Make std::get_temporary_buffer respect overaligned types when possible

[EricWF]

This LGTM minus nits.

Is there a LWG issue or paper that specifies this change? or is it just a general bug fix?

include/memory

2007

Please use the alignment_of trait; since that's always available within the library, but different spellings of the alignof keyword aren't.

2048

alignment_of.

[ckennelly]

In D41746#973941, @EricWF wrote:

This LGTM minus nits.

Is there a LWG issue or paper that specifies this change? or is it just a general bug fix?

This is just a general bug fix.

include/memory

2007

I also guarded for STDCPP_DEFAULT_NEW_ALIGNMENT

[ckennelly]

I don't have commit access, so can this be committed to trunk?

include/memory

get_temporary_buffer(ptrdiff_t __n) _NOEXCEPT
     pair<_Tp*, ptrdiff_t> __r(0, 0);
     const ptrdiff_t __m = (~ptrdiff_t(0) ^
                            ptrdiff_t(ptrdiff_t(1) << (sizeof(ptrdiff_t) * __CHAR_BIT__ - 1)))
                            / sizeof(_Tp);
     if (__n > __m)
         __n = __m;
     while (__n > 0)
     {
+#if !defined(_LIBCPP_HAS_NO_ALIGNED_ALLOCATION)
+        if (alignof(_Tp) > __STDCPP_DEFAULT_NEW_ALIGNMENT__)
+        {
+            std::align_val_t __al = std::align_val_t(alignof(_Tp));
+            __r.first = static_cast<_Tp*>(::operator new(
+                __n * sizeof(_Tp), __al, nothrow));
+        } else {
+            __r.first = static_cast<_Tp*>(::operator new(
+                __n * sizeof(_Tp), nothrow));
+        }
+#else
+#if defined(__STDCPP_DEFAULT_NEW_ALIGNMENT__)
+        if (__alignof__(_Tp) > __STDCPP_DEFAULT_NEW_ALIGNMENT__)
+#else
+        if (__alignof__(_Tp) > __alignof__(std::max_align_t))
+#endif
+        {
+            // Since aligned operator new is unavailable, return an empty
+            // buffer rather than one with invalid alignment.
+            return __r;
+        }
+
         __r.first = static_cast<_Tp*>(::operator new(__n * sizeof(_Tp), nothrow));
+#endif
+
         if (__r.first)
         {
             __r.second = __n;
             break;
         }
         __n /= 2;
     }
     return __r;
 }
 
 template <class _Tp>
 inline _LIBCPP_INLINE_VISIBILITY
-void return_temporary_buffer(_Tp* __p) _NOEXCEPT {::operator delete(__p);}
+void return_temporary_buffer(_Tp* __p) _NOEXCEPT
+{
+#if !defined(_LIBCPP_HAS_NO_ALIGNED_ALLOCATION)
+    if (alignof(_Tp) > __STDCPP_DEFAULT_NEW_ALIGNMENT__)
+    {
+        std::align_val_t __al = std::align_val_t(alignof(_Tp));
+        ::operator delete(__p, __al);
+        return;
+    }
+#endif
+    ::operator delete(__p);
+}
 
 #if _LIBCPP_STD_VER <= 14 || defined(_LIBCPP_ENABLE_CXX17_REMOVED_AUTO_PTR)
 template <class _Tp>
 struct auto_ptr_ref
 {
     _Tp* __ptr_;
 };
 

test/std/utilities/memory/temporary.buffer/overaligned.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <memory>
+
+// template <class T>
+//   pair<T*, ptrdiff_t>
+//   get_temporary_buffer(ptrdiff_t n);
+//
+// template <class T>
+//   void
+//   return_temporary_buffer(T* p);
+
+#include <memory>
+#include <cassert>
+
+struct alignas(32) A {
+    int field;
+};
+
+int main()
+{
+    std::pair<A*, std::ptrdiff_t> ip = std::get_temporary_buffer<A>(5);
+    assert(!(ip.first == nullptr) ^ (ip.second == 0));
+    assert(reinterpret_cast<uintptr_t>(ip.first) % alignof(A) == 0);
+    std::return_temporary_buffer(ip.first);
+}