<functional>: std::function doesn't handle over-aligned types

[StephanTLavavej]

The Small Functor Optimization in std::function was implemented before we started to respect over-aligned types. Currently, we emit silent bad codegen when we decide to locally store an over-aligned function object.

C:\Temp>type meow.cpp
#include <functional>
#include <stdint.h>
#include <stdio.h>
using namespace std;

struct alignas(16) Meow {
    int i;

    void operator()() {
        printf("This should be 0: %d\n", static_cast<int>(reinterpret_cast<uintptr_t>(this) % alignof(Meow)));
    }
};

int main() {
    function<void()> f(Meow{});
    int x = 1729;
    function<void()> g(Meow{});
    f();
    g();
    (void) x;
}

C:\Temp>cl /EHsc /nologo /W4 /wd4324 meow.cpp
meow.cpp

C:\Temp>meow
This should be 0: 0
This should be 0: 8

Casey Carter (@CaseyCarter) observed: "I think all that's required here is to add || alignof(_Impl) > alignof(max_align_t) to _Is_large and write a quick regression test."

STL/stl/inc/functional

Lines 797 to 798 in 2239101

	template <class _Impl> // determine whether _Impl must be dynamically allocated
	_INLINE_VAR constexpr bool _Is_large = (_Space_size < sizeof(_Impl)) || !_Impl::_Nothrow_move::value;

STL/stl/inc/functional

Lines 1065 to 1069 in 2239101

	union _Storage { // storage for small objects (basic_string is small)
	max_align_t _Dummy1; // for maximum alignment
	char _Dummy2[_Space_size]; // to permit aliasing
	_Ptrt* _Ptrs[_Small_object_num_ptrs]; // _Ptrs[_Small_object_num_ptrs - 1] is reserved
	};

Tracked by Microsoft-internal VSO-157296 and VSO-1062649.

[AlexGuteniev]

I think this is a bit more complex than just changing the alignment.
There should be _ENABLE_FUNCTION_ALIGNMENT_FIX macro.
See similar for `:

STL/stl/inc/atomic

Lines 1508 to 1516 in 2239101

	#ifndef _ENABLE_ATOMIC_ALIGNMENT_FIX
	static_assert(alignof(_Ty) >= sizeof(_Ty) || (sizeof(_Ty) != 2 && sizeof(_Ty) != 4 && sizeof(_Ty) != 8),
	"You've instantiated std::atomic<T> with sizeof(T) equal to 2/4/8 and alignof(T) < sizeof(T). "
	"Before VS 2015 Update 2, this would have misbehaved at runtime. "
	"VS 2015 Update 2 was fixed to handle this correctly, "
	"but the fix inherently changes layout and breaks binary compatibility. "
	"Please define _ENABLE_ATOMIC_ALIGNMENT_FIX to acknowledge that you understand this, "
	"and that everything you're linking has been compiled with VS 2015 Update 2 (or later).");
	#endif // _ENABLE_ATOMIC_ALIGNMENT_FIX

[StephanTLavavej]

The difference is that std::function performs type erasure via virtual while atomic does not.

[AlexGuteniev]

I've put a test in #698 , let's see it fail here, then proceed with the fix

[AlexGuteniev]

Seen test fail, added fix, #698 is ready

[AlexGuteniev]

Stephan T. Lavavej (@StephanTLavavej) , I did what was suggested and can't understand x86 tests failure.
To me it looks like aligned allocation of underlying function object is still not aligned.

[AlexGuteniev]

Figured it out. Legitimately fails if __cpp_aligned_new is not there (C++14 and below). Now should pass.