[Python-Dev] mingw32 and gc-header weirdness

[Christian Tismer]

On 7/23/09 2:04 AM, Antoine Pitrou wrote:
> Christian Tismer<tismer<at>  stackless.com>  writes:
>> Despite the fact that Python probably has to be changed:
>> If it is true then all the 32-bit Linux Pythons have a 12
>> byte GC head, IOW they are *all* badly aligned.
>
> Why are they badly aligned?
> The fact that long double is 12 bytes long doesn't mean it will force a 12-byte
> alignment - just whatever alignment is enough for a long double on the target
> machine. This could be 4, 8 or 16 bytes.

Things are a bit different:

Alignment is not the primary concern of the gc header structure.
Note that all the objects are created by malloc (system or python's
arena allocator), and therefore all objects are correctly aligned
by construction.

The point is: The GC header is a structure invisible to the "real"
gc allocated objects. It is opaquely prepended to every gc aware
object. Therefore, it *needs* to have the correct size, in order
to propagate its (already correct) alignment to the real object.

It appears that python, compiled with gcc for all x64 32bit Linuxen
(and mingw32) produces a 12 byte GC header. Not relevant for the
header itself, but all GC objects are misaligned.

This may not be recognized so far, because there is no builtin
GC-enabled type that embeds a double.
But if you create such a type, then the double will be correctly
aligned in your object's structure, but then, when the object
gets allocated, it is misaligned, because the header size is not a
multiple of 8.

To Martin: So I disagree. The gc header is not responsible for
alignment in the first place, but to propagate it, correctly.
And this fails miserably (in principle) since years.

Proposal: We should use a simple construct that makes the
gc header size simply a multiple of 8 or 16, whatever needed.
Even a byte array would be ok.

But please no long double :-)

cheers - chris
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