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• Where to find algorithms for standard math functions?

  - by dsimcha

  I'm looking to submit a patch to the D programming language standard library that will allow much of std.math to be evaluated at compile time using the compile-time function evaluation facilities of the language. Compile-time function evaluation has several limitations, the most important ones being: You can't use assembly language. You can't call C code or code for which the source is otherwise unavailable. Several std.math functions violate these and compile-time versions need to be written. Where can I get information on good algorithms for computing things such as logarithms, exponents, powers, and trig functions? I prefer just high level descriptions of algorithms to actual code, for two reasons: To avoid legal ambiguity and the need to make my code look "different enough" from the source to make sure I own the copyright. I want simple, portable algorithms. I don't care about micro-optimization as long as they're at least asymptotically efficient. Edit: D's compile time function evaluation model allows floating point results computed at compile time to differ from those computed at runtime anyhow, so I don't care if my compile-time algorithms don't give exactly the same result as the runtime version as long as they aren't less accurate to a practically significant extent.

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• Atomic Instructions and Variable Update visibility

  - by dsimcha

  On most common platforms (the most important being x86; I understand that some platforms have extremely difficult memory models that provide almost no guarantees useful for multithreading, but I don't care about rare counter-examples), is the following code safe? Thread 1: someVariable = doStuff(); atomicSet(stuffDoneFlag, 1); Thread 2: while(!atomicRead(stuffDoneFlag)) {} // Wait for stuffDoneFlag to be set. doMoreStuff(someVariable); Assuming standard, reasonable implementations of atomic ops: Is Thread 1's assignment to someVariable guaranteed to complete before atomicSet() is called? Is Thread 2 guaranteed to see the assignment to someVariable before calling doMoreStuff() provided it reads stuffDoneFlag atomically? Edits: The implementation of atomic ops I'm using contains the x86 LOCK instruction in each operation, if that helps. Assume stuffDoneFlag is properly cleared somehow. How isn't important. This is a very simplified example. I created it this way so that you wouldn't have to understand the whole context of the problem to answer it. I know it's not efficient.

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• Function overloading by return type?

  - by dsimcha

  Why don't more mainstream statically typed languages support function/method overloading by return type? I can't think of any that do. It seems no less useful or reasonable than supporting overload by parameter type. How come it's so much less popular?

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• Why is there so much poorly indented code out there?

  - by dsimcha

  The more I browse the code to open source projects in languages that aren't Python, the more I realize that it seems a lot of programmers don't believe in proper indentation. (I won't mention any projects specifically to avoid having anyone take this question too personally.) Usually code is indented, but in a way just different enough from the standard style that it drives me crazy, especially in old/crufty code. I've noticed that when I write in C-like languages, I tend to indent correctly as religiously as when I'm writing in Python, with the exception of debugging code that I actually want to stick out like a sore thumb. Given how easy it is with a modern IDE to fix incorrect indentation, what are some rationales for not religiously keeping indentation in sync with braces?

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• Most interesting and challenging programming tasks

  - by dsimcha

  Some programmers enjoy optimizing code to make the implementation as fast as humanly possible; or golfing to make code as compact as possible. Others enjoy metaprogramming to make code generic, or designing algorithms to be asymptotically efficient. What do you find most interesting and challenging as a programmer?

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• Self-Configuring Classes W/ Command Line Args: Pattern or Anti-Pattern?

  - by dsimcha

  I've got a program where a lot of classes have really complicated configuration requirements. I've adopted the pattern of decentralizing the configuration and allowing each class to take and parse the command line/configuration file arguments in its c'tor and do whatever it needs with them. (These are very coarse-grained classes that are only instantiated a few times, so there is absolutely no performance issue here.) This avoids having to do shotgun surgery to plumb new options I add through all the levels they need to be passed through. It also avoids having to specify each configuration option in multiple places (where it's parsed and where it's used). What are some advantages/disadvantages of this style of programming? It seems to reduce separation of concerns in that every class is now doing configuration stuff, and to make programs less self-documenting because what parameters a class takes becomes less explicit. OTOH, it seems to increase encapsulation in that it makes each class more self-contained because no other part of the program needs to know exactly what configuration parameters a class might need.

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• When are global variables acceptable?

  - by dsimcha

  Everyone here seems to hate global variables, but I see at least one very reasonable use for them: They are great for holding program parameters that are determined at program initialization and not modified afterwords. Do you agree that this is an exception to the "globals are evil" rule? Is there any other exception that you can think of, besides in quick and dirty throwaway code where basically anything goes? If not, why are globals so fundamentally evil that you do not believe that there are any exceptons?

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• Testing When Correctness is Poorly Defined?

  - by dsimcha

  I generally try to use unit tests for any code that has easily defined correct behavior given some reasonably small, well-defined set of inputs. This works quite well for catching bugs, and I do it all the time in my personal library of generic functions. However, a lot of the code I write is data mining code that basically looks for significant patterns in large datasets. Correct behavior in this case is often not well defined and depends on a lot of different inputs in ways that are not easy for a human to predict (i.e. the math can't reasonably be done by hand, which is why I'm using a computer to solve the problem in the first place). These inputs can be very complex, to the point where coming up with a reasonable test case is near impossible. Identifying the edge cases that are worth testing is extremely difficult. Sometimes the algorithm isn't even deterministic. Usually, I do the best I can by using asserts for sanity checks and creating a small toy test case with a known pattern and informally seeing if the answer at least "looks reasonable", without it necessarily being objectively correct. Is there any better way to test these kinds of cases?

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• Faking a Single Address Space

  - by dsimcha

  I have a large scientific computing task that parallelizes very well with SMP, but at too fine grained a level to be easily parallelized via explicit message passing. I'd like to parallelize it across address spaces and physical machines. Is it feasible to create a scheduler that would parallelize already multithreaded code across multiple physical computers under the following conditions: The code is already multithreaded and can scale pretty well on SMP configurations. The fact that not all of the threads are running in the same address space or on the same physical machine must be transparent to the program, even if this comes at a significant performance penalty in some use cases. You may assume that all of the physical machines involved are running operating systems and CPU architectures that are binary compatible. Things like locks and atomic operations may be slow (having network latency to deal with and all) but must "just work".

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• C++0x implementation guesstimates?

  - by dsimcha

  The C++0x standard is on its way to being complete. Until now, I've dabbled in C++, but avoided learning it thoroughly because it seems like it's missing a lot of modern features that I've been spoiled by in other languages. However, I'd be very interested in C++0x, which addresses a lot of my complaints. Any guesstimates, after the standard is ratified, as to how long it will take for major compiler vendors to provide reasonably complete, production-quality implementations? Will it happen soon enough to reverse the decline in C++'s popularity, or is it too little, too late? Do you believe that C++0x will become "the C++" within a few years, or do you believe that most people will stick to the earlier standard in practice and C++0x will be somewhat of a bastard stepchild, kind of like C99?

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• Are spinlocks a good choice for a memory allocator?

  - by dsimcha

  I've suggested to the maintainers of the D programming language runtime a few times that the memory allocator/garbage collector should use spinlocks instead of regular OS critical sections. This hasn't really caught on. Here are the reasons I think spinlocks would be better: At least in synthetic benchmarks that I did, it's several times faster than OS critical sections when there's contention for the memory allocator/GC lock. Edit: Empirically, using spinlocks didn't even have measurable overhead in a single-core environment, probably because locks need to be held for such a short period of time in a memory allocator. Memory allocations and similar operations usually take a small fraction of a timeslice, and even a small fraction of the time a context switch takes, making it silly to context switch in the case of contention. A garbage collection in the implementation in question stops the world anyhow. There won't be any spinning during a collection. Are there any good reasons not to use spinlocks in a memory allocator/garbage collector implementation?

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• Fast, cross-platform timer?

  - by dsimcha

  I'm looking to improve the D garbage collector by adding some heuristics to avoid garbage collection runs that are unlikely to result in significant freeing. One heuristic I'd like to add is that GC should not be run more than once per X amount of time (maybe once per second or so). To do this I need a timer with the following properties: It must be able to grab the correct time with minimal overhead. Calling core.stdc.time takes an amount of time roughly equivalent to a small memory allocation, so it's not a good option. Ideally, should be cross-platform (both OS and CPU), for maintenance simplicity. Super high resolution isn't terribly important. If the times are accurate to maybe 1/4 of a second, that's good enough. Must work in a multithreaded/multi-CPU context. The x86 rdtsc instruction won't work.

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