build system: add COMPILER_SAVE_RESTORE_LIBCALLS option

Add new Kconfig option to enable -msave-restore flag for RISC-V
targets. This option can be used to reduce binary size by replacing
inlined register save/restore sequences with library calls.

CMakeLists.txt

@@ -61,6 +61,10 @@ else()
     list(APPEND link_options "-fno-rtti")           # used to invoke correct multilib variant (no-rtti) during linking
 endif()
 
+if(CONFIG_COMPILER_SAVE_RESTORE_LIBCALLS)
+    list(APPEND compile_options "-msave-restore")
+endif()
+
 if(CONFIG_COMPILER_DISABLE_GCC8_WARNINGS)
     list(APPEND compile_options "-Wno-parentheses"
                                 "-Wno-sizeof-pointer-memaccess"

Kconfig

@@ -394,6 +394,18 @@ mainmenu "Espressif IoT Development Framework Configuration"
                 For C++, this warns about the deprecated conversion from string
                 literals to ``char *``.
 
+        config COMPILER_SAVE_RESTORE_LIBCALLS
+            bool "Enable -msave-restore flag to reduce code size"
+            depends on IDF_TARGET_ARCH_RISCV
+            help
+                Adds -msave-restore to C/C++ compilation flags.
+
+                When this flag is enabled, compiler will call library functions to
+                save/restore registers in function prologues/epilogues. This results
+                in lower overall code size, at the expense of slightly reduced performance.
+
+                This option can be enabled for RISC-V targets only.
+
         config COMPILER_DISABLE_GCC8_WARNINGS
             bool "Disable new warnings introduced in GCC 6 - 8"
             default "n"

components/newlib/system_libs.lf

@@ -4,6 +4,8 @@ entries:
     lib2funcs (noflash_text)
     if IDF_TARGET_ESP32 = n:
         _divsf3 (noflash)
+    if IDF_TARGET_ARCH_RISCV:
+        save-restore (noflash)
 
 [mapping:gcov]
 archive: libgcov.a

docs/en/api-guides/performance/size.rst

@@ -298,6 +298,7 @@ The following configuration options will reduce the final binary size of almost
     - Don't enable :ref:`CONFIG_COMPILER_CXX_EXCEPTIONS`, :ref:`CONFIG_COMPILER_CXX_RTTI`, or set the :ref:`CONFIG_COMPILER_STACK_CHECK_MODE` to Overall. All of these options are already disabled by default, but they have a large impact on binary size.
     - Disabling :ref:`CONFIG_ESP_ERR_TO_NAME_LOOKUP` will remove the lookup table to translate user-friendly names for error values (see :doc:`/api-guides/error-handling`) in error logs, etc. This saves some binary size, but error values will be printed as integers only.
     - Setting :ref:`CONFIG_ESP_SYSTEM_PANIC` to "Silent reboot" will save a small amount of binary size, however this is *only* recommended if no one will use UART output to debug the device.
+    :CONFIG_IDF_TARGET_ARCH_RISCV: - Set :ref:`CONFIG_COMPILER_SAVE_RESTORE_LIBCALLS` to reduce binary size by replacing inlined prologues/epilogues with library calls.
 
 .. note::