now might be a good time to clean stuff up.
-    __asan_handle_no_return();
+  // Inform the ASan runtime that now might be a good time to clean stuff up.
+  __asan_handle_no_return();
 #endif
 
 #ifdef __USING_SJLJ_EXCEPTIONS__