The Sacrifices of Portablility?

hackwrench asks: "There is lots of talk about writing portable programs, but this pursuit has resulted in a lot of processor features going unused. One example is being able to write a program that purposely uses a combination of 16-bit and 32 bit. I know there are arguments that writing solely in one or the other is a performance advantage, but what are the factors involved? Is the slowness of such a combination inherent in its design or is it a result of current hardware. We are beginning to replace systems and programs designed primarily to run in pure 32-bit mode with systems designed to run in pure 64-bit mode, so I ask: Is such purity really worth it?"

Comments

Ideally, your code is clean enough

[Frumious Wombat]

that this transition isn't all that painful.

My personal experience with this was Linux on Alpha, where certain programs assumed a 32-bit environment, rather than querying the system they were built on for size of int, pointer, etc. As a result many programs were funky on the Alpha, and the 'pc-isms' (what we once would have called Vaxocentrisms) caused great waste of time as they had to be tracked down an eliminated.

Your code, if you've been worrying about anything other than 32-bit PCs, should already be 64-bit clean, as you've had 15 years of Alpha, SGI, Power, Itanium, and Sun 64-bit systems to support. If it isn't, hopefully it's something such as user interface which will still run in the 32-bit environment, though not necessarily optimally.

Personally, I think that writing robust, portable, code is worth the effort. Unless you're talking about running on an embedded system where every byte counts, it doesn't hurt you at all to design clean algorithms and data structures, and put in checks to actually determine the size of ints, longs, pointers, etc, rather than just assuming that everyone will run x86 (or MIPS-64 or whatever) from now until the end of time. I have research programs that were written in the 70s (in their original form), on Cyber 205 and similar long-gone architectures, which still work because they were written in a mostly portable manner, with only the most critical nasty bits tied specifically to that machine. Your code is going to be in use longer than you think; be nice to your successors and make it portable now.

Re:It depends

[forkazoo]

A lot of people are writing responses that tend to assume it is impossible to write code that is portable, and also optimised for a specific platform. I recently read a book called "Vector Game Math Processors" (everybody needs a hobby, right?). Looking at how the examples were coded in that book sort of shifted my assumptions about how I should do things.

Basically, the book covers the major vector instruction sets: Altivec, PS2, SSE, etc. Naturally, a program written with hand optimised SSE assembly won't run very well on a PowerMac G4. So, the approach the author used was to start by coding a vector math function in plain C. He only calls this function by a function pointer. So, instead of calling sw_vector_foo directly, he calls vector_foo. He then goes on to write altivec_foo, and sse_foo, and gamecube_foo. With some simple #ifdefs at compile time, the function pointer is assigned to the most optimal code path for the platform.

So, the result is that by thinking about portability going in, he doesn't have to do hardly any work to have fairly optimal hand-tuned vector routines for a new architecture.

In general, code written to be portable is also much cleaner, and better commented, and whatnot, just because the author was forced tos pend an extra few minutes thinking about how things ought to be put together. I really can't think of any normal case where portability shouldn't be a consideration. On some obscure embedded systems, you might really want to optimise to a super specific piece of hardware, but it is seldom worth it.

Think about writing GUI apps for a Palm pilot before the switch to ARM CPU's. A programmer could have said, "hey, I'm using the Palm OS API's, and they only run on Coldfire CPU's, so I have no reason to make anything portable." Then, a little while later, Palms OS starts running on ARM. If he had invested a smidgen of extra effort to write his code in a portable way, he could easily start to take advantage of the ARM stuff right away. Since most of the issues of portability are in the planning phase, and get handled at compile time, the difference in memory footprint need not be appreciably larger. (Like a bunch of hand coded ASM for a different platform, which get's #ifdef'd away, or sizeof() operators...)