Re: ARM packed structures

[Dimitry Andric <dimitry at andric dot com>]

Caputo, Tim wrote:
> I am having a problem with structures using the arm cross complier I've
> built.  Initially I noticed that my structures were not being packed in
> memory.  I realize now that this is meant to be efficient, however, I
> need to share these structures with legacy code on another processor and
> with specialty hardware that both expect/need them to be packed.  I
> tried the pragma route, the attribute route and finally was successful
> packing my structures with the directive -fpack_struct.

Don't do that.  Read the gcc manual, and use __attribute__((packed))
instead.


> Unfortunately this resulted the compiler being extremely conservative in
> how it accessed elements of the struct.  It now reads one byte at a time
> from memory and reconstructs longwords in registers (I assume it is
> worried about the whole structure being unaligned in memory.)

It is not "worried", arm processors just *require* you to do this
dance.  It depends on the specific cpu model how complicated the dance
is. :)

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