Add option to compile Eigen with alignment enabled

Eigen is most optimal when using SIMD instructions which require
overaligned buffers. In C++14 this is a problem because there's no
built-in support for correctly aligned allocations, so this Eigen
feature is currently disabled in AliceVision. C++17 solves Eigen
alignment issues completely because there now exist additional overloads
of operator new() which support correct allocation alignment. We can't
upgrade to C++17 yet, but certain compilers added support for this
feature earlier, such as -faligned-new in GCC 7.1. Therefore it makes
sense to add a configuration option to allow enabling alignment support
in Eigen in cases when the compiler is good enough.

CMakeLists.txt

@@ -19,6 +19,7 @@ option(AV_BUILD_LAPACK "Enable building an embedded Lapack" ON)
 option(AV_BUILD_SUITESPARSE "Enable building an embedded SuiteSparse" ON)
 option(AV_BUILD_FFMPEG "Enable building an embedded FFMpeg" ON)
 option(AV_BUILD_ALICEVISION "Enable building of AliceVision" ON)
+option(AV_EIGEN_NO_ALIGN "Disable Eigen alignment" ON)
 
 option(AV_USE_CUDA "Enable CUDA" ON)
 

src/CMakeLists.txt

@@ -199,6 +199,15 @@ if(CMAKE_COMPILER_IS_GNUCXX)
   AddCompilerFlag("-Werror=return-type")
   AddCompilerFlag("-Werror=switch")
   AddCompilerFlag("-Werror=return-local-addr")
+# Eigen requires overaligned buffers for maximum efficiency (e.g. on AVX512 buffers may need to
+# be aligned to 64 bytes). AliceVision currently does not support this. Fortunately this is fixed
+# in C++17. While we can't upgrade to C++17 just yet, some compilers support overaligned
+# allocation feature with a separate flag, so use it if alignment is enabled in Eigen.
+# See https://eigen.tuxfamily.org/dox/group__TopicUnalignedArrayAssert.html
+if(NOT AV_EIGEN_NO_ALIGN)
+    if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 7.1)
+        AddCompilerFlag("-faligned-new")
+    endif()
 endif()
 
 # ==============================================================================
@@ -346,8 +355,11 @@ endif()
 find_package(Eigen3 3.3 REQUIRED)
 if(Eigen3_FOUND OR EIGEN3_FOUND)
   # message(STATUS "EIGEN_INCLUDE_DIR: ${EIGEN_INCLUDE_DIR}")
-  # See https://eigen.tuxfamily.org/dox/group__TopicUnalignedArrayAssert.html
-  set(AV_EIGEN_DEFINITIONS -DEIGEN_MAX_ALIGN_BYTES=0 -DEIGEN_MAX_STATIC_ALIGN_BYTES=0)
+  if(AV_EIGEN_NO_ALIGN)
+    set(AV_EIGEN_DEFINITIONS -DEIGEN_MAX_ALIGN_BYTES=0 -DEIGEN_MAX_STATIC_ALIGN_BYTES=0)
+  else()
+    set(AV_EIGEN_DEFINITIONS -DALICEVISION_EIGEN_REQUIRE_ALIGNMENT=1)
+  endif()
 else()
   message(FATAL_ERROR " EIGEN NOT FOUND. EIGEN_INCLUDE_DIR: ${EIGEN_INCLUDE_DIR}")
 endif()

src/aliceVision/numeric/numeric.hpp

@@ -8,18 +8,20 @@
 
 #pragma once
 
-// AliceVision does not support Eigen with alignment,
+// AliceVision does not support Eigen with alignment, unless C++17 aligned new feature is enabled.
 // So ensure Eigen is used with the correct flags.
-#ifndef EIGEN_MAX_ALIGN_BYTES
-#error "EIGEN_MAX_ALIGN_BYTES is not defined"
-#elif EIGEN_MAX_ALIGN_BYTES != 0
-#error "EIGEN_MAX_ALIGN_BYTES is defined but not 0"
-#endif
-
-#ifndef EIGEN_MAX_STATIC_ALIGN_BYTES
-#error "EIGEN_MAX_STATIC_ALIGN_BYTES is not defined"
-#elif EIGEN_MAX_STATIC_ALIGN_BYTES != 0
-#error "EIGEN_MAX_STATIC_ALIGN_BYTES is defined but not 0"
+#ifndef ALICEVISION_EIGEN_REQUIRE_ALIGNMENT
+    #ifndef EIGEN_MAX_ALIGN_BYTES
+        #error "EIGEN_MAX_ALIGN_BYTES is not defined"
+    #elif EIGEN_MAX_ALIGN_BYTES != 0
+        #error "EIGEN_MAX_ALIGN_BYTES is defined but not 0"
+    #endif
+
+    #ifndef EIGEN_MAX_STATIC_ALIGN_BYTES
+        #error "EIGEN_MAX_STATIC_ALIGN_BYTES is not defined"
+    #elif EIGEN_MAX_STATIC_ALIGN_BYTES != 0
+        #error "EIGEN_MAX_STATIC_ALIGN_BYTES is defined but not 0"
+    #endif
 #endif