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.

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

@@ -5,18 +5,23 @@
 COMPONENT_SRCDIRS := .
 
 ifdef CONFIG_SPIRAM_ALLOW_BSS_SEG_EXTERNAL_MEMORY
-   # This linker script must come before esp32.project.ld
-   LINKER_SCRIPTS += esp32.extram.bss.ld
+	# This linker script must come before esp32.project.ld
+	LINKER_SCRIPTS += esp32.extram.bss.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.
+ifndef CONFIG_COMPILER_CXX_RTTI
+	# This linker script must come before esp32.project.ld
+	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
-#linker will ignore panic_highint_hdl.S as it has no other files depending on any
-#symbols in it.
+# 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
+# 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

@@ -98,6 +98,11 @@ 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)