[Python-Dev] cpython: Using 'long double' to force this structure to be worst case aligned is no (original) (raw)
Antoine Pitrou solipsis at pitrou.net
Fri Dec 14 10:41:41 CET 2012
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Le Fri, 14 Dec 2012 01:14:04 -0800, "Gregory P. Smith" <greg at krypto.org> a écrit :
Yes, see the followup. My comments before were all misinterpreting sizet.
Same result on x8664 linux. On a 64-bit platform the 24 byte structure now occupies 24 bytes instead of being padded to 32. Nice. On a 32-bit platform it should remain 16 bytes.
But you are losing the 16-byte alignment that the union was precisely designed to enforce.
The PyGCHead union structure is NOT part of the ABI laid out in http://www.python.org/dev/peps/pep-0384/ and is accurately excluded from the .h file when PyLIMITEDAPI is defined so changing this in 3.4 should not be a problem.
Not an ABI problem in 3.4 indeed (except that it might break platforms with strict alignment requirements).
It should be noted that the GC head isn't part of standard atomic types (int, float, str...), so the memory gain will not be very noticeable IMO.
Regards
Antoine.
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