[issue27987] obmalloc's 8-byte alignment causes undefined behavior
STINNER Victor
report at bugs.python.org
Mon Apr 15 07:24:14 EDT 2019
STINNER Victor <vstinner at redhat.com> added the comment:
The x86-64 ABI requires that memory allocated on the heap is aligned to 16 bytes.
On x86-64, glibc malloc(size) aligns on 16 bytes for size >= 16, otherwise align to 8 bytes. So the glibc doesn't respect exactly the ABI. I understand that a compiler will not use instructions which require 16B align on a memory block smaller than 16B, so align to 8B for size < 16B should be fine *in practice*.
Python objects are at least 16B because of PyObject header. Moreover, objects tracked by the GC gets additional 16B header from PyGC_Head.
But pymalloc is also used for PyMem_Malloc() since Python 3.6, and PyMem_Malloc() is used to allocate things which are not PyObject.
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<https://bugs.python.org/issue27987>
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