On Thu, 12 Feb 2009, Max Laier wrote:
That's the tricky part. Rearranging the structs itself is not that
difficult, but identifying which should be rearranged and if, how ... that's
the problem. The fact that gaps might be different for 64 vs. 32 bit
architectures has already been mentioned.
In addition one needs to keep in mind that changing a struct layout is a ABI
change. So if we do identify structs that we want to change we should do
them all at once to keep the different versions down to a minimum.
Leaving aside the potential memory savings, etc, another potential use for a
tool like this is actually in ABI monitoring and maintenance. I.e., run a
tool that generates a description of the structure ABI of the kernel and user
interfaces of the system, then run it intermittently to detect and report
changes, perhaps correlated with symbol version information, etc. Right now we
discover ABI changes in three ways: a developer changing the ABI realizes it
has changed and (probably) deals with it, a developer reviews changes later
and finds it, or a user finds it the hardware way due to application crashes,
data loss, etc. Mechanizing tracking of API signatures, structure layout,
etc, would help make all of this a lot more deterministic.
Robert N M Watson
Computer Laboratory
University of Cambridge
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