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

16 bit is often slower than 32 bit

[dtfinch]

In 32 bit protected mode, 16 bit instructions require a prefix to tell it that the following instruction is 16 bit, wasting a byte and a CPU cycle. In 16 bit real mode, the same is true of 32 bit instructions. But modern processors aren't optimized to preserve 16 bit performance. If they can improve 32 bit performance just a little, they'd be willing to sacrifice a lot of 16 bit performance to do it. Also, if you're mixing 16 and 32 bit variables in C/C++, it'll do a lot of expensive conversions to make it all work. I've done very little with 64 bit though, aside from playing with MMX on one occasion.

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.

It depends

[sfcat]

There are many factors that go into deciding how to write code. Portability is just one consideration of many. I would say that it is worth it if speed is of critical importantance and development expenses are of no concequence.

For instance, consider a video game. The faster it is the more likely it is that players will like it. But there are many more important factors including is the game just plain fun. So in video games, there is really a basic threshold of speed that needs to be met and after that is met, other factors are more important.

Next consider a real time system for trading stocks. This system is all about speed and reliability. You can control the deployment hardware and it is economically worthwhile to spent a lot in development if it makes more money in the long run. So coding your own memory pooler that uses the size of the pointer and a specific struct to make the code allocate and deallocate memory in constant time (it is very possible) is worthwhile because it can save alot of time per transaction.

But all of these issues come down to what exactly you are writing and both the technical and business requirements of your project. Without knowning those in advance, we can't really answer your question.

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...)

Does it matter?

[Jah-Wren Ryel]

It used to be that computers were expensive and people were relatively cheap. Nowadays, the reverse is generally true.

So, unless these systems have performance critical portions, like high-speed digital signal processing where every FLOP counts, it really isn't worth the extra effort to optimize your code for the platform - you'll just end up having to hand-tweak (or even worse, un-tweak) it again on the next hardware upgrade.

Re:Does it matter?

[richg74]

It used to be that computers were expensive and people were relatively cheap. Nowadays, the reverse is generall

For most applications, the potential performance gains from hand optimization for a specific platform aren't enough to matter. (And, as I think Brian Kernighan said, trying to outsmart the compiler defeats the purpose of using one.) Big performance gains come, in most cases, from figuring out a better way (~algorithm) to solve the problem, not from tweaks.

There's another aspect of portability that doesn't get mentioned too much: the portability of the programmer. If you are in the habit of writing portable code, it's much easier to shift to working on a different platform. (I'd also say, from my own experience, that it makes your work less error-prone.) That versatility is potentially of significant value to your employer, and of course is of value to you personally.

The performance question

[be-fan]

A couple of points about optimization.

1) Premature optimization is evil. Everybody says this, but so many people do not take it to heart. I'd rather have software that works, than software that is fast but crashes. As a programmer, its nice to work on non-buggy software, even if its not as fast as it could be.

2) Target-specific optimization is generally evil, unless you're sure your code will not live very long (eg: a game). The thing is that micro-optimizations generally tune for a particular processor, and actually pessimizes the code in the long run. In comparison, if you write good general code, it'll still be fast ten years from now when processors look very different.

3) The bottlenecks that people, especially C/C++ programmers worry about, are usually not the bottlenecks that usually matter. If you worry that your code could be faster/more memory efficient if you use a 16-bit field here or there instead of a 32-bit one, your algorithms better be absolutely perfect. Most code does not use perfect algorithms. That's why so much software is still so slow. Most programmers just don't get the time to use the best algorithms, much less get down to the level of micro-optimizations.

That's why I always find language performance debates entertaining. C/C++ programmers will freak out if you tell them language X is very productive, but is maybe two-thirds as fast as C (something that is true of a number of high-level, but compiled, languages). Meanwhile, they will write code that runs at maybe 1/3 of what the machine is capable of, because they spend so much time writing the code they have little time to optimize it.