Am 20.06.2011 19:10, schrieb Nicolas Pitre:
On Mon, 20 Jun 2011, Alan Stern wrote:
On Mon, 20 Jun 2011, Nicolas Pitre wrote:
On Mon, 20 Jun 2011, Alan Stern wrote:
On Sun, 19 Jun 2011, Nicolas Pitre wrote:
The question is: does the structure really has to be packed?
What do you mean? The structure really does need to be allocated
without padding between the fields; is that the same thing? So do a
bunch of other structures that currently have no annotations at all.
Yes, that's the same thing. The packed attribute tells the compiler
that you don't want it to insert padding in it as it sees fit.
I thought the packed attribute does more than that. For example, on
some architectures doesn't it also force the compiler to use
byte-oriented instructions for accessing the structure's fields?
Yes, but that's a consequence of not being able to access those fields
in their naturally aligned position anymore. Hence the addition of the
align attribute to tell the compiler that we know that the structure is
still aligned to a certain degree letting the compiler to avoid
byte-oriented instructions when possible.
Not exactly. As far as I can tell, the ((packed)) attribute caused the
compiler to change the structure's alignment from its natural value to
1. That's why the fields weren't in their naturally aligned positions
and why removing ((packed)) fixed the problem.
Are we talking past each other?
Remember that I was the one asking if the align attribute was needed in
the first place. If it is not then by all means please get rid of it!
But if it _is_ needed, then the generated code can be much better if the
packed attribute is _also_ followed by the align attribute to
increase it from 1.
That reminds me of some time where I had fun asking someone with deeper
ppc- and xlC-knowledge than I, if he is sure that assignments, are
atomic. He always said yes. Got something like a running gag. ;)
I see it that way: packed is needed to be sure that at least for struct
ehci_regs there are no padding bytes inbetween the members. It might
work without, but that depends on the compiler (-version, architecture,
whatever).
That packed without an additional aligned() caused errors on ARM with
gcc 4.6 is another problem which got (currently) fixed by removing packed.
But this introduces imho doubts and uncertainty about if padding bytes
could be between the members, therefore I would prefer to use packed
with aligned instead of removing the packed.
Regards,
Alexander
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