Byte swapping support

[Eric Botcazou]

> To support applications that assume big-endian memory layout on little-
> endian systems, I'm considering adding support for reversing the
> storage order to GCC.

That was also the goal of the scalar_storage_order attribute.

> In contrast to the existing scalar storage order support for structs, the
> goal is to reverse the storage order for all memory operations to achieve
> maximum compatibility with the behavior on big-endian systems, as far as
> observable by the application.

I presume that you'll well aware of this, but you cannot just reverse the 
storage order for any memory operation; for example, an array of 4 chars in C 
is stored the same way in big-endian and little-endian order, so you ought not 
to do byte swapping when you access it as a whole.  So the above sentence must 
be read as "to reverse the storage order for all scalar memory operations".

When the scalar_storage_order attribute was designed, discussions lead to the 
conclusion that doing the swapping for any scalar memory operation, as opposed 
to any access to a scalar within a structure, would not be a significant step 
forward to warrant the significantly more complex implementation (or the big 
performance penalty if you do things very roughly).

> The plan is to insert byte swapping instructions as part of the RTL
> expansion of GIMPLE assignments that access memory. This would leverage
> code that was added for -fsso-struct, keeping the code simple and
> maintainable.

How do you discriminate scalars stored in native order and scalars stored in 
reverse order though?  That's the main difficulty of the implementation.

-- 
Eric Botcazou