Auto-VectorizationJim HoggProgram ManagerVisual C++ CompilerMicrosoft Corporation

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

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?

ADD RAX, RBX

1.10

1.20

RAX

RBX

2.30

ADDPS XMM1, XMM2

XMM1

XMM2

1.10 2.10 3.10 4.10

RAX

1.20 2.20 3.20 4.20

2.30 4.30 6.30 8.30XMM1

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

Auto-Vectorization : Exampleconst 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-Vectorized

Speedup

108.7 millisecs 37.3 millisecs 2.9X

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!

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/auto-vectorizer-in-visual-studio-11-overview.aspx

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