[libcxx-commits] [PATCH] D152208: [libc++] Introduce __make_uninitialized_buffer and use it instead of get_temporary_buffer

[Nikolas Klauser via Phabricator via libcxx-commits]

philnik added inline comments.


================
Comment at: libcxx/include/__memory/uninitialized_buffer.h:71-73
+#else
+  _Tp* __ptr = static_cast<_Tp*>(::operator new(sizeof(_Tp) * __count, align_val_t(_LIBCPP_ALIGNOF(_Tp)), nothrow));
+#endif
----------------
mingmingl wrote:
> Hi, a regression is observed in a benchmark that uses `std::stable_sort` intensively. 
> 
> Comparing baseline and the regressed case, the baseline (using `get_temporary_buffer`) uses unaligned alloc ( `void* operator new[](size_t size) `) when objects are not over aligned, and the regressed uses aligned alloc for all objects. This may increase the memory usage as well.
> 
> Given `std::stable_sort` is pretty important, I wonder if this regression warrants a fix to have consistent usage of (aligned or unaligned) new operator?
I don't understand how using the aligned alloc would increase memory usage. `void* operator new[](size_t size)` is basically `void* operator new[](size_t size, align_val_t{__STDCPP_DEFAULT_NEW_ALIGNMENT__})`. So unless you have a bad implementation it shouldn't make any difference.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D152208/new/

https://reviews.llvm.org/D152208