6 ms·

If anyone's interested, this is the (epic!) discussion thread for "Getting explicit SIMD on stable Rust": https://internals.rust-lang.org/t/getting-explicit-si

by mrec 9y ago

If anyone's interested, this is the (epic!) discussion thread for "Getting explicit SIMD on stable Rust": https://internals.rust-lang.org/t/getting-explicit-simd-on-stable-rust/4380/334 https://internals.rust-lang.org/t/getting-explicit-simd-on-s... It still wouldn't compete with really good hand-tuned asm, but it might help reduce the perf/usability tradeoff a bit.

phkahler 9y ago
I'd just like to see 2,3, and 4 element vectors as a first class citizen in C, C++, or Rust. These are incredibly common for so many things it's hard for me to understand this omission. I want to be able to pass them by value and as return values from function. I want to do operations like a=b+c with vectors without creating classes or overloading operators. For a lot of people the SIMD instructions are about parallel computation, but for me they represent vector primitives.
- Zelizz 9y ago
  Look into the Clang vector extensions. They're very similar to GLSL/OpenCL vectors. They implement some very basic operator overloading too, so they're much less annoying that calling a function for everything. I made a simple linear algebra library using them, if you want to see an example: https://github.com/GavinHigham/glla https://github.com/GavinHigham/glla
- mrec 9y ago
  What would baking them into the language add that a library solution couldn't? I'm not keeping track of Rust very closely these days, but it supports arithmetic operator overloading and the Copy trait. What's missing? I agree that this needs to be standardized for vector-ey crates to be able to talk to each other seamlessly. Otherwise we'll end up with a rerun of the C++ strings fiasco, with char* and wchar_t* and std::string and std::wstring and BSTR and _bstr_t and CString and CComBSTR and QString and GString and...
  - phkahler 9y ago
    OOP often involves overhead. If someone defined a really clean vector type/object in a library that let me write expressions the natural way, that could be added to the language I suppose. And that's what I want.
    - dbaupp 9y ago
      Rust, C and C++ all possess ways (it's the even default/only way in C and Rust, and almost so in C++) to write types like this that don't involve the typical pointer-soup/dynamic-call overhead of typical OOP. People can and do write vector in libraries right now, usually using existing compiler support that gives them guarantees about SIMD.
      - StillBored 9y ago
        What I find odd about your reply, is that you downgrade C++, yet its really the only language that has sufficient operator overloading to do what the poster was requesting (transparent support without OO overhead). AKA, its completely possible, and there are quite a number of C++ libraries that make vectors look like native types complete with long lists of global operator overloads for interaction with other base types. Generally these libraries are just thin wrappers around inline assembly or intel intrinsics for SSE/AVX and generally don't bring any form of OO syntax into the picture. That said, even with C++ the libraries tend to fall down a bit when it comes to individual element operation/masking because the closest method is generally using array syntax for the elements which limits the operations to a single element in the vector at a time. Which means you end up with OO method calling syntax for element masking (or creative solutions with operator() )
        dbaupp 9y ago
        I didn't denigrate C++, I'm specifically talking about the behaviour of the data types/mention of OOP which is orthogonal to the surface syntax like using symbolic operators instead of textual names. It is factual that the default data declaration in C++ has a little of the "pointer soup" due to methods and `this`. This can often get inlined away and so is usually irrelevant (hence "almost so"), but the poster did emphasise their desire for pass-by-value. This can be avoided by e.g. using friends more than methods, but this isn't the default way a lot of people write C++. (Also, Rust has pretty transparent support in the same manner also without OO overhead, but differs slightly because methods can and often do use pass-by-value. This is the distinction I was drawing.)
- nwmcsween 9y ago
  It doesn't really make sense in C as any modern optimizing compiler will turn it into SIMD. IIRC rust needs explicit SIMD due to bounds checking.
  - steveklabnik 9y ago
    > IIRC rust needs explicit SIMD due to bounds checking. Bounds checks can be eliminated and code can be vectorized if the optimizer can prove it; explicit SIMD is useful for the cases where it can't.
  - blt 9y ago
    Optimizing compilers are rarely successful at turning non trivial code into simd.
  - dbaupp 9y ago
    Bounds checking gets in the way for simple cases but anything even slightly more complicated needs to be written extremely carefully in any language, for autovectorisation to work. It is like, essentially, writing explicit SIMD without using intrinsics and without guarantees it will work as desired. And, that is assuming that the autovectoriser is able to synthesise the desired instructions, e.g. I believe SSE2's packssdw & packusdw ("pack with signed/unsigned saturation") and pmaddwd ("multiply and add packed integers") are useful in a JPEG codec but I find it extremely unlikely that any compiler will autovectorise to them.
  - phkahler 9y ago
    I want it to turn it into SIMD instructions. What I don't want is to write classes, functions, or loops that have to be automatically converted to SIMD. I want a simple built-in type for these three size vectors. I also mentioned that they should be passed by value and be able to be returned by value in a (SIMD) register. This is the most efficient way to write and execute vector math.
    - kbenson 9y ago
      Can't a library just add that? Make some types, implement some functions and/or overload some ops. If you're defining the special type anyway, I'm not sure why it has to be built in.
- deleted 9y ago
  [deleted]
- cbhl 9y ago
  Have you tried Halide? http://halide-lang.org/ http://halide-lang.org/ I've seen it used for various video/photo processing operations.
  - test9753 9y ago
    Off-topic: the "Halide Talk" video in that page was very good: https://www.youtube.com/watch?v=3uiEyEKji0M https://www.youtube.com/watch?v=3uiEyEKji0M
- pertymcpert 9y ago
  What if your target architecture has a different vector width and hence you're using 4 element vectors but wasting 12? If you want to use vectors explicitly, you can do so using intrinsics for most vector architectures.
  - phkahler 9y ago
    The sizes I mentioned are extremely common in 2d and 3d geometry. This is due to the number of dimensions visible in our world. While someone may want to run 11 dimensional calculations in string theory, there are a large number of common real-world applications of the lengths 2,3,4. In C and C++ you can often use intrinsics but the big 3 - ARM, Intel, PPC - all define them differently. I want this common stuff to be part of the language. Sure go ahead and support general vectors via class definitions and such, but give me direct support for the common sizes.
    - pertymcpert 9y ago
      Your issue with intrinsics are that the different ISAs have different specs. Fine. But if that was your only issue, then your use case is that you're manually vectoring hot loops correct? Assuming that's true, you want to maximise performance by using as much of the parallelism that vectors give you. So if you're dealing with [4 x int32], on a 128 bit vector ISA you would be fully utilising your vector registers, but moving to say AVX-512 you're now only using 1/4 of your potential parallelism. Your architecture independent vector types would have to target the lowest common denominator, and completely defeat the purpose of vectorisation.
      - phkahler 9y ago
        I want to do math with 2,3, or 4-element vectors. These will typically represent 2d or 3d coordinates or velocities. 4-element vectors may be used for homogeneous coordinates or similar. My point is that these are very common mathematical entities and should be explicitly supported by the language. How these map to any particular processors resources is not my problem - though the three major vector extensions today all have 4-element vectors. Some support more, but that's not terribly relevant to the math I want to do. A smart compiler could pack multiple small vectors into a wide vector register just like they try to pack multiple scalars in there today. I am not interested in vectorizing loops. I want to write code like this: Vec3double position = {5.8,3.9,2.1}; Vec3double velocity = {1.0,0.0,0.0}; double timestep = 0.01; position += timestep*velocity; and so on. Yes, I also want the common use case of multiplication of vector and a scalar to be that easy. Any paint program or graphics library (including font rendering) does a ton of this stuff. So does every 2d or 3d physics engine. Ray tracing. FEA software. CAD. The list of uses for these vector sizes is long and has nothing to do with auto-vectorizing loops. Of course there are plenty of applications where loop vectorization is valuable and I don't want to take anything away from that. I just want built-in support for these common mathematical entities in the base language.
- ryl00 9y ago
  Fortran?
- FrozenVoid 9y ago
  https://gcc.gnu.org/onlinedocs/gcc/Vector-Extensions.html https://gcc.gnu.org/onlinedocs/gcc/Vector-Extensions.html
  - phkahler 9y ago
    I'm familiar with that. The problem is that it's not standard. Having the actual types v4float or v4double as part of the language would also make it easier to mix and match code from different places/libraries.
- pjmlp 9y ago
  They are coming to ANSI C++. http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0193r1.html http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p019...
- Sean1708 9y ago
  Do you mean something like this[0] (probably the worst possible implementation but I was in a rush)? I suppose you still have to create a "class" but you could set it up to use syntax like `V4(1, 2, 3, 4)` if you prefer. [0]: https://play.rust-lang.org/?gist=989fe1c05e3e68df896420327431b52f&version=stable https://play.rust-lang.org/?gist=989fe1c05e3e68df89642032743...

by mrec 9y ago

phkahler 9y ago

I'd just like to see 2,3, and 4 element vectors as a first class citizen in C, C++, or Rust. These are incredibly common for so many things it's hard for me to understand this omission. I want to be able to pass them by value and as return values from function. I want to do operations like a=b+c with vectors without creating classes or overloading operators. For a lot of people the SIMD instructions are about parallel computation, but for me they represent vector primitives.

Zelizz 9y ago
Look into the Clang vector extensions. They're very similar to GLSL/OpenCL vectors. They implement some very basic operator overloading too, so they're much less annoying that calling a function for everything. I made a simple linear algebra library using them, if you want to see an example: https://github.com/GavinHigham/glla https://github.com/GavinHigham/glla
mrec 9y ago
What would baking them into the language add that a library solution couldn't? I'm not keeping track of Rust very closely these days, but it supports arithmetic operator overloading and the Copy trait. What's missing? I agree that this needs to be standardized for vector-ey crates to be able to talk to each other seamlessly. Otherwise we'll end up with a rerun of the C++ strings fiasco, with char* and wchar_t* and std::string and std::wstring and BSTR and _bstr_t and CString and CComBSTR and QString and GString and...
- phkahler 9y ago
  OOP often involves overhead. If someone defined a really clean vector type/object in a library that let me write expressions the natural way, that could be added to the language I suppose. And that's what I want.
  - dbaupp 9y ago
    Rust, C and C++ all possess ways (it's the even default/only way in C and Rust, and almost so in C++) to write types like this that don't involve the typical pointer-soup/dynamic-call overhead of typical OOP. People can and do write vector in libraries right now, usually using existing compiler support that gives them guarantees about SIMD.
    - StillBored 9y ago
      What I find odd about your reply, is that you downgrade C++, yet its really the only language that has sufficient operator overloading to do what the poster was requesting (transparent support without OO overhead). AKA, its completely possible, and there are quite a number of C++ libraries that make vectors look like native types complete with long lists of global operator overloads for interaction with other base types. Generally these libraries are just thin wrappers around inline assembly or intel intrinsics for SSE/AVX and generally don't bring any form of OO syntax into the picture. That said, even with C++ the libraries tend to fall down a bit when it comes to individual element operation/masking because the closest method is generally using array syntax for the elements which limits the operations to a single element in the vector at a time. Which means you end up with OO method calling syntax for element masking (or creative solutions with operator() )
      - dbaupp 9y ago
        I didn't denigrate C++, I'm specifically talking about the behaviour of the data types/mention of OOP which is orthogonal to the surface syntax like using symbolic operators instead of textual names. It is factual that the default data declaration in C++ has a little of the "pointer soup" due to methods and `this`. This can often get inlined away and so is usually irrelevant (hence "almost so"), but the poster did emphasise their desire for pass-by-value. This can be avoided by e.g. using friends more than methods, but this isn't the default way a lot of people write C++. (Also, Rust has pretty transparent support in the same manner also without OO overhead, but differs slightly because methods can and often do use pass-by-value. This is the distinction I was drawing.)
nwmcsween 9y ago
It doesn't really make sense in C as any modern optimizing compiler will turn it into SIMD. IIRC rust needs explicit SIMD due to bounds checking.
- steveklabnik 9y ago
  > IIRC rust needs explicit SIMD due to bounds checking. Bounds checks can be eliminated and code can be vectorized if the optimizer can prove it; explicit SIMD is useful for the cases where it can't.
- blt 9y ago
  Optimizing compilers are rarely successful at turning non trivial code into simd.
- dbaupp 9y ago
  Bounds checking gets in the way for simple cases but anything even slightly more complicated needs to be written extremely carefully in any language, for autovectorisation to work. It is like, essentially, writing explicit SIMD without using intrinsics and without guarantees it will work as desired. And, that is assuming that the autovectoriser is able to synthesise the desired instructions, e.g. I believe SSE2's packssdw & packusdw ("pack with signed/unsigned saturation") and pmaddwd ("multiply and add packed integers") are useful in a JPEG codec but I find it extremely unlikely that any compiler will autovectorise to them.
- phkahler 9y ago
  I want it to turn it into SIMD instructions. What I don't want is to write classes, functions, or loops that have to be automatically converted to SIMD. I want a simple built-in type for these three size vectors. I also mentioned that they should be passed by value and be able to be returned by value in a (SIMD) register. This is the most efficient way to write and execute vector math.
  - kbenson 9y ago
    Can't a library just add that? Make some types, implement some functions and/or overload some ops. If you're defining the special type anyway, I'm not sure why it has to be built in.
deleted 9y ago
[deleted]
cbhl 9y ago
Have you tried Halide? http://halide-lang.org/ http://halide-lang.org/ I've seen it used for various video/photo processing operations.
- test9753 9y ago
  Off-topic: the "Halide Talk" video in that page was very good: https://www.youtube.com/watch?v=3uiEyEKji0M https://www.youtube.com/watch?v=3uiEyEKji0M
pertymcpert 9y ago
What if your target architecture has a different vector width and hence you're using 4 element vectors but wasting 12? If you want to use vectors explicitly, you can do so using intrinsics for most vector architectures.
- phkahler 9y ago
  The sizes I mentioned are extremely common in 2d and 3d geometry. This is due to the number of dimensions visible in our world. While someone may want to run 11 dimensional calculations in string theory, there are a large number of common real-world applications of the lengths 2,3,4. In C and C++ you can often use intrinsics but the big 3 - ARM, Intel, PPC - all define them differently. I want this common stuff to be part of the language. Sure go ahead and support general vectors via class definitions and such, but give me direct support for the common sizes.
  - pertymcpert 9y ago
    Your issue with intrinsics are that the different ISAs have different specs. Fine. But if that was your only issue, then your use case is that you're manually vectoring hot loops correct? Assuming that's true, you want to maximise performance by using as much of the parallelism that vectors give you. So if you're dealing with [4 x int32], on a 128 bit vector ISA you would be fully utilising your vector registers, but moving to say AVX-512 you're now only using 1/4 of your potential parallelism. Your architecture independent vector types would have to target the lowest common denominator, and completely defeat the purpose of vectorisation.
    - phkahler 9y ago
      I want to do math with 2,3, or 4-element vectors. These will typically represent 2d or 3d coordinates or velocities. 4-element vectors may be used for homogeneous coordinates or similar. My point is that these are very common mathematical entities and should be explicitly supported by the language. How these map to any particular processors resources is not my problem - though the three major vector extensions today all have 4-element vectors. Some support more, but that's not terribly relevant to the math I want to do. A smart compiler could pack multiple small vectors into a wide vector register just like they try to pack multiple scalars in there today. I am not interested in vectorizing loops. I want to write code like this: Vec3double position = {5.8,3.9,2.1}; Vec3double velocity = {1.0,0.0,0.0}; double timestep = 0.01; position += timestep*velocity; and so on. Yes, I also want the common use case of multiplication of vector and a scalar to be that easy. Any paint program or graphics library (including font rendering) does a ton of this stuff. So does every 2d or 3d physics engine. Ray tracing. FEA software. CAD. The list of uses for these vector sizes is long and has nothing to do with auto-vectorizing loops. Of course there are plenty of applications where loop vectorization is valuable and I don't want to take anything away from that. I just want built-in support for these common mathematical entities in the base language.
ryl00 9y ago
Fortran?
FrozenVoid 9y ago
https://gcc.gnu.org/onlinedocs/gcc/Vector-Extensions.html https://gcc.gnu.org/onlinedocs/gcc/Vector-Extensions.html
- phkahler 9y ago
  I'm familiar with that. The problem is that it's not standard. Having the actual types v4float or v4double as part of the language would also make it easier to mix and match code from different places/libraries.
pjmlp 9y ago
They are coming to ANSI C++. http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0193r1.html http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p019...
Sean1708 9y ago
Do you mean something like this[0] (probably the worst possible implementation but I was in a rush)? I suppose you still have to create a "class" but you could set it up to use syntax like `V4(1, 2, 3, 4)` if you prefer. [0]: https://play.rust-lang.org/?gist=989fe1c05e3e68df896420327431b52f&version=stable https://play.rust-lang.org/?gist=989fe1c05e3e68df89642032743...