Add support form emulated<N> arch

As a side effect:

- provide a scalar version for xsimd::bitwise_cast.
- reorder some functions in xsimd_sse2.hpp

Fix #998

.github/workflows/emulated.yml

@@ -0,0 +1,74 @@
+name: Linux emulated build
+on: [push, pull_request]
+concurrency:
+  group: ${{ github.workflow }}-${{ github.job }}-${{ github.ref }}
+  cancel-in-progress: true
+defaults:
+  run:
+    shell: bash -l {0}
+jobs:
+  build:
+    runs-on: ubuntu-20.04
+    name: '${{ matrix.sys.compiler }} ${{ matrix.sys.version }} - emulated'
+    strategy:
+      matrix:
+        sys:
+          - { compiler: 'gcc',   version: '7'}
+          - { compiler: 'clang', version: '8'}
+    steps:
+    - name: Setup compiler
+      if: ${{ matrix.sys.compiler == 'gcc' }}
+      run: |
+        GCC_VERSION=${{ matrix.sys.version }}
+        sudo apt-get update
+        sudo apt-get --no-install-suggests --no-install-recommends install g++-$GCC_VERSION
+        CC=gcc-$GCC_VERSION
+        echo "CC=$CC" >> $GITHUB_ENV
+        CXX=g++-$GCC_VERSION
+        echo "CXX=$CXX" >> $GITHUB_ENV
+        CXXFLAGS="-Wno-noexcept-type -Wno-stringop-overflow"
+        echo "CXXFLAGS=$CXXFLAGS" >> $GITHUB_ENV
+    - name: Setup compiler
+      if: ${{ matrix.sys.compiler == 'clang' }}
+      run: |
+        LLVM_VERSION=${{ matrix.sys.version }}
+        #sudo add-apt-repository ppa:ubuntu-toolchain-r/test || exit 1
+        wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add - || exit 1
+        sudo add-apt-repository "deb http://apt.llvm.org/focal/ llvm-toolchain-focal main" || exit 1
+        sudo apt-get update || exit 1
+        sudo apt-get --no-install-suggests --no-install-recommends install clang-$LLVM_VERSION || exit 1
+        sudo apt-get --no-install-suggests --no-install-recommends install g++-9 g++-9-multilib || exit 1
+        sudo ln -s /usr/include/asm-generic /usr/include/asm
+        CC=clang-$LLVM_VERSION
+        echo "CC=$CC" >> $GITHUB_ENV
+        CXX=clang++-$LLVM_VERSION
+        echo "CXX=$CXX" >> $GITHUB_ENV
+    - name: Checkout xsimd
+      uses: actions/checkout@v3
+    - name: Install mamba
+      uses: mamba-org/provision-with-micromamba@main
+      with:
+        environment-file: environment.yml
+    - name: Configure build
+      env:
+        CC: ${{ env.CC }}
+        CXX: ${{ env.CXX }}
+      run: |
+
+        mkdir _build
+        cd _build
+        cmake  .. -DBUILD_TESTS=ON \
+                  -DBUILD_BENCHMARK=ON \
+                  -DBUILD_EXAMPLES=ON \
+                  -DCMAKE_BUILD_TYPE=Release \
+                  -DCMAKE_C_COMPILER=$CC \
+                  -DCMAKE_CXX_COMPILER=$CXX \
+                  -DXSIMD_ENABLE_WERROR=ON \
+                  -DCMAKE_CXX_FLAGS="-DXSIMD_DEFAULT_ARCH=emulated\<128\> -DXSIMD_WITH_EMULATED=1 ${CXXFLAGS}" \
+                  -G Ninja
+    - name: Build
+      run: ninja -C _build
+    - name: Test
+      run: |
+        cd _build/test
+        ./test_xsimd

docs/source/api/arch.rst

@@ -17,3 +17,12 @@ The best available architecture is available at compile time through
 .. doxygengroup:: architectures
    :project: xsimd
    :members:
+
+
+Emulated mode
+-------------
+
+When compiled with the macro ``XSIMD_WITH_EMULATED`` set to ``1``, xsimd also
+exhibits a specific architecture ``xsimd::emulated<N>``, which consists of a
+vector of ``N`` bits emulated using scalar mode.
+It is mostly available for testing and debugging.

include/xsimd/arch/generic/xsimd_generic_memory.hpp

@@ -412,6 +412,12 @@ namespace xsimd
                 return true;
             }
 
+            template <typename ITy>
+            constexpr bool is_zip_lo(size_t, ITy)
+            {
+                return false;
+            }
+
             template <typename ITy0, typename ITy1, typename... ITys>
             constexpr bool is_zip_lo(size_t bsize, ITy0 index0, ITy1 index1, ITys... indices)
             {
@@ -423,6 +429,12 @@ namespace xsimd
                 return true;
             }
 
+            template <typename ITy>
+            constexpr bool is_zip_hi(size_t, ITy)
+            {
+                return false;
+            }
+
             template <typename ITy0, typename ITy1, typename... ITys>
             constexpr bool is_zip_hi(size_t bsize, ITy0 index0, ITy1 index1, ITys... indices)
             {