Make python extension automatically enable AVX instructions if the host machine

supports them.

dlib/cmake_utils/check_if_avx_instructions_executable_on_host.cmake

@@ -0,0 +1,19 @@
+# This script checks if your compiler and host processor can generate and then run programs with AVX instructions.
+
+cmake_minimum_required(VERSION 2.8.12)
+
+# Don't rerun this script if its already been executed.
+if (DEFINED AVX_IS_AVAILABLE_ON_HOST)
+   return()
+endif()
+
+# Set to false unless we find out otherwise in the code below.
+set(AVX_IS_AVAILABLE_ON_HOST 0)
+
+try_compile(test_for_avx_worked ${PROJECT_BINARY_DIR}/avx_test_build ${CMAKE_CURRENT_LIST_DIR}/test_for_avx 
+	avx_test)
+
+if(test_for_avx_worked)
+	message (STATUS "AVX instructions can be executed by the host processor.")
+	set(AVX_IS_AVAILABLE_ON_HOST 1)
+endif()

dlib/cmake_utils/test_for_avx/CMakeLists.txt

@@ -0,0 +1,23 @@
+
+cmake_minimum_required(VERSION 2.8.12)
+project(avx_test)
+
+set(USE_AVX_INSTRUCTIONS ON CACHE BOOL "Use SSE4 instructions")
+
+# Pull this in since it sets the AVX compile options by putting that kind of stuff into the active_compile_opts list.
+include(../set_compiler_specific_options.cmake)
+
+
+try_run(run_result compile_result ${PROJECT_BINARY_DIR}/avx_test_try_run_build ${CMAKE_CURRENT_LIST_DIR}/avx_test.cpp
+   COMPILE_DEFINITIONS ${active_compile_opts})
+
+message(STATUS "run_result = ${run_result}")
+message(STATUS "compile_result = ${compile_result}")
+
+if (${run_result} EQUAL 0 AND compile_result)
+   message(STATUS "Ran AVX test program successfully, you have AVX available.")
+else()
+   message(STATUS "Unable to run AVX test program, you don't seem to have AVX instructions available.")
+   # make this build fail so that calling try_compile statements will error in this case.
+   add_library(make_this_build_fail ${CMAKE_CURRENT_LIST_DIR}/this_file_doesnt_compile.cpp)
+endif()

dlib/cmake_utils/test_for_avx/avx_test.cpp

@@ -0,0 +1,13 @@
+
+#include <immintrin.h> 
+
+int main()
+{
+    __m256 x;
+    x = _mm256_set1_ps(1.23);
+    x = _mm256_add_ps(x,x);
+    return 0;
+}
+
+// ------------------------------------------------------------------------------------
+

dlib/cmake_utils/test_for_avx/this_file_doesnt_compile.cpp

@@ -0,0 +1,3 @@
+
+#error "This file doesn't compile!"
+

tools/python/CMakeLists.txt

@@ -3,10 +3,15 @@ CMAKE_MINIMUM_REQUIRED(VERSION 2.8.12)
 
 set(USE_SSE4_INSTRUCTIONS ON CACHE BOOL "Use SSE4 instructions")
 
+include(../../dlib/cmake_utils/check_if_avx_instructions_executable_on_host.cmake)
+if (AVX_IS_AVAILABLE_ON_HOST)
+   set(USE_AVX_INSTRUCTIONS ON CACHE BOOL "Use AVX instructions")
+endif()
+
 # Set this to disable link time optimization. The only reason for
 # doing this to make the compile faster which is nice when developing
 # new modules.
- set(PYBIND11_LTO_CXX_FLAGS "")
+# set(PYBIND11_LTO_CXX_FLAGS "")
 
 
 # Avoid cmake warnings about changes in behavior of some Mac OS X path