From 2e2e7fe7825b58a910dcdde442e61307b16907d6 Mon Sep 17 00:00:00 2001
From: Alexey Spiridonov <lesha@meta.com>
Date: Fri, 24 May 2024 23:57:06 -0700
Subject: [PATCH] Save ~2ns from GLIBCXX `exception_ptr_get_type` and
`exception_ptr_get_object`
Summary:
I noticed `get_type` was unexpectedly slow at ~2.5ns. yfeldblum pointed out that `ptr.__cxa_exception_type` probably incurs a PLT lookup, and that we can safely elide this.
The new `exception_ptr_get_type_` implementation is exactly the same as the FreeBSD version.
I also visually checked the implementation against:
- https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/libsupc%2B%2B/eh_ptr.cc#L138
- https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/libsupc%2B%2B/unwind-cxx.h
Reviewed By: yfeldblum
Differential Revision: D57792972
fbshipit-source-id: 1658ac7ef1f1f33cead11bf68747948cd522de33
---
folly/lang/Exception.cpp | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)
diff --git a/folly/lang/Exception.cpp b/folly/lang/Exception.cpp
index 34ecbc7f978..320f73c9820 100644
--- a/folly/lang/Exception.cpp
+++ b/folly/lang/Exception.cpp
@@ -243,7 +243,12 @@ bool exception_ptr_access_rt_v_() noexcept {
std::type_info const* exception_ptr_get_type_(
std::exception_ptr const& ptr) noexcept {
- return !ptr ? nullptr : ptr.__cxa_exception_type();
+ if (!ptr) {
+ return nullptr;
+ }
+ auto object = reinterpret_cast<void* const&>(ptr);
+ auto exception = static_cast<abi::__cxa_exception*>(object) - 1;
+ return exception->exceptionType;
}
void* exception_ptr_get_object_(
@@ -253,7 +258,7 @@ void* exception_ptr_get_object_(
return nullptr;
}
auto object = reinterpret_cast<void* const&>(ptr);
- auto type = ptr.__cxa_exception_type();
+ auto type = exception_ptr_get_type_(ptr);
return !target || target->__do_catch(type, &object, 1) ? object : nullptr;
}