INSTALL.md: add section discussing how to enable CPU specific optimizations

diff --git a/INSTALL.md b/INSTALL.md
index b3b293dc06115fe4a2502d6b7055e1a52a6d78db..aa0e88dfe7f04881f938b094b895713c6e34f5bf 100644 (file)
--- a/INSTALL.md
+++ b/INSTALL.md
@@ -9,7 +9,7 @@ To build the library, type from source tree directory:
 ```
 mkdir build
 cd build
-cmake ..
+cmake .. -DCMAKE_BUILD_TYPE=Release
 make
 ```
 Binaries are then located in the 'bin' directory.
@@ -63,6 +63,28 @@ Note 4 : On MacOS, if it does not work, try adding the following flag to the cma
 You can use cmake to generate the project files for the IDE you are using (VC2010, XCode, etc).
 Type 'cmake --help' for available generators on your platform.
 
+# Enabling CPU specific optimizations
+
+For Intel/AMD processors, OpenJPEG implements optimizations using the SSE4.1
+instruction set (for example, for the 9x7 inverse MCT transform) and the AVX2
+instruction set (for example, for the 5x3 inverse discrete wavelet transform).
+Currently, those optimizations are only available if OpenJPEG is built to
+use those instruction sets (and the resulting binary will only run on compatible
+CPUs)
+
+With gcc/clang, it is possible to enable those instruction sets with the following :
+
+cmake -DCMAKE_C_FLAGS="-O3 -msse4.1 -DNDEBUG" ..
+
+cmake -DCMAKE_C_FLAGS="-O3 -mavx2 -DNDEBUG" ..
+
+(AVX2 implies SSE4.1)
+
+Or if the binary is dedicated to run on the machine where it has
+been compiled :
+
+cmake -DCMAKE_C_FLAGS="-O3 -march=native -DNDEBUG" ..
+
 # Modifying OpenJPEG
 
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