doc: toolchain: add a few words about XCC

This adds a few words about setting up XCC.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>

doc/getting_started/toolchain_3rd_party_x_compilers.rst

@@ -210,6 +210,84 @@ You can build toolchains from source code using crosstool-NG.
       $ echo $XTOOLS_TOOLCHAIN_PATH
       /Volumes/CrossToolNGNew/build/output/
 
+Cadence Tensilica Xtensa C/C++ Compiler (XCC)
+*********************************************
+
+#. Obtain Tensilica Software Development Toolkit targeting the specific SoC
+   on hand. This usually contains two parts:
+
+   * The Xtensa Xplorer which contains the necessary executables and
+     libraries.
+
+   * A SoC-specific add-on to be installed on top of Xtensa Xplorer.
+
+     * This add-on allows the compiler to generate code for the SoC on hand.
+
+#. Install Xtensa Xplorer and then the SoC add-on.
+
+   * Follow the instruction from Cadence on how to install the SDK.
+
+   * Depending on the SDK, there are two set of compilers:
+
+     * GCC-based compiler: ``xt-xcc`` and its friends.
+
+     * Clang-based compiler: ``xt-clang`` and its friends.
+
+     * By default, Zephyr would use GCC-based ``xt-xcc``.
+
+#. Make sure you have obtained a license to use the SDK, or has access to
+   a remote licensing server.
+
+#. :ref:`Set these environment variables <env_vars>`:
+
+   * Set :envvar:`ZEPHYR_TOOLCHAIN_VARIANT` to ``xcc``.
+   * Set :envvar:`XTENSA_TOOLCHAIN_PATH` to the toolchain installation
+     directory.
+   * Set :envvar:`XTENSA_CORE` to the SoC ID where application is being
+     targeting.
+   * Set :envvar:`TOOLCHAIN_VER` to the Xtensa SDK version.
+
+#. For example, assuming the SDK is installed in ``/opt/xtensa``, and
+   using the SDK for application development on ``intel_s1000_crb``,
+   setup the environment using:
+
+   .. code-block:: console
+
+      # Linux
+      export ZEPHYR_TOOLCHAIN_VARIANT=xcc
+      export XTENSA_TOOLCHAIN_PATH=/opt/xtensa/XtDevTools/install/tools/
+      export XTENSA_CORE=X6H3SUE_RI_2018_0
+      export TOOLCHAIN_VER=RI-2018.0-linux
+
+#. To use Clang-based compiler:
+
+   * Set :envvar:`XCC_USE_CLANG` to ``1``.
+
+     * For example:
+
+       .. code-block:: console
+
+          # Linux
+          export XCC_USE_CLANG=1
+
+   * Note that the Clang-based compiler may contain an old LLVM bug which
+     results in the following error:
+
+     .. code-block:: console
+
+        /tmp/file.s: Assembler messages:
+        /tmp/file.s:20: Error: file number 1 already allocated
+        clang-3.9: error: Xtensa-as command failed with exit code 1
+
+     If this happens, set :envvar:`XCC_NO_G_FLAG` to ``1``.
+
+     * For example:
+
+       .. code-block:: console
+
+          # Linux
+          export XCC_NO_G_FLAG=1
+
 .. _GNU Arm Embedded: https://developer.arm.com/open-source/gnu-toolchain/gnu-rm
 .. _crosstool-ng site: http://crosstool-ng.org
 .. _Arm Compiler 6: https://developer.arm.com/tools-and-software/embedded/arm-compiler/downloads/version-6