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<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<link rel="stylesheet" href="../common-revealjs/css/reveal.css">
<link rel="stylesheet" href="../common-revealjs/css/theme/white.css">
<link rel="stylesheet" href="../common-revealjs/css/custom.css">
<script>
// This is needed when printing the slides to pdf
var link = document.createElement( 'link' );
link.rel = 'stylesheet';
link.type = 'text/css';
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<script>
// This is used to display the static images on each slide,
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</head>
<body>
<div class="reveal">
<div class="slides">
<div id="global-images" class="global-images">
<img src="../common-revealjs/images/sycl_academy.png" />
<img src="../common-revealjs/images/sycl_logo.png" />
<img src="../common-revealjs/images/trademarks.png" />
<img src="../common-revealjs/images/codeplay.png" />
</div>
<!--Slide 1-->
<section class="hbox">
<div class="hbox" data-markdown>
## Vectorization
</div>
</section>
<!--Slide 2-->
<section class="hbox" data-markdown>
## Learning Objectives
* Learn about scalar and vector instructions
* Learn about horizontal and vertical vectorization
* Learn how to write explicit vector code
* learn how to use swizzles
</section>
<!--Slide 3-->
<section>
<div class="hbox" data-markdown>
#### Vector instructions
</div>
<div class="container" data-markdown>
</div>
<div class="container" data-markdown>
* Data parallel devices such as GPUs, SIMD CPUs and other accelerators are vector processors.
* This means they can execute vector instructions.
* Vector instructions are single instructions which perform loads,
stores, or operations such as add or multiply on multiple elements
at once.
</div>
</section>
<!--Slide 4-->
<section>
<div class="hbox" data-markdown>
#### Vectorization
</div>
<div class="container" data-markdown>
* Vectorization is the process of converting scalar code into vectorized code.
* In a SPMD programming model like SYCL vectorization is important.
* Vectorization can be performed in two ways, and it depends on how you write your code and can impact the mapping to hardware.
</div>
</section>
<!--Slide 5-->
<section>
<div class="hbox" data-markdown>
#### Horizontal vectorization
</div>
<div class="container">
<div class="col" data-markdown>
</div>
<div class="col" data-markdown>
* Horizontal (or auto-) vectorization is done automatically by the compiler.
* It maps the scalar operation of each work-item to a single processing element, or element of a vector instruction.
</div>
</div>
</section>
<!--Slide 6-->
<section>
<div class="hbox" data-markdown>
#### Vertical vectorization
</div>
<div class="container">
<div class="col" data-markdown>
</div>
<div class="col" data-markdown>
* Vertical (or explicit) vectorization is done by using vector types explicitly.
* It maps the vector instruction of each work-item to multiple processing elements, or elements of vector instructions.
</div>
</div>
</section>
<!--Slide 7-->
<section>
<div class="hbox" data-markdown>
#### Horizontal vs vertical vectorization
</div>
<div class="container" data-markdown>
* Both horizontal and vertical vectorization generally achieve the same result.
* It can be useful to specify vectorization explicitly, particularly for describing aligned loads and stores.
* An important distinction to make is that whether a kernel function uses explicit vector types can impact the mapping of work-items to processing elements.
* It's not always a 1:1 mapping.
</div>
</section>
<!--Slide 8-->
<section>
<div class="hbox" data-markdown>
#### Vec class
</div>
<div class="container">
<code class="code-100pc"><pre>
template <typename dataT, int numElements>
class vec;
</code></pre>
</div>
<div class="container" data-markdown>
* The `vec` class template is used to represent explicit vectors in SYCL.
* It has a type which represents the type of elements it stores and a number of elements.
* The valid number of elements are 1, 2, 3, 4, 8, 16.
* Note that vectors of 3 elements are padded to the size of 4.
</div>
</section>
<!--Slide 9-->
<section>
<div class="hbox" data-markdown>
#### Aliases
</div>
<div class="container">
<code class="code-100pc"><pre>
using float4 = vec<float, 4>;
...
</code></pre>
</div>
<div class="container" data-markdown>
* A number of aliases are provided for shorthand with the notation of the type followed by the size, such as `float4`.
</div>
</section>
<!--Slide 10-->
<section>
<div class="hbox" data-markdown>
#### Vec constructors
</div>
<div class="container">
<code class="code-100pc"><pre>
auto f4 = sycl::float4{1.0f, 2.0f, 3.0f, 4.0f}; // {1.0f, 2.0f, 3.0f, 4.0f}
</code></pre>
</div>
<div class="container">
<code class="code-100pc"><pre>
auto f2 = sycl::float4{2.0f, 3.0f}; // {2.0f, 3.0f}
auto f4 = sycl::float4{1.0f, f2, 4.0f}; // {1.0f, 2.0f, 3.0f, 4.0f}
</code></pre>
</div>
<div class="container">
<code class="code-100pc"><pre>
auto f4 = sycl::float4{0.0f}; // {0.0f, 0.0f, 0.0f, 0.0f}
</code></pre>
</div>
<div class="container" data-markdown>
* A `vec` can be constructed with any combination of scalar and vector values which add up to the correct number of elements.
* A `vec`can also be constructed from a single scalar in which case it will initialize ever element to that value.
</div>
</section>
<!--Slide 11-->
<section>
<div class="hbox" data-markdown>
#### Vec operators
</div>
<div class="container">
<code class="code-100pc"><pre>
auto f4a = sycl::float4{1.0f, 2.0f, 3.0f, 4.0f}; // {1.0f, 2.0f, 3.0f, 4.0f}
auto f4b = sycl::float4{2.0f}; // {2.0f, 2.0f, 2.0f, 2.0f}
auto f4r = f4a * f4b; // {2.0f, 4.0f, 6.0f, 8.0f}
</code></pre>
</div>
<div class="container" data-markdown>
* The `vec` class provides a number of operators such as `+`, `-`, `*`, `/` and many more, which perform the operation elemeent-wise.
</div>
</section>
<!--Slide 12-->
<section>
<div class="hbox" data-markdown>
#### Swizzles
</div>
<div class="container">
<code class="code-100pc"><pre>
auto f4 = sycl::float4{1.0f, 2.0f, 3.0f, 4.0f}; // {1.0f, 2.0f, 3.0f, 4.0f}
auto f2 = f4.swizzle<0, 3>(); // {1.0f, 4.0f}
</code></pre>
</div>
<div class="container">
<code class="code-100pc"><pre>
auto f4 = sycl::float4{1.0f, 2.0f, 3.0f, 4.0f}; // {1.0f, 2.0f, 3.0f, 4.0f}
f4.swizzle<1, 2>() = sycl::float2{9.0f, 9.0f}; // f4 becomes {1.0f, 9.0f, 9.0f, 4.0f}
</code></pre>
</div>
<div class="container" data-markdown>
* The `swizzle` function returns a representation of the specified elements of a `vec` which can be used on the lhs or rhs of an expression.
</div>
</section>
<!--Slide 13-->
<section>
<div class="hbox" data-markdown>
#### Simple swizzles
</div>
<div class="container">
<code class="code-100pc"><pre>
auto f4 = sycl::float4{1.0f, 2.0f, 3.0f, 4.0f}; // {1.0f, 2.0f, 3.0f, 4.0f}
auto f2 = f4.xw(); // {1.0f, 4.0f}
</code></pre>
</div>
<div class="container">
<code class="code-100pc"><pre>
auto f4 = sycl::float4{1.0f, 2.0f, 3.0f, 4.0f}; // {1.0f, 2.0f, 3.0f, 4.0f}
f4.yz() = sycl::float2{9.0f, 9.0f}; // f4 becomes {1.0f, 9.0f, 9.0f, 4.0f}
</code></pre>
</div>
<div class="container" data-markdown>
* If `SYCL_SIMPLE_SWIZZLES` is defined before including `sycl/sycl.hpp` simplified swizzle member functions can also be used in place of `swizzle`.
</div>
</section>
<!--Slide 14-->
<section>
<div class="hbox" data-markdown>
#### Vectorized image convolution performance
</div>
<div class="container"data-markdown>
</div>
</section>
<!--Slide 15-->
<section>
<div class="hbox" data-markdown>
## Questions
</div>
</section>
<!--Slide 16-->
<section>
<div class="hbox" data-markdown>
#### Exercise
</div>
<div class="container" data-markdown>
Code_Exercises/Exercise_17_Vectors/source
</div>
<div class="container" data-markdown>
Update the image convolution application to use vectors
types.
</div>
</section>
</div>
</div>
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