doc: update documentation for APPDIRS makefile variable feature

- Add comprehensive documentation for the new APPDIRS variable in
  buildsystem.rst, including its usage and behavior with APP_*FLAGS.
- Update appdev.rst with practical usage examples and reference the
  demo_profiling application.
- Update changelog.rst to document the new feature.

Signed-off-by: Huaqi Fang <578567190@qq.com>

Author: fanghuaqi

doc/source/changelog.rst

@@ -86,6 +86,7 @@ This is release version of ``0.9.0`` of Nuclei SDK, which is still under develop
   - Add :ref:`design_app_demo_ecc` to demonstrate ECC error injection feature on Nuclei RISC-V CPU.
   - Disable ECLIC v2 hardware context auto save/restore feature in ``demo_clint_timer``, ``demo_plic``, ``demo_smode_clint`` and ``demo_smode_plic`` applications when using CLINT/PLIC interrupt modes to ensure proper interrupt handling
   - Add :ref:`design_app_demo_smode_clint` to demonstrate timer interrupt handling in S-Mode with CLINT interrupt mode.
+  - Update :ref:`design_app_demo_profiling` to demonstrate the usage of ``APPDIRS`` variable for application-specific compilation flags.
   - Remove ``nuclei_cache`` npk configuration from ``demo_cache``, ``demo_cidu``, ``demo_ecc``, ``demo_pma``, ``demo_smpcc``, and ``smphello`` applications ``npk.yml`` since it is now deprecated
   - Add ``get_cpu_feature`` in ``cpuinfo`` to list CPU features for the future hardware ci testing.
 
@@ -116,6 +117,7 @@ This is release version of ``0.9.0`` of Nuclei SDK, which is still under develop
 
 * Build System
 
+  - Add new ``APPDIRS`` variable to apply application-specific compilation flags only to specified source directories. When defined, ``APP_XXXFLAGS`` and ``APP_COMMON_FLAGS`` are applied only to source code in ``APPDIRS`` directories, enabling fine-grained control for profiling, coverage, and other app-specific optimizations. See :ref:`develop_buildsystem_var_appdirs` for details.
   - Add new ``n300e`` CPU core support in build system and npk.yml for evalsoc
   - Add ``-isystem=/include/libncrt`` for nuclei llvm toolchain when using libncrt_xxx library, but ``-isystem=`` feature is not
     the same as gcc, see report here https://github.com/llvm/llvm-project/pull/82084#discussion_r2387373311, so when you use libncrt library with llvm toolchain, the header file could be wrongly included using newlib ones

doc/source/develop/appdev.rst

@@ -38,6 +38,10 @@ To develop a Nuclei SDK application from scratch, you can do the following steps
 
 4. Follow :ref:`develop_buildsystem` to change your application Makefile.
 
+.. note::
+
+   * Starting from version 0.9.0, you can use :ref:`develop_buildsystem_var_appdirs` to apply compilation flags specifically to your application source code.
+
 .. _develop_appdev_addsrc:
 
 Add Extra Source Code
@@ -154,4 +158,48 @@ If you want to change the application level Make options,
 you can add the :ref:`develop_buildsystem_makefile_local`.
 
 
+.. _develop_appdev_appdirs_usage:
+
+Application-Specific Compilation Flags with APPDIRS
+---------------------------------------------------
+
+Starting from Nuclei SDK version 0.9.0, you can use the :ref:`develop_buildsystem_var_appdirs` variable to apply compilation flags specifically to your application source code, without affecting the SDK library code. This is particularly useful for features like profiling, code coverage, or other application-specific optimizations.
+
+The **APPDIRS** variable allows you to specify directories that need application-specific compilation flags. It can support multiple directories separated by space, e.g. ``APPDIRS = . src``.
+
+If **APPDIRS** is not defined or empty, the **APP_XXXFLAGS** and **APP_COMMON_FLAGS** will be added to global flags and applied to all source files in the project.
+
+If **APPDIRS** is defined, the **APP_XXXFLAGS** and **APP_COMMON_FLAGS** will be applied only to source code located in the directories specified by **APPDIRS**.
+
+The following flags can be used with **APPDIRS**:
+
+* :ref:`develop_buildsystem_var_app_common_flags`: Common flags for C/C++/ASM compilation
+* :ref:`develop_buildsystem_var_app_cflags`: Flags specific to C compilation
+* :ref:`develop_buildsystem_var_app_cxxflags`: Flags specific to C++ compilation
+* :ref:`develop_buildsystem_var_app_asmflags`: Flags specific to ASM compilation
+
+For example, if you want to append specific profiling or coverage flags only to your application source code while keeping the SDK source code compiled with default flags, you can set ``APPDIRS`` to point to your application source directories.
+
+A practical example of this feature can be found in the ``application/baremetal/demo_profiling`` application, which uses ``APPDIRS`` to enable profiling instrumentation only for the application code:
+
+.. code-block:: makefile
+
+    # Application-specific compilation options that can be used for
+    # profiling, coverage, and other app-specific settings
+    # -pg enables profiling instrumentation
+    APP_COMMON_FLAGS := -pg
+
+    # App-specific C compilation flags
+    APP_CFLAGS :=
+    # App-specific C++ compilation flags
+    APP_CXXFLAGS :=
+    # App-specific Assembly compilation flags
+    APP_ASMFLAGS :=
+
+    # Specify directories that need app-specific compilation flags
+    # If left empty, APP_XXXFLAGS will be applied to all source files
+    APPDIRS := . src
+
+This approach allows for fine-grained control of compilation flags for specific parts of an application while maintaining clean separation from SDK source code.
+
 .. _Options That Control Optimization in GCC: https://gcc.gnu.org/onlinedocs/gcc-9.2.0/gcc/Optimize-Options.html#Optimize-Options

doc/source/develop/buildsystem.rst

@@ -1510,6 +1510,7 @@ APP_COMMON_FLAGS
 .. note::
 
     * Added in 0.4.0 release.
+    * Behavior changed in 0.9.0 with introduction of APPDIRS - the behavior of this variable now depends on the :ref:`develop_buildsystem_var_appdirs` setting.
 
 This variable is used to define app common compiler flags to all c/asm/cpp compiler.
 You can pass it via make command to define extra flags to compile application.
@@ -1520,6 +1521,10 @@ You can pass it via make command to define extra flags to compile application.
 APP_ASMFLAGS
 ~~~~~~~~~~~~
 
+.. note::
+
+    * Behavior changed in 0.9.0 with introduction of APPDIRS - the behavior of this variable now depends on the :ref:`develop_buildsystem_var_appdirs` setting.
+
 This variable is similiar to **APP_COMMON_FLAGS** but used to pass extra app asm flags.
 
 
@@ -1528,13 +1533,21 @@ This variable is similiar to **APP_COMMON_FLAGS** but used to pass extra app asm
 APP_CFLAGS
 ~~~~~~~~~~
 
+.. note::
+
+    * Behavior changed in 0.9.0 with introduction of APPDIRS - the behavior of this variable now depends on the :ref:`develop_buildsystem_var_appdirs` setting.
+
 This variable is similiar to **APP_COMMON_FLAGS** but used to pass extra app c flags.
 
 .. _develop_buildsystem_var_app_cxxflags:
 
 APP_CXXFLAGS
 ~~~~~~~~~~~~
 
+.. note::
+
+    * Behavior changed in 0.9.0 with introduction of APPDIRS - the behavior of this variable now depends on the :ref:`develop_buildsystem_var_appdirs` setting.
+
 This variable is similiar to **APP_COMMON_FLAGS** but used to pass extra app cxx flags.
 
 .. _develop_buildsystem_var_app_ldflags:
@@ -1544,6 +1557,36 @@ APP_LDFLAGS
 
 This variable is similiar to **APP_COMMON_FLAGS** but used to pass extra app linker flags.
 
+.. _develop_buildsystem_var_appdirs:
+
+APPDIRS
+~~~~~~~
+
+.. note::
+
+    * This variable is introduced in version 0.9.0
+
+This variable is used to specify directories that need application-specific compilation flags.
+It can support multiple directories separated by space, e.g. ``APPDIRS = . src``.
+
+If **APPDIRS** is not defined or empty, the **APP_XXXFLAGS** and **APP_COMMON_FLAGS** will be added to global flags
+and applied to all source files in the project.
+
+If **APPDIRS** is defined, the **APP_XXXFLAGS** and **APP_COMMON_FLAGS** will be applied only to source code
+located in the directories specified by **APPDIRS**. This allows for fine-grained control of compilation flags
+for specific parts of an application.
+
+For example, if you want to apply specific profiling or coverage flags only to your application source code
+while keeping the SDK library code compiled with default flags, you can set ``APPDIRS`` to point to your
+application source directories.
+
+When **APPDIRS** is defined, the following flags will be applied only to source files in those directories:
+
+* **APP_COMMON_FLAGS**: Common flags for C/C++/ASM compilation
+* **APP_CFLAGS**: Flags specific to C compilation
+* **APP_CXXFLAGS**: Flags specific to C++ compilation
+* **APP_ASMFLAGS**: Flags specific to ASM compilation
+
 .. _develop_buildsystem_var_nogc:
 
 NOGC