Idea - big and little endian data areas using named address spaces

[David Brown]

On 10/11/10 18:05, Paul Koning wrote:
> C++ lets you define explicit-order integer types and hide all the
> conversions.  I used that a couple of jobs ago, back around 1996 or
> so -- worked nicely, and should work even better now that C++ is more
> mature.
>

Yes, a lot of such things can be done with C++ classes.  But I see 
several advantages in implementing it in the compiler rather than classes.

First, it will be available in C as well as C++.  For lots of reasons, 
many programs are written in C and not C++.

Secondly, the best way to implement endian re-ordering varies from 
target to target.  Some targets have assembly instructions that can be 
used, requiring inline assembly.  Some work best using shifts and masks, 
others using union types.  You can't make a C++ class that will give the 
best code on many targets without making it very messy, so typically it 
will be re-implemented for each target.

It's easy to make mistakes with C++ class using casting operators to do 
the endian swapping.  Miss out some details, and you might find the 
compiler omitting the endian swap somewhere.

Finally, if you want a struct with endian-swapped members, then each 
member must be explicitly declared as the appropriate class.  You can't 
swap the entire struct at once, as you could do with a memory space (or 
attribute) solution.

mvh.,

David


> paul
>
> On Nov 10, 2010, at 7:00 AM, David Brown wrote:
>
>> Would it be possible to use the named address space syntax to
>> implement reverse-endian data?  Conversion between little-endian
>> and big-endian data structures is something that turns up regularly
>> in embedded systems, where you might well be using two different
>> architectures with different endianness.  Some compilers offer
>> direct support for endian swapping, but gcc has no neat solution.
>> You can use the __builtin_bswap32 (but no __builtin_bswap16?)
>> function in recent versions of gcc, but you still need to handle
>> the swapping explicitly.
>>
>> Named address spaces would give a very neat syntax for using such
>> byte-swapped areas.  Ideally you'd be able to write something
>> like:
>>
>> __swapendian stuct { int a; short b; } data;
>>
>> and every access to data.a and data.b would be endian-swapped.  You
>> could also have __bigendian and __litteendian defined to
>> __swapendian or blank depending on the native ordering of the
>> target.
>>
>>
>> I've started reading a little about how named address spaces work,
>> but I don't know enough to see whether this is feasible or not.
>>
>>
>> Another addition in a similar vein would be __nonaligned, for
>> targets which cannot directly access non-aligned data.  The loads
>> and stores would be done byte-wise for slower but correct
>> functionality.
>>
>
>