How to change bit-field allocation

[tm_gccmail@kloo.net]

On Mon, 6 Sep 2004, James E Wilson wrote:

> Min ZOU wrote:
> > As I know, GCC allocates bit-fields inside a structure from the lowest
> > addressed byte, i.e. LSB, for little endian. However, I want to reverse
> > the ordering for the bit-fields allocation, i.e. always from MSB.
> 
> You can't change the allocation order for bit-fields without changing 
> the endianness of the processor.  A few moments thought should be enough 
> to see the problem.  The normal rule is that a bitfield goes in the 
> first avaiable bit.  You want it to go in the last available bit 
> instead.  But the last bit of what?  If you want the last bit of the 
> structure, then that is impossible to compute at this point because we 
> don't know the size of the structure yet.  If you put it in the last bit 
> of the current byte, then you run into problems if you have more than 8 
> bits of bitfields.  If you put it in the last bit of the current word, 
> then this probably works, but will add more padding then is absolutely 
> necessary, and hence does not give exactly the same result as you would 
> get for the other endianness.
> 
> What ever you are trying to do is probably wrong.  You probably should 
> say what problem you are trying to solve, rather than asking how to 
> implement particular solutions for it.

The SH is bi-endian.

I don't know what he's doing, but in the context of this, it may make
sense.

Toshi