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# How to enable Halide backend for improve efficiency {#tutorial_dnn_halide}
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## Introduction
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This tutorial guidelines how to run your models in OpenCV deep learning module
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using Halide language backend. Halide is an open-source project that let us
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write image processing algorithms in well-readable format, schedule computations
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according to specific device and evaluate it with a quite good efficiency.
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An official website of the Halide project: http://halide-lang.org/.
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An up to date efficiency comparison: https://github.com/opencv/opencv/wiki/DNN-Efficiency
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## Requirements
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### LLVM compiler
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@note LLVM compilation might take a long time.
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- Download LLVM source code from http://releases.llvm.org/4.0.0/llvm-4.0.0.src.tar.xz.
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Unpack it. Let **llvm_root** is a root directory of source code.
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- Create directory **llvm_root**/tools/clang
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- Download Clang with the same version as LLVM. In our case it will be from
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http://releases.llvm.org/4.0.0/cfe-4.0.0.src.tar.xz. Unpack it into
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**llvm_root**/tools/clang. Note that it should be a root for Clang source code.
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- Build LLVM on Linux
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@code
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cd llvm_root
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mkdir build && cd build
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cmake -DLLVM_ENABLE_TERMINFO=OFF -DLLVM_TARGETS_TO_BUILD="X86" -DLLVM_ENABLE_ASSERTIONS=ON -DCMAKE_BUILD_TYPE=Release ..
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make -j4
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@endcode
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- Build LLVM on Windows (Developer Command Prompt)
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@code
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mkdir \\path-to-llvm-build\\ && cd \\path-to-llvm-build\\
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cmake.exe -DLLVM_ENABLE_TERMINFO=OFF -DLLVM_TARGETS_TO_BUILD=X86 -DLLVM_ENABLE_ASSERTIONS=ON -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=\\path-to-llvm-install\\ -G "Visual Studio 14 Win64" \\path-to-llvm-src\\
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MSBuild.exe /m:4 /t:Build /p:Configuration=Release .\\INSTALL.vcxproj
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@endcode
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@note `\\path-to-llvm-build\\` and `\\path-to-llvm-install\\` are different directories.
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### Halide language.
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- Download source code from GitHub repository, https://github.com/halide/Halide
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or using git. The root directory will be a **halide_root**.
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@code
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git clone https://github.com/halide/Halide.git
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@endcode
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- Build Halide on Linux
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@code
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cd halide_root
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mkdir build && cd build
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cmake -DLLVM_DIR=llvm_root/build/lib/cmake/llvm -DCMAKE_BUILD_TYPE=Release -DLLVM_VERSION=40 -DWITH_TESTS=OFF -DWITH_APPS=OFF -DWITH_TUTORIALS=OFF ..
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make -j4
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@endcode
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- Build Halide on Windows (Developer Command Prompt)
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@code
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cd halide_root
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mkdir build && cd build
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cmake.exe -DLLVM_DIR=\\path-to-llvm-install\\lib\\cmake\\llvm -DLLVM_VERSION=40 -DWITH_TESTS=OFF -DWITH_APPS=OFF -DWITH_TUTORIALS=OFF -DCMAKE_BUILD_TYPE=Release -G "Visual Studio 14 Win64" ..
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MSBuild.exe /m:4 /t:Build /p:Configuration=Release .\\ALL_BUILD.vcxproj
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@endcode
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## Build OpenCV with Halide backend
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When you build OpenCV add the following configuration flags:
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- `ENABLE_CXX11` - enable C++11 standard
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- `WITH_HALIDE` - enable Halide linkage
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- `HALIDE_ROOT_DIR` - path to Halide build directory
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## Set Halide as a preferable backend
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@code
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net.setPreferableBackend(DNN_BACKEND_HALIDE);
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@endcode
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