C++: add provisions for optional RTTI support

Ref. https://github.com/espressif/esp-idf/issues/1684

This change allows RTTI to be enabled in menuconfig. For full RTTI
support, libstdc++.a in the toolchain should be built without
-fno-rtti, as it is done now.

Generally if libstdc++.a is built with RTTI, applications which do not
use RTTI (and build with -fno-rtti) could still include typeinfo
structures referenced from STL classes’ vtables. This change works
around this, by moving all typeinfo structures from libstdc++.a into
a non-loadable section, placed into a non-existent memory region
starting at address 0. This can be done because when the application
is compiled with -fno-rtti, typeinfo structures are not used at run
time. This way, typeinfo structures do not contribute to the
application binary size.

If the application is build with RTTI support, typeinfo structures are
linked into the application .rodata section as usual.

Note that this commit does not actually enable RTTI support.
The respective Kconfig option is hidden, and will be made visible when
the toolchain is updated.

CMakeLists.txt

@@ -22,6 +22,12 @@ else()
     list(APPEND cxx_compile_options "-fno-exceptions")
 endif()
 
+if(CONFIG_COMPILER_CXX_RTTI)
+    list(APPEND cxx_compile_options "-frtti")
+else()
+    list(APPEND cxx_compile_options "-fno-rtti")
+endif()
+
 if(CONFIG_COMPILER_DISABLE_GCC8_WARNINGS)
     list(APPEND compile_options "-Wno-parentheses"
                                 "-Wno-sizeof-pointer-memaccess"

Kconfig

@@ -149,6 +149,15 @@ mainmenu "Espressif IoT Development Framework Configuration"
                 Size (in bytes) of the emergency memory pool for C++ exceptions. This pool will be used to allocate
                 memory for thrown exceptions when there is not enough memory on the heap.
 
+        config COMPILER_CXX_RTTI
+            # Invisible option, until the toolchain with RTTI support is released.
+            # Use prompt "Enable C++ run-time type info (RTTI)" when updating.
+            bool
+            help
+                Enabling this option compiles all C++ files with RTTI support enabled.
+                This increases binary size (typically by tens of kB) but allows using
+                dynamic_cast conversion and typeid operator.
+
         choice COMPILER_STACK_CHECK_MODE
             prompt "Stack smashing protection mode"
             default COMPILER_STACK_CHECK_MODE_NONE

components/esp32/CMakeLists.txt

@@ -56,6 +56,11 @@ else()
         target_linker_script(${COMPONENT_LIB} INTERFACE "ld/esp32.extram.bss.ld")
     endif()
 
+    if(NOT CONFIG_COMPILER_CXX_RTTI)
+        # This has to be linked before esp32.project.ld
+        target_linker_script(${COMPONENT_LIB} INTERFACE "ld/esp32.discard-rtti.ld")
+    endif()
+
     # Process the template file through the linker script generation mechanism, and use the output for linking the
     # final binary
     target_linker_script(${COMPONENT_LIB} INTERFACE "${CMAKE_CURRENT_LIST_DIR}/ld/esp32.project.ld.in" 

components/esp32/component.mk

@@ -9,14 +9,19 @@ ifdef CONFIG_SPIRAM_ALLOW_BSS_SEG_EXTERNAL_MEMORY
 	LINKER_SCRIPTS += esp32.extram.bss.ld
 endif
 
-#Linker scripts used to link the final application.
+ifndef CONFIG_COMPILER_CXX_RTTI
-#Warning: These linker scripts are only used when the normal app is compiled; the bootloader
+	# This linker script must come before esp32.project.ld
-#specifies its own scripts.
+	LINKER_SCRIPTS += esp32.discard-rtti.ld
+endif
+
+# Linker scripts used to link the final application.
+# Warning: These linker scripts are only used when the normal app is compiled; the bootloader
+# specifies its own scripts.
 LINKER_SCRIPTS += $(COMPONENT_BUILD_DIR)/esp32.project.ld esp32.peripherals.ld
 
-#ld_include_panic_highint_hdl is added as an undefined symbol because otherwise the
+# ld_include_panic_highint_hdl is added as an undefined symbol because otherwise the
-#linker will ignore panic_highint_hdl.S as it has no other files depending on any
+# linker will ignore panic_highint_hdl.S as it has no other files depending on any
-#symbols in it.
+# symbols in it.
 COMPONENT_ADD_LDFLAGS += -L $(COMPONENT_PATH)/ld \
                          -T esp32_out.ld \
                          -u ld_include_panic_highint_hdl \

components/esp32/ld/esp32.discard-rtti.ld

@@ -0,0 +1,11 @@
+/* This is only included if CONFIG_COMPILER_CXX_RTTI is not set, to
+ * move RTTI sections of libstdc++ to an unused non-loadable memory region.
+ */
+
+SECTIONS
+{
+  .rodata.discard-rtti (NOLOAD):
+  {
+    *libstdc++.a:(.rodata._ZTI* .rodata._ZTS*)
+  } > discard_seg
+}

components/esp32/ld/esp32.ld

@@ -86,6 +86,11 @@ MEMORY
   /* external memory ,including data and text */
   extern_ram_seg(RWX)  :             org = 0x3F800000,
                                      len = 0x400000
+
+  /* This is not a memory range which can really be accessed; we use it as a "bitbucket"
+     where non-loadable sections, which aren't used at run time, can be discarded.
+   */
+  discard_seg (R) :                  org = 0x00000000, len = 0x10000000
 }
 
 #if defined(CONFIG_ESP32_USE_FIXED_STATIC_RAM_SIZE)

make/project.mk

@@ -452,7 +452,6 @@ CXXFLAGS ?=
 EXTRA_CXXFLAGS ?=
 CXXFLAGS := $(strip \
 	-std=gnu++11 \
-	-fno-rtti \
 	$(OPTIMIZATION_FLAGS) $(DEBUG_FLAGS) \
 	$(COMMON_FLAGS) \
 	$(COMMON_WARNING_FLAGS) \
@@ -465,6 +464,12 @@ else
 CXXFLAGS += -fno-exceptions
 endif
 
+ifdef CONFIG_COMPILER_CXX_RTTI
+CXXFLAGS += -frtti
+else
+CXXFLAGS += -fno-rtti
+endif
+
 ARFLAGS := cru
 
 export CFLAGS CPPFLAGS CXXFLAGS ARFLAGS

tools/cmake/build.cmake

@@ -114,8 +114,7 @@ function(__build_set_default_build_specifications)
     list(APPEND c_compile_options   "-std=gnu99"
                                     "-Wno-old-style-declaration")
 
-    list(APPEND cxx_compile_options "-std=gnu++11"
+    list(APPEND cxx_compile_options "-std=gnu++11")
-                                    "-fno-rtti")
 
     idf_build_set_property(COMPILE_DEFINITIONS "${compile_definitions}" APPEND)
     idf_build_set_property(COMPILE_OPTIONS "${compile_options}" APPEND)