[issue27987] obmalloc's 8-byte alignment causes undefined behavior

[Neil Schemenauer]

Neil Schemenauer <nas-python at arctrix.com> added the comment:

> sys.getsizeof(3.14) is 24.  And it becomes 32 byte in 16byte aligned pymalloc. (+33%)

I've been doing some reading and trying to understand this issue.  My understanding is that malloc() needs to return pointers that are 16-byte aligned on AMD64 but, in general, pointers don't have the be aligned that way.  If you have a structure that contains a "long double" then that member also has to be 16-bit aligned.

It seems to me that we don't need to have the PyObject structure containing a Python float to be 16-byte aligned.  If so, could we introduce a new obmalloc API that returns memory with 8-byte alignment, for use by objects that know they don't require 16-byte alignment?  floatobject.c could use this API to avoid the 33% overhead.

The new obmalloc API could initially be internal use only until we can come up with a design we know we can live with long term.

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue27987>
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