Very simple SIMD programming

Paulo Pinto pjmlp at progtools.org
Wed Oct 24 05:39:22 PDT 2012


On Wednesday, 24 October 2012 at 02:41:53 UTC, bearophile wrote:
> I have found a nice paper, "Extending a C-like Language for 
> Portable SIMD Programming", (2012), by Roland L., Sebastian 
> Hack and Ingo Wald:
>
> http://www.cdl.uni-saarland.de/projects/vecimp/vecimp_tr.pdf
>
> SIMD programming is necessary in a system language, or in any 
> language that wants to use the modern CPUs well. So languages 
> like C, C++, D (and Mono-C#) support such wider registers.
>
> The authors of this paper have understood that it's also 
> important to make SIMD programming easy, almost as easy as 
> scalar code, so most programmers are able to write such kind of 
> correct code.
>
> So this this paper presents ideas to better express SIMD 
> semantics in a C-like language. They introduce few new 
> constructs in a large subset of C language, with few ideas. The 
> result coding patterns seem easy enough (they are surely look 
> simpler than most multi-core coding patterns I've seen, 
> including Cilk+).
>
>
> They present a simple scalar program in C:
>
> struct data_t {
>     int key;
>     int other;
> };
>
> int search(data_t* data , int N) {
>     for (int i = 0; i < N; i++) {
>         int x = data[i].key;
>         if (4 < x & x <= 8) return x;
>     }
>     return -1;
> }
>
>
> Then they explain the three most common ways to represent an 
> array of structs, here a struct that contains 3 values:
>
> x0 y0 z0 x1 y1 z1 x2 y2 z2 x3 y3 z3 x4 y4 z4 x5 y5 z5 x6 y6 z6 
> x7 y7 z7
> (a) Array of Structures (AoS)
>
> x0 x1 x2 x3 x4 x5 x6 x7   y0 y1 y2 y3 y4 y5 y6 y7   z0 z1 z2 z3 
> z4 z5 z6 z7
> (b) Structure of Arrays (SoA)
>
> x0 x1 x2 x3 y0 y1 y2 y3 z0 z1 z2 z3 x4 x5 x6 x7 y4 y5 y6 y7 z4 
> z5 z6 z7
> (c) Hybrid Structure of Arrays (Hybrid SoA)
>
> They explain how the (c) is the preferred pattern in SIMD 
> programming.
>
>
> Using the (c) data pattern they show how in C with (nice) SIMD 
> intrinsics you write vectorized code (a simd_data_t struct 
> instance contains 8 int values):
>
> struct simd_data_t {
>     simd_int key;
>     simd_int other;
> };
>
> int search(simd_data_t* data , int N) {
>     for (int i = 0; i < N/L; ++i) {
>         simd_int x = load(data[i].key);
>         simd_int cmp = simd_and(simd_lt(4, x),
>         simd_le(x, 8));
>         int mask = simd_to_mask(cmp);
>         if (mask != 0) {
>             simd_int result = simd_and(mask , x);
>             for (int j = 0; j < log2(L); j++)
>                 result = simd_or(result ,
>                 whole_reg_shr(result , 1 << j));
>                 return simd_extract(result , 0);
>             }
>         }
>     return -1;
> }
>
>
> D should do become able to do this (that is not too much bad), 
> or better.
>
>
> Their C language extensions allow to write nicer code like:
>
> struct data_t {
>     int key;
>     int other;
> };
>
> int search(data_t *scalar data , int scalar N) {
>     int L = lengthof(*data);
>     for (int i = 0; i < N/L; ++i) {
>         int x = data[i].key;
>         if (4 < x & x <= 8)
>             int block[L] result = [x, 0];
>         scalar {
>             for (int j = 0; j < log2(L); ++j)
>                 result |= whole_reg_shr(result , 1 << j);
>             return get(x, 0);
>         }
>     }
>     return -1;
> }
>
>
> This is based on just few simple ideas, explained in the paper 
> (they are interesting, but quoting here those parts of the 
> paper is not a good idea). Such ideas are not directly portable 
> to D (unless the front-end is changed. Their compiler works by 
> lowering, and emits regular C++ code with intrinsics).
>
>
> Near the end of the paper they also propose some C++ library 
> code:
>
>>the C++ template mechanism would allow to define a hybrid SoA 
>>container class: Similar to std::vector which abstracts a 
>>traditional C array, one could implement a wrapper around a T 
>>block[N]*:<
>
>
> // scalar context throughout this example
> struct vec3 { float x, y, z; };
> // vec3 block[N]* pointing to ceil(n/N) elements
> hsoa <vec3 > vecs(n);
> // preferred vector length of vec3 automatically derived
> static const int N = hsoa <vec3 >::vector_length;
> int i = /*...*/
> hsoa <vec3 >::block_index ii = /*...*/
> vec3 v = vecs[i]; // gather
> vecs[i] = v; // scatter
> vec3 block[N] w = vecs[ii]; // fetch whole block
> hsoa <vec3 >::ref r = vecs[i]; // get proxy to a scalar
> r = v; // pipe through proxy
> // for each element
> vecs.foreach([](vec3& scalar v) { /*...*/ });
>
>
> Regardless of the other ideas of their C-like language, a 
> similar struct should be added to Phobos once a bit higher 
> level SIMD support is in better shape in D. Supporting 
> Hybrid-SoA and few operations on it will be an important but 
> probably quite short and simple addition to Phobos collections 
> (it's essentially an struct that acts like an array, with few 
> simple extra operations).
>
> I think no commonly used language allows both very simple and 
> quite efficient SIMD programming (Scala, CUDA, C, C++, C#, 
> Java, Go, and currently Rust too, are not able to support SIMD 
> programming well. I think currently Haskell too is not 
> supporting it well, but Haskell is very flexible, and it's 
> compiled by a native compiler, so such things are maybe 
> possible to add). So supporting it well in D will be an 
> interesting selling point of D. (Supporting a very simple SIMD 
> coding in D will make D more widespread, but such kind of 
> programming will probably keep being a small niche).
>
> Bye,
> bearophile


Actually, I am yet to see any language that has SIMD as part of 
the language standard and not as an extension where each vendor 
does its own way.



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