[libc++] Mark libc++ deallocation helpers as noexcept

[ldionne]

They already can't throw exceptions and they are called from noexcept functions, but they were not marked as noexcept. Depending on compiler inlining, this might not make a difference or this might improve the codegen a bit by removing the implicit try-catch block that Clang generates around non-noexcept functions called from noexcept functions.

The original issue also mentioned that one occurrence of std::allocator::deallocate was missing noexcept, however it has since then been removed.

Fixes #66100

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Louis Dionne (ldionne)

Changes

They already can't throw exceptions and they are called from noexcept functions, but they were not marked as noexcept. Depending on compiler inlining, this might not make a difference or this might improve the codegen a bit by removing the implicit try-catch block that Clang generates around non-noexcept functions called from noexcept functions.

The original issue also mentioned that one occurrence of std::allocator::deallocate was missing noexcept, however it has since then been removed.

Fixes #66100

---

Full diff: https://github.com/llvm/llvm-project/pull/110884.diff

1 Files Affected:

• (modified) libcxx/include/new (+4-4)

diff --git a/libcxx/include/new b/libcxx/include/new
index 3252b0bb1abcdb..8d7d1eb5fdc7b9 100644
--- a/libcxx/include/new
+++ b/libcxx/include/new
@@ -275,7 +275,7 @@ _LIBCPP_HIDE_FROM_ABI void* __libcpp_operator_new(_Args... __args) {
 }
 
 template <class... _Args>
-_LIBCPP_HIDE_FROM_ABI void __libcpp_operator_delete(_Args... __args) {
+_LIBCPP_HIDE_FROM_ABI void __libcpp_operator_delete(_Args... __args) _NOEXCEPT {
 #if __has_builtin(__builtin_operator_new) && __has_builtin(__builtin_operator_delete)
   __builtin_operator_delete(__args...);
 #else
@@ -296,7 +296,7 @@ inline _LIBCPP_HIDE_FROM_ABI void* __libcpp_allocate(size_t __size, size_t __ali
 }
 
 template <class... _Args>
-_LIBCPP_HIDE_FROM_ABI void __do_deallocate_handle_size(void* __ptr, size_t __size, _Args... __args) {
+_LIBCPP_HIDE_FROM_ABI void __do_deallocate_handle_size(void* __ptr, size_t __size, _Args... __args) _NOEXCEPT {
 #ifdef _LIBCPP_HAS_NO_SIZED_DEALLOCATION
   (void)__size;
   return std::__libcpp_operator_delete(__ptr, __args...);
@@ -305,7 +305,7 @@ _LIBCPP_HIDE_FROM_ABI void __do_deallocate_handle_size(void* __ptr, size_t __siz
 #endif
 }
 
-inline _LIBCPP_HIDE_FROM_ABI void __libcpp_deallocate(void* __ptr, size_t __size, size_t __align) {
+inline _LIBCPP_HIDE_FROM_ABI void __libcpp_deallocate(void* __ptr, size_t __size, size_t __align) _NOEXCEPT {
 #if defined(_LIBCPP_HAS_NO_ALIGNED_ALLOCATION)
   (void)__align;
   return __do_deallocate_handle_size(__ptr, __size);
@@ -319,7 +319,7 @@ inline _LIBCPP_HIDE_FROM_ABI void __libcpp_deallocate(void* __ptr, size_t __size
 #endif
 }
 
-inline _LIBCPP_HIDE_FROM_ABI void __libcpp_deallocate_unsized(void* __ptr, size_t __align) {
+inline _LIBCPP_HIDE_FROM_ABI void __libcpp_deallocate_unsized(void* __ptr, size_t __align) _NOEXCEPT {
 #if defined(_LIBCPP_HAS_NO_ALIGNED_ALLOCATION)
   (void)__align;
   return __libcpp_operator_delete(__ptr);

[EricWF]

What effect does this have on codegen?

[ldionne]

What effect does this have on codegen?

Depending on compiler inlining, this might not make a difference or this might improve the codegen a bit by removing the implicit try-catch block that Clang generates around non-noexcept functions called from noexcept functions.

Here's an example using Godbolt: https://godbolt.org/z/47d5P86Kz

You'll have to zoom in, but on the left you have without my patch, and on the right with my patch. You can see that after my patch, we don't generate a try-catch block that calls __clang_call_terminate inside __libcpp_deallocate. The moment you enable optimizations (even -O1), there's no difference anymore.

So this is almost NFC, but the code is more correct this way anyway so we might as well do it.