1. Auto-Vectorization Jim Hogg Program Manager Visual C++ Compiler Microsoft Corporation

2. Auto-Vectorization : What is it? A feature in the Visual Studio 11 C++ native compiler Tries to make your C++ loops run faster, by using vector registers and instructions Speedup varies with application. In tight loops, can reach 4X on ints or floats You don’t need to request this option - it’s on-by-default Just recompile your code and measure the speedup

3. All modern PC chips include vector registers and instructions Vector registers are called XMM0 – XMM15 Each vector register holds 4 ints or 4 floats Vector instructions are called SSE Instructions perform 4 operations in parallel Auto-Vectorization : how does it work?

4. ADD RAX, RBX 1.10 1.20 RAX RBX 2.30 ADDPS XMM1, XMM2 XMM1 XMM2 1.102.103.104.10 RAX 1.202.203.204.20 2.304.306.308.30 XMM1 SCALAR VECTOR for (int i = 0; i < 1000; ++i) a[i] += b[i] for (int i = 0; i < 1000; i += 4) a[i : i+3] += b[i : i+3] Auto-Vectorization : Simple Loop

5. Auto-Vectorization : Example const int dim = 1000000; float a[dim], b[dim]; int main() { for (int n = 0; n < dim; ++n) a[n] += sin(b[n]) * cos(b[n]); } Vectorization Disabled Auto-VectorizedSpeedup 108.7 millisecs37.3 millisecs2.9X

6. Auto-Vectorization : Summary Prior to Visual Studio 11, the C++ compiler emitted code that performed only scalar operations In Visual Studio 11, the C++ compiler tries to use vector registers and instructions to speed up your C++ loops  Emits code to uses SSE vector registers and instructions automatically  No need to change your C++ source code  The feature is on-by-default  Adjusts to your computer at runtime (SSE2 or SSE4.1)  Transformations are always safe! To try it out, just recompile your code, and run!

7. Auto-Vectorization : Links Channel 9 Video http://channel9.msdn.com/Shows/C9-GoingNative/GoingNative-7- VC11-Auto-Vectorizer-C-NOW-LangNEXT Blog Posts http://blogs.msdn.com/b/nativeconcurrency/archive/2012/04/12/au to-vectorizer-in-visual-studio-11-overview.aspx

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