docs(ulp): Add description about -msave-restore option

docs/en/api-reference/system/ulp-lp-core.rst

@@ -34,7 +34,9 @@ The first argument to ``ulp_embed_binary`` specifies the ULP binary name. The na
 
 1. Enable both :ref:`CONFIG_ULP_COPROC_ENABLED` and :ref:`CONFIG_ULP_COPROC_TYPE` in menucofig, and set :ref:`CONFIG_ULP_COPROC_TYPE` to ``CONFIG_ULP_COPROC_TYPE_LP_CORE``. The :ref:`CONFIG_ULP_COPROC_RESERVE_MEM` option reserves RTC memory for the ULP, and must be set to a value big enough to store both the ULP LP-Core code and data. If the application components contain multiple ULP programs, then the size of the RTC memory must be sufficient to hold the largest one.
 
-2. Build the application as usual (e.g., ``idf.py app``).
+2. If you want to reduce the code size at the expense of possible performance loss, you can turn on :ref:`CONFIG_ULP_LP_RISCV_SAVE_RESTORE_LIBCALLS` to enable GCC's ``-msave-restore`` feature.
+
+3. Build the application as usual (e.g., ``idf.py app``).
 
 During the build process, the following steps are taken to build ULP program:
 

docs/en/api-reference/system/ulp-risc-v.rst

@@ -120,7 +120,9 @@ Starting the ULP RISC-V Program
 
 To run a ULP RISC-V program, the main application needs to load the ULP program into RTC memory using the :cpp:func:`ulp_riscv_load_binary` function, and then start it using the :cpp:func:`ulp_riscv_run` function.
 
-Note that the ``CONFIG_ULP_COPROC_ENABLED`` and ``CONFIG_ULP_COPROC_TYPE_RISCV`` options must be enabled in menuconfig to work with ULP RISC-V. To reserve memory for the ULP, the ``RTC slow memory reserved for coprocessor`` option must be set to a value big enough to store ULP RISC-V code and data. If the application components contain multiple ULP programs, then the size of the RTC memory must be sufficient to hold the largest one.
+Note that the :ref:`CONFIG_ULP_COPROC_ENABLED` and :ref:`CONFIG_ULP_COPROC_TYPE_RISCV` options must be enabled in menuconfig to work with ULP RISC-V. To reserve memory for the ULP, the ``RTC slow memory reserved for coprocessor`` option must be set to a value big enough to store ULP RISC-V code and data. If the application components contain multiple ULP programs, then the size of the RTC memory must be sufficient to hold the largest one.
+
+If you want to reduce the code size at the expense of some performance loss, you can turn on :ref:`CONFIG_ULP_LP_RISCV_SAVE_RESTORE_LIBCALLS` to enable GCC's ``-msave-restore`` feature.
 
 Each ULP RISC-V program is embedded into the ESP-IDF application as a binary blob. The application can reference this blob and load it in the following way (suppose ULP_APP_NAME was defined to ``ulp_app_name``):
 

docs/zh_CN/api-reference/system/ulp-lp-core.rst

@@ -34,7 +34,9 @@ ULP LP-Core 代码会与 ESP-IDF 项目共同编译，生成一个单独的二
 
 1. 在 menuconfig 中启用 :ref:`CONFIG_ULP_COPROC_ENABLED` 和 :ref:`CONFIG_ULP_COPROC_TYPE` 选项，并将 :ref:`CONFIG_ULP_COPROC_TYPE` 设置为 ``CONFIG_ULP_COPROC_TYPE_LP_CORE``。:ref:`CONFIG_ULP_COPROC_RESERVE_MEM` 选项为 ULP 保留 RTC 内存，因此必须设置为一个足够大的值，以存储 ULP LP-Core 代码和数据。如果应用程序组件包含多个 ULP 程序，那么 RTC 内存的大小必须足够容纳其中最大的程序。
 
-2. 按照常规步骤构建应用程序（例如 ``idf.py app``）。
+2. 如须减少程序体积，可启用 :ref:`CONFIG_ULP_LP_RISCV_SAVE_RESTORE_LIBCALLS` 。此选项会开启GCC的 ``-msave-restore`` 功能，并可能些微影响程序性能。
+
+3. 按照常规步骤构建应用程序（例如 ``idf.py app``）。
 
 在构建过程中，采取以下步骤来构建 ULP 程序：
 

docs/zh_CN/api-reference/system/ulp-risc-v.rst

@@ -120,7 +120,9 @@ ULP 中的所有硬件指令都不支持互斥，所以 Lock API 需通过一种
 
 要运行 ULP RISC-V 程序，主程序需要调用 :cpp:func:`ulp_riscv_load_binary` 函数，将 ULP 程序加载到 RTC 内存中，然后调用 :cpp:func:`ulp_riscv_run` 函数，启动 ULP RISC-V 程序。
 
-注意，必须在 menuconfig 中启用 ``CONFIG_ULP_COPROC_ENABLED`` 和 ``CONFIG_ULP_COPROC_TYPE_RISCV`` 选项，以便正常运行 ULP RISC-V 程序。``RTC slow memory reserved for coprocessor`` 选项设置的值必须足够存储 ULP RISC-V 代码和数据。如果应用程序组件包含多个 ULP 程序，RTC 内存必须足以容纳最大的程序。
+注意，必须在 menuconfig 中启用 :ref:`CONFIG_ULP_COPROC_ENABLED` 和 :ref:`CONFIG_ULP_COPROC_TYPE_RISCV` 选项，以便正常运行 ULP RISC-V 程序。``RTC slow memory reserved for coprocessor`` 选项设置的值必须足够存储 ULP RISC-V 代码和数据。如果应用程序组件包含多个 ULP 程序，RTC 内存必须足以容纳最大的程序。
+
+如须减少程序体积，可启用 :ref:`CONFIG_ULP_LP_RISCV_SAVE_RESTORE_LIBCALLS` 。此选项会开启GCC的 ``-msave-restore`` 功能，并可能些微影响程序性能。
 
 每个 ULP RISC-V 程序均以二进制 BLOB 的形式嵌入到 ESP-IDF 应用程序中。应用程序可以引用此 BLOB，并以下面的方式加载此 BLOB（假设 ULP_APP_NAME 已被定义为 ``ulp_app_name``）：