Are we sure the memoryview atomics need aligning

[da-woods]

Describe your issue

We have a slightly complicated scheme to make sure memoryview atomics are aligned:

cython/Cython/Utility/MemoryView.pyx

Lines 353 to 356 in 90e9f6a

	# the following array will contain a single __pyx_atomic int with
	# suitable alignment
	cdef __pyx_atomic_int acquisition_count[2]
	cdef __pyx_atomic_int *acquisition_count_aligned_p

cython/Cython/Utility/MemoryView.pyx

Lines 386 to 387 in 90e9f6a

	self.acquisition_count_aligned_p = <__pyx_atomic_int *> align_pointer(
	<void *> &self.acquisition_count[0], sizeof(__pyx_atomic_int))

I've had a quick try at removing it, and the indirection from this scheme seems to cause something like a 20% performance loss, for a toy example doing memoryview slicing in a loop.

My view is that:

• __pyx_atomic_int should already be aligned appropriately within the __pyx_memoryview struct (because I'm pretty sure the C compiler guarantees this).
• The allocated __pyx_memoryview should be aligned appropriately because
  1. malloc guarantees this (by allocating to the maximum alignment the system requires)
  2. Although Python itself will be responsible for the positioning of the struct within allocated memory,
     it's pretty much got to be right, otherwise it'd break a lot of code (especially on platforms that require alignment)

Therefore, I think this scheme is an unnecessary pessimization. I've tried putting in an assert to check the alignment and can't get it to trip locally (which doesn't prove it's right, but suggests it might be at least on some platforms).

Am I missing anything?