I am doing DSP in Haxe. Some of my DSP includes recursive algorithms that may generate denormal (aka subnormal) numbers. Some platforms perform poorly when encountering such numbers, making real-time processing impossible (and even offline processing, in some cases, dramatically more difficult). Obviously, only algorithms that produce very small numbers (eg, via recursive multiplication) are effected, but I am working with these.
One very common procedure for dealing with the problem is simply this:
if r is a denormal
r <- 0
This works fine when denormals are too small to have any effect on the the given algorithm, which is (pretty much) always.
I am looking to build for a number of platforms and would like to avoid these headaches before they happen to the greatest extent possible. So the question is, how do I identify/eliminate denormals in Haxe quickly and efficiently?
This might break down to other questions like: does Haxe have a language-specific method of handling denormals, or is it up to the platform? (I see nothing in the docs -- not even an isDenormal function) If it's up to the platform, is there a flag or something? How do I know which platforms need special handling, and which do not?
Many thanks!
Haxe doesn't support these operations. The problem is that most of the native platforms it addresses do not have any support for that either. I am talking mainly of JavaScript, Flash, PHP and Neko here.
You can certainly build your own library and try to optimize things where possible using inlines.
Related
for a study on genetic programming, I would like to implement an evolutionary system on basis of llvm and apply code-mutations (possibly on IR level).
I found llvm-mutate which is quite useful executing point mutations.
As far as I have understood, the instructions get count/numbered, one can then e.g. delete a numbered instruction.
However, introduction of new instructions seems to be possible as one of the availeable statements in the code.
Real mutation however would allow to insert any of the allowed IR instructions, irrespective of it beeing used in the code to be mutated.
In addition, it should be possible to insert library function calls of linked libraries (not used in the current code, but possibly available, because the lib has been linked in clang).
Did I overlook this in the llvm-mutate or is it really not possible so far?
Are there any projects trying to /already have implement(ed) such mutations for llvm?
llvm has lots of code analysis tools which should allow the implementation of the afore mentioned approach. llvm is huge, so I'm a bit disoriented. Any hints which tools could be helpful (e.g. getting a list of available library functions etc.)?
Thanks
Alex
Very interesting question. I have been intrigued by the possibility of doing binary-level genetic programming for a while. With respect to what you ask:
It is apparent from their documentation that LLVM-mutate can't do what you are asking. However, I think it is wise for it not to. My reasoning is that any machine-language genetic program would inevitably face the "Halting Problem", e.g. it would be impossible to know if a randomly generated instruction would completely crash the whole computer (for example, by assigning a value to a OS-reserved pointer), or it might run forever and take all of your CPU cycles. Turing's theorem tells us that it is impossible to know in advance if a given program would do that. Mind you, LLVM-mutate can cause for a perfectly harmless program to still crash or run forever, but I think their approach makes it less likely by only taking existing instructions.
However, such a thing as "impossibility" only deters scientists, not engineers :-)...
What I have been thinking is this: In nature, real mutations work a lot more like LLVM-mutate that like what we do in normal Genetic Programming. In other words, they simply swap letters out of a very limited set (A,T,C,G) and every possible variation comes out of this. We could have a program or set of programs with an initial set of instructions, plus a set of "possible functions" either linked or defined in the program. Most of these functions would not be actually used, but they will be there to provide "raw DNA" for mutations, just like in our DNA. This set of functions would have the complete (or semi-complete) set of possible functions for a problem space. Then, we simply use basic operations like the ones in LLVM-mutate.
Some possible problems though:
Given the amount of possible variability, the only way to have
acceptable execution times would be to have massive amounts of
computing power. Possibly achievable in the Cloud or with GPUs.
You would still have to contend with Mr. Turing's Halting Problem.
However I think this could be resolved by running the solutions in a
"Sandbox" that doesn't take you down if the solution blows up:
Something like a single-use virtual machine or a Docker-like
container, with a time limitation (to get out of infinite loops). A
solution that crashes or times out would get the worst possible
fitness, so that the programs would tend to diverge away from those
paths.
As to why do this at all, I can see a number of interesting applications: Self-healing programs, programs that self-optimize for an specific environment, program "vaccination" against vulnerabilities, mutating viruses, quality assurance, etc.
I think there's a potential open source project here. It would be insane, dangerous and a time-sucking vortex: Just my kind of project. Count me in if someone doing it.
I have an image processing routine that I believe could be made very parallel very quickly. Each pixel needs to have roughly 2k operations done on it in a way that doesn't depend on the operations done on neighbors, so splitting the work up into different units is fairly straightforward.
My question is, what's the best way to approach this change such that I get the quickest speedup bang-for-the-buck?
Ideally, the library/approach I'm looking for should meet these criteria:
Still be around in 5 years. Something like CUDA or ATI's variant may get replaced with a less hardware-specific solution in the not-too-distant future, so I'd like something a bit more robust to time. If my impression of CUDA is wrong, I welcome the correction.
Be fast to implement. I've already written this code and it works in a serial mode, albeit very slowly. Ideally, I'd just take my code and recompile it to be parallel, but I think that that might be a fantasy. If I just rewrite it using a different paradigm (ie, as shaders or something), then that would be fine too.
Not require too much knowledge of the hardware. I'd like to be able to not have to specify the number of threads or operational units, but rather to have something automatically figure all of that out for me based on the machine being used.
Be runnable on cheap hardware. That may mean a $150 graphics card, or whatever.
Be runnable on Windows. Something like GCD might be the right call, but the customer base I'm targeting won't switch to Mac or Linux any time soon. Note that this does make the response to the question a bit different than to this other question.
What libraries/approaches/languages should I be looking at? I've looked at things like OpenMP, CUDA, GCD, and so forth, but I'm wondering if there are other things I'm missing.
I'm leaning right now to something like shaders and opengl 2.0, but that may not be the right call, since I'm not sure how many memory accesses I can get that way-- those 2k operations require accessing all the neighboring pixels in a lot of ways.
Easiest way is probably to divide your picture into the number of parts that you can process in parallel (4, 8, 16, depending on cores). Then just run a different process for each part.
In terms of doing this specifically, take a look at OpenCL. It will hopefully be around for longer since it's not vendor specific and both NVidia and ATI want to support it.
In general, since you don't need to share too much data, the process if really pretty straightforward.
I would also recommend Threading Building Blocks. We use this with the Intel® Integrated Performance Primitives for the image analysis at the company I work for.
Threading Building Blocks(TBB) is similar to both OpenMP and Cilk. And it uses OpenMP to do the multithreading, it is just wrapped in a simpler interface. With it you don't have to worry about how many threads to make, you just define tasks. It will split the tasks, if it can, to keep everything busy and it does the load balancing for you.
Intel Integrated Performance Primitives(Ipp) has optimized libraries for vision. Most of which are multithreaded. For the functions we need that aren't in the IPP we thread them using TBB.
Using these, we obtain the best result when we use the IPP method for creating the images. What it does is it pads each row so that any given cache line is entirely contained in one row. Then we don't divvy up a row in the image across threads. That way we don't have false sharing from two threads trying to write to the same cache line.
Have you seen Intel's (Open Source) Threading Building Blocks?
I haven't used it, but take a look at Cilk. One of the big wigs on their team is Charles E. Leiserson; he is the "L" in CLRS, the most widely/respected used Algorithms book on the planet.
I think it caters well to your requirements.
From my brief readings, all you have to do is "tag" your existing code and then run it thru their compiler which will automatically/seamlessly parallelize the code. This is their big selling point, so you dont need to start from scratch with parallelism in mind, unlike other options (like OpenMP).
If you already have a working serial code in one of C, C++ or Fortran, you should give serious consideration to OpenMP. One of its big advantages over a lot of other parallelisation libraries / languages / systems / whatever, is that you can parallelise a loop at a time which means that you can get useful speed-up without having to re-write or, worse, re-design, your program.
In terms of your requirements:
OpenMP is much used in high-performance computing, there's a lot of 'weight' behind it and an active development community -- www.openmp.org.
Fast enough to implement if you're lucky enough to have chosen C, C++ or Fortran.
OpenMP implements a shared-memory approach to parallel computing, so a big plus in the 'don't need to understand hardware' argument. You can leave the program to figure out how many processors it has at run time, then distribute the computation across whatever is available, another plus.
Runs on the hardware you already have, no need for expensive, or cheap, additional graphics cards.
Yep, there are implementations for Windows systems.
Of course, if you were unwise enough to have not chosen C, C++ or Fortran in the beginning a lot of this advice will only apply after you have re-written it into one of those languages !
Regards
Mark
Another thread had this quote
Erlang VM BEAM and HiPE is written mostly in C.
Linked-in drivers are written mostly in C. (They are plugged to VM
and serves communication with outside
world.)
I've read some opinions that Ada's strong typing, modularity, run-time checking, parallel processing etc. etc. are better than that of C.
Would Erlang compiled using Ada be noticably better or worse than the Erlang made with C?
Just a hypothetical Q.
Normally I'd throw a bunch of weasel-words at you on a topic like this, but it turns out this exact question has been studied (it's a .doc file. Sorry).
Rational had a compiler with a large amount of code in both languages, and a large amount of data compiled over several years on bug rates, fix times, etc. Out of curiosity, one of their engineers crunched the numbers.
The answer was "Development Costs of C Exceed Those of Ada". If you read past the summary title, you'd see that they figured writing the same code in Ada cost them about half what writing it in C cost.
I know that everyone reading this is anxious to poke holes in that conclusion. I was too. But they looked at darn near every angle I could think of in the report.
"Better" in what way? Better as in faster? Better as in less bugs? Better as in more portable? Better as in more readable? Better as in more extensible?
For any suitable definition of "better" arguments can be made either way. However, it is just about sacred writ that no compiled language is more portable than C. Thus, if one of your goals is to make your application highly portable, C is an excellent choice.
More people understand C than Ada. Writing erlang extensions might be much harder if it was written in Ada, simply because fewer people are conversant with the language.
C code can be highly performant, but I am aware of no comparisons between C and Ada w/r to compiler optimizations.
Ada's type checking might be useful, or it could be a real problem. One presumes that a VM does it's own type checking on the pieces that matter to it. The overhead of RTTC in Ada could impose a completely unnecessary burden.
Perhaps, but what language was your Ada compiler written in? Ada? What about the compiler that write your FIRST Ada compiler?
At some point when you are building software, formal semantics and software processes are much more important than what language something was coded in.
Is it possible that Microsoft will be able to make F# programs, either at VM execution time, or more likely at compile time, detect that a program was built with a functional language and automatically parallelize it better?
Right now I believe there is no such effort to try and execute a program that was built as single threaded program as a multi threaded program automatically.
That is to say, the developer would code a single threaded program. And the compiler would spit out a compiled program that is multi-threaded complete with mutexes and synchronization where needed.
Would these optimizations be visible in task manager in the process thread count, or would it be lower level than that?
I think this is unlikely in the near future. And if it does happen, I think it would be more likely at the IL level (assembly rewriting) rather than language level (e.g. something specific to F#/compiler). It's an interesting question, and I expect that some fine minds have been looking at this and will continue to look at this for a while, but in the near-term, I think the focus will be on making it easier for humans to direct the threading/parallelization of programs, rather than just having it all happen as if by magic.
(Language features like F# async workflows, and libraries like the task-parallel library and others, are good examples of near-term progress here; they can do most of the heavy lifting for you, especially when your program is more declarative than imperative, but they still require the programmer to opt-in, do analysis for correctness/meaningfulness, and probably make slight alterations to the structure of the code to make it all work.)
Anyway, that's all speculation; who can say what the future will bring? I look forward to finding out (and hopefully making some of it happen). :)
Being that F# is derived from Ocaml and Ocaml compilers can optimize your programs far better than other compilers, it probably could be done.
I don't believe it is possible to autovectorize code in a generally-useful way and the functional programming facet of F# is essentially irrelevant in this context.
The hardest problem is not detecting when you can perform subcomputations in parallel, it is determining when that will not degrade performance, i.e. when the subtasks will take sufficiently long to compute that it is worth taking the performance hit of a parallel spawn.
We have researched this in detail in the context of scientific computing and we have adopted a hybrid approach in our F# for Numerics library. Our parallel algorithms, built upon Microsoft's Task Parallel Library, require an additional parameter that is a function giving the estimated computational complexity of a subtask. This allows our implementation to avoid excessive subdivision and ensure optimal performance. Moreover, this solution is ideal for the F# programming language because the function parameter describing the complexity is typically an anonymous first-class function.
Cheers,
Jon Harrop.
I think the question misses the point of the .NET architecture-- F#, C# and VB (etc.) all get compiled to IL, which then gets compiled to machine code via the JIT compiler. The fact that a program was written in a functional language isn't relevant-- if there are optimizations (like tail recursion, etc.) available to the JIT compiler from the IL, the compiler should take advantage of it.
Naturally, this doesn't mean that writing functional code is irrelevant-- obviously, there are ways to write IL which will parallelize better-- but many of these techniques could be used in any .NET language.
So, there's no need to flag the IL as coming from F# in order to examine it for potential parallelism, nor would such a thing be desirable.
There's active research for autoparallelization and auto vectorization for a variety of languages. And one could hope (since I really like F#) that they would concive a way to determine if a "pure" side-effect free subset was used and then parallelize that.
Also since Simon Peyton-Jones the father of Haskell is working at Microsoft I have a hard time not beliving there's some fantastic stuff comming.
It's possible but unlikely. Microsoft spends most of it's time supporting and implementing features requested by their biggest clients. That usually means C#, VB.Net, and C++ (not necessarily in that order). F# doesn't seem like it's high on the list of priorities.
Microsoft is currently developing 2 avenues for parallelisation of code: PLINQ (Pararllel Linq, which owes much to functional languages) and the Task Parallel Library (TPL) which was originally part of Robotics Studio. A beta of PLINQ is available here.
I would put my money on PLINQ becoming the norm for auto-parallelisation of .NET code.
I am trying to build out a useful 3d game engine out of the Ogre3d rendering engine for mocking up some of the ideas i have come up with and have come to a bit of a crossroads. There are a number of scripting languages that are available and i was wondering if there were one or two that were vetted and had a proper following.
LUA and Squirrel seem to be the more vetted, but im open to any and all.
Optimally it would be best if there were a compiled form for the language for distribution and ease of loading.
One interesting option is stackless-python. This was used in the Eve-Online game.
The syntax is a matter of taste, Lua is like Javascript but with curly braces replaced with Pascal-like keywords. It has the nice syntactic feature that semicolons are never required but whitespace is still not significant, so you can even remove all line breaks and have it still work. As someone who started with C I'd say Python is the one with esoteric syntax compared to all the other languages.
LuaJIT is also around 10 times as fast as Python and the Lua interpreter is much much smaller (150kb or around 15k lines of C which you can actually read through and understand). You can let the user script your game without having to embed a massive language. On the other hand if you rip the parser part out of Lua it becomes even smaller.
The Python/C API manual is longer than the whole Lua manual (including the Lua/C API).
Another reason for Lua is the built-in support for coroutines (co-operative multitasking within the one OS thread). It allows one to have like 1000's of seemingly individual scripts running very fast alongside each other. Like one script per monster/weapon or so.
( Why do people write Lua in upper case so much on SO? It's "Lua" (see here). )
One more vote for Lua. Small, fast, easy to integrate, what's important for modern consoles - you can easily control its memory operations.
I'd go with Lua since writing bindings is extremely easy, the license is very friendly (MIT) and existing libraries also tend to be under said license. Scheme is also nice and easy to bind which is why it was chosen for the Gimp image editor for example. But Lua is simply great. World of Warcraft uses it, as a very high profile example. LuaJIT gives you native-compiled performance. It's less than an order of magnitude from pure C.
I wouldn't recommend LUA, it has a peculiar syntax so takes some time to get used to. Depending on who will be doing the scripting, this may not be a problem, but I would try to use something fairly accessible.
I would probably choose python. It normally compiles to bytecode, so you would need to embed the interpreter. However, if you must you can use PyPy to for example translate the code to C, and then compile it.
Embedding the interpreter is no issue. I am more interested in features and performance at this point in time. LUA and Squirrel are both interpreted, which is nice because one of the games i am building out is to include modifiable code, which has an editor in game.
I would love to hear about python, as i have seen its use within the battlefield series i believe.
python is also nice because it has actual OGRE bindings, just in case you need to modify something lower-level on the fly. I don't know of any equivalent bindings for Lua.
Since it's a C++ library, I would suggest either JavaScript or Squirrel, the latter being my personal favorite of the two for being even closer to C++, in particular to how it handles tables/structs and classes. It would be the easiest to get used to for a C++ coder because of all the similarities.
However, if you go with JavaScript and find an HTML5 version of Ogre3D, you should be able to port your game code directly into the web version with minimal (if any) changes necessary.
Both of these are a good choice, and both have their pros and cons, but both would definitely be the easiest to learn since you're likely already working in C++. If you're working with Java, the same may hold true, and if it's Game Maker, you wouldn't need either one unless you're trying to make an executable that people wouldn't need Game Maker itself to run, in which case, good luck finding an extension to run either of these.