Adding macro XTENSOR_SELECT_ALIGN (#2152)

Adding macro XTENSOR_SELECT_ALIGN

Author: tdegeus

docs/source/index.rst

@@ -70,6 +70,7 @@ for details.
    histogram
    random
    sfinae
+   xsimd
    file_loading
    build-options
    pitfall

docs/source/sfinae.rst

@@ -4,7 +4,7 @@
 
    The full license is in the file LICENSE, distributed with this software.
 
-.. _histogram:
+.. _sfinae:
 
 SFINAE
 ======

docs/source/xsimd.rst

@@ -0,0 +1,52 @@
+.. Copyright (c) 2016, Johan Mabille, Sylvain Corlay and Wolf Vollprecht
+
+   Distributed under the terms of the BSD 3-Clause License.
+
+   The full license is in the file LICENSE, distributed with this software.
+
+.. _xsimd:
+
+xsimd
+=====
+
+Alignment of fixed-size members
+-------------------------------
+
+.. note::
+
+    If you are using ``C++ >= 17`` you should not have to worry about this.
+
+If you define a structure having members of fixed-size xtensor types,
+you must ensure that the buffers properly aligned.
+For this you can use the macro ``XTENSOR_SELECT_ALIGN`` available in
+``xtensor/xtensor_config.hpp``.
+Consider the following example:
+
+.. code-block:: cpp
+
+    template <typename T>
+    class alignas(XTENSOR_SELECT_ALIGN(T)) Foo
+    {
+    public:
+
+        using allocator_type = std::conditional_t<XTENSOR_SELECT_ALIGN(T) != 0,
+                                                  xt_simd::aligned_allocator<T, XTENSOR_SELECT_ALIGN(T)>,
+                                                  std::allocator<T>>;
+
+        Foo(T fac) : m_fac(fac)
+        {
+            m_bar.fill(fac);
+        }
+
+        auto get() const
+        {
+            return m_bar;
+        }
+
+    private:
+
+        xt::xtensor_fixed<T, xt::xshape<10, 10>> m_bar;
+        T m_fac;
+    };
+
+Whereby it is important to store the fixed-sized xtensor type (in this case ``xt::xtensor_fixed<T, xt::xshape<10, 10>>``) as first member.

include/xtensor/xstorage.hpp

@@ -1368,8 +1368,6 @@ namespace xt
         lhs.swap(rhs);
     }
 
-    #define XTENSOR_SELECT_ALIGN (XTENSOR_DEFAULT_ALIGNMENT != 0 ? XTENSOR_DEFAULT_ALIGNMENT : alignof(T))
-
     template <class X, class T, std::size_t N, class A, bool B>
     struct rebind_container<X, svector<T, N, A, B>>
     {
@@ -1383,7 +1381,7 @@ namespace xt
      *
      * To be moved to xtl, along with the rest of xstorage.hpp
      */
-    template <class T, std::size_t N, std::size_t Align = XTENSOR_SELECT_ALIGN>
+    template <class T, std::size_t N, std::size_t Align = XTENSOR_SELECT_ALIGN(T)>
     class alignas(Align) aligned_array : public std::array<T, N>
     {
     public:
@@ -1936,6 +1934,5 @@ namespace std
 #endif
 
 #undef XTENSOR_CONST
-#undef XTENSOR_SELECT_ALIGN
 
 #endif

include/xtensor/xtensor_config.hpp

@@ -110,6 +110,10 @@
 #define XTENSOR_OPENMP_TRESHOLD 0
 #endif
 
+#ifndef XTENSOR_SELECT_ALIGN
+#define XTENSOR_SELECT_ALIGN(T) (XTENSOR_DEFAULT_ALIGNMENT != 0 ? XTENSOR_DEFAULT_ALIGNMENT : alignof(T))
+#endif
+
 #ifdef IN_DOXYGEN
 namespace xtl
 {

test/CMakeLists.txt

@@ -219,6 +219,7 @@ set(XTENSOR_TESTS
     test_xrandom.cpp
     test_xrepeat.cpp
     test_xsort.cpp
+    test_xsimd.cpp
     test_xvectorize.cpp
     test_extended_xmath_interp.cpp
     test_extended_broadcast_view.cpp

test/test_xsimd.cpp

@@ -0,0 +1,51 @@
+/***************************************************************************
+* Copyright (c) Johan Mabille, Sylvain Corlay and Wolf Vollprecht          *
+* Copyright (c) QuantStack                                                 *
+*                                                                          *
+* Distributed under the terms of the BSD 3-Clause License.                 *
+*                                                                          *
+* The full license is in the file LICENSE, distributed with this software. *
+****************************************************************************/
+
+#include <complex>
+#include <limits>
+
+#include "gtest/gtest.h"
+#include "xtensor/xfixed.hpp"
+#include "xtensor/xtensor_config.hpp"
+
+template <typename T>
+class alignas(XTENSOR_SELECT_ALIGN(T)) Foo
+{
+public:
+
+    using allocator_type = std::conditional_t<XTENSOR_SELECT_ALIGN(T) != 0,
+                                              xt_simd::aligned_allocator<T, XTENSOR_SELECT_ALIGN(T)>,
+                                              std::allocator<T>>;
+
+    Foo(T fac) : m_fac(fac)
+    {
+        m_bar.fill(fac);
+    }
+
+    auto get() const
+    {
+        return m_bar;
+    }
+
+private:
+
+    xt::xtensor_fixed<T, xt::xshape<10, 10>> m_bar;
+    T m_fac;
+};
+
+namespace xt
+{
+
+    TEST(xsimd, alignas)
+    {
+        int fac = 10;
+        Foo<int> foo(10);
+        EXPECT_TRUE(xt::sum(foo.get())() == fac * 10 * 10);
+    }
+}