Re: [boost] [endian] Refresh based on comments received

From: Beman Dawes (bdawes_at_[hidden])
Date: 2006-06-12 15:38:02

Tomas Puverle wrote:
>> A refresh of the .zip file for the Endian library, based on comments 
>> received so far, is available at 
>> http://mysite.verizon.net/~beman/endian-0.2.zip
>>
>> The docs are online at 
>> http://mysite.verizon.net/~beman/endian-0.2/libs/endian/index.html
> 
> I was very interested in this library/thread but unfortunately didn't have the 
> time to read the old thread and this one until now.
> 
> I'd like to put up a few suggestions for consideration:
> 
> 1) There is some mention of supporting custom swapping routines.  This is 
> fairly important, as the 3 architectures that I use the most (x86, x86-64 and 
> sparc) all support some harware primitives to perform endian swapping.  x86's 
> have a bswap instruction.  sparc allows you to load/store memory operands as 
> either little or big endian.  Other architectures I occasionaly touch have 
> similar capabilities.  I would like to be able to take advantage of this.

In theory it should be possible to provide assemble language 
optimizations for specific platforms. In practice..., well, we will have 
to see what surfaces.

The first step is to finalize the interface, docs, etc, and have a 
formal review. Once that is complete, it would be great if platform 
specific assembler optimizations were contributed, particularly when 
timing tests show considerable speedups.

You might want to check the assembly language output from your favorite 
compiler. I have seen a compiler that recognized byte-swaps and 
generated code that used a byte-swap instruction.

> 2)  Could the library include functionality to make it more useful with other 
> types?  This suggestion is not a replacement for what you are doing, it's more 
> like a supplement.  It's useful when you need to do a fair amount of 
> processing on your data after you read it in but before you write it back to 
> the stream/storage etc.
> This is how I've implemented similar things in the past:
> 
> template<typename T>
> struct endian_swapper
> {
>   static void swap_in_place(T&);
> };
> 
> The class is specialised for builtin types and arrays, where the member does 
> the obvious thing.  There is also an inline template free function with the 
> same signature as the member, which just does "return 
> endian_swapper<T>::swap_in_place(t);"
> 
> Now consider a struct, e.g. some sort of network message:
> 
> struct A
> {
>    char k;
>    int i;
>    float x;   
> };
> 
> BOOST_IMPLEMENT_ENDIAN(A, (k)(i)(x));
> 
> The macro expands to a specialisation of endian_swapper<A> with swap_in_place 
> just calls the free function swap_in_place:
> 
> template<> 
> struct endian_swapper<A>
> {
>      void swap_in_place(A& t_)
>      {
>         swap_in_place(t_.(&A::k));
>         swap_in_place(t_.(&A::i));
>         swap_in_place(t_.(&A::x));
>      }
> };                       
> 
> You can see how this also recursively works for structs/arrays inside other 
> structs/arrays.
> 
> Final piece is the free function swap, which gives you back a swapped copy of 
> the original:
> 
> template<class T>
> T swap(const T & t_)
> {
>    T temp(t_)
>    swap_in_place(temp);
>    return temp;
> }
> 
> I just tried to outline the general idea here but I hope I managed to get the 
> point across.
> 
> It would be even nicer if it could also understand the native/little/big 
> endian semantics you've already discussed but I thought trying to work that 
> into the examples would just make them too complicated.

I'm not adverse to a more general endian swapper, but that is beyond my 
needs. (I'm basically trying to get endian types into Boost so I can use 
them in a B-tree library, and don't want to spend the whole summer just 
talking about endian possibilities.)

> Thanks for the good work.

Well, thank you for the comments!

--Beman