[lwip-users] Re: [lwip] Anyone Using An Architecture That Isn't little or big endian?

[Shaun Jackman]

Having to redefine every constant for each architecture does not sit 
well with me. Are you certain you can't coerce your compiler to 
optimise away constants? I thought most every compiler does this.

Rod's code snippet will not produce compiler errors so long as the 
xtoxx() calls are macros. eg
        case ntohs( ETHTYPE_IP):
evaluates to
        case (ETHTYPE_IP << 8) | (ETHTYPE_IP >> 8):
which is constant, thus good C.

Unless you're *very* constrained (in code space or time), I would not 
sweat the little things such as where and how byte swapping is done. 
I believe it has very little effect on the overall performance of the 
stack, although I'd be interested to hear if someone disagrees. If 
the call is implemented as a macro, the majority of calls will be 
eliminated when they're called on a constant. When the call is on a 
variable, the function is so simple that the compiler should produce 
near-perfect assembler.

Cheers,
Shaun


> Thanks for the input. This looks like a good idea, but the compiler
> I'm using would then insert byte-swapping code for each use of
> ETHTYPE_ARP or ETHTYPE_IP, and it wouldn't be optimized out, and it
> ends up being more code than the current approach. Would other C
> compilers really optimize this away? Anyways, this particular code
> snippet would produce compile errors in any C compiler since a case
> statement must contain a constant. It would compile with an if then
> else statement, but might result in two byte swaps taking place
> instead of one  ...
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