On Tue, Jan 3, 2017 at 12:19 PM, Doug McIlroy <doug at cs.dartmouth.edu> wrote:

> > keeping the code I work on portable between Linux and the Mac requires
> > more than a bit of ‘ifdef’ hell.
>
> Curmudgeonly comment: I bristle at the coupling of "ifdef" and "portable".
> Ifdefs that adjust code for different systems are prima facie
> evidence of NON-portability. I'll buy "configurable" as a descriptor
> for such ifdef'ed code, but not "portable".
>
> And, while I am venting about ifdef:
> As a matter of sytle, ifdefs are global constructs. Yet they often
> have local effects like an if statement. Why do we almost always write
>
> #ifdef LINUX
>         linux code
> #else
>         default unix code
> #endif
>
> instead of the much cleaner
>
>         if(LINUX)
>                 linux code
>         else
>                 default unix code
>
> In early days the latter would have cluttered precious memory
> with unfreachable code, but now we have optimizing compilers
> that will excise the useless branch just as effectively as cpp.
>
>
I have seen an interesting failure for the latter construct; I was
compiling some [unremembered] chess program for some [unremembered] UNIX
workstation in the late '80s.

The code had bit array data structures with get/set routines that optimized
for host word size, with code something like:

    if (sizeof (unsigned int) == 64)
        {
           // cast structure into array of 64 bit unsigned ints and use bit
operators
        }
    else  // sizeof (int) == 32
        {
           // cast structure into array of 32 bit unsigned ints and use bit
operators
        }

(It might of been 32/16; I don't remember, but it isn't relevant)

The '64' branch had an expression containing something like  '1u << 60' in
it.

I was compiling on a 32 bit int machine; the compiler flagged the '1u <<
60' as a fatal error due to the size of the shift -- on this compiler the
expression evaluator was running before the dead code remover.

-- Charles
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