Re: Byte-reverse instructions in PowerPC Processors

[David Daney <ddaney@avtrex.com>]

Roberto Barbieri Carrera wrote:
> Hello,
> 
> I don´t actually know if this is the proper place for asking this.
> Please excuse me if I´m wrong.
> 
> I´m developing a system that works in two stages: configuration time
> and run time.
> 
> Configuration processing is performed in a little endian machine, with
> code compiled with gcc.
> 
> Run time processing is performed in a ´603´ PowerPC processor, namely,
> an MPC8248. Also compiled with gcc.
> 
> The output of the configuration stage is a binary file containing
> several struct´s. This means that the endianness of the data has to be
> converted somewhere.
> 
> Of course I can do this with some software, but this brings a
> maintainability issue, i.e., if a struct changes in the future, the
> conversion software will have to be changed also. This usually brings
> bugs.
> 
> I though that, as the PowerPC processor has byte-reverse instructions,
> one could declare, for example, a big struct containing all the file´s
> structs, as byte-order reversed, so that the compiler could use the
> byte-reversed instructions each time it accesses something within this
> struct. Declaring my struct like this in the PowerPC code could make
> my binary file little endian, as well as its memory image in the
> PowerPC processor, and no conversions would be necessary.
> 
> Is it possible to do something like this with gcc? If not, would it be
> too hard to implement?
> 
> Thank you in advance for your answer.

Portable device drivers often have similar issues.

One I have recently seen is in the Linux kernel: drivers/net/e100.c  You 
may be able to borrow the techniques used there.

David Daney