arch, boards, cmake: Build C++ ELF modules without __cxa_atexit.

A C++ module with a static object does not link.  GCC registers each such
object's destructor with __cxa_atexit(dtor, obj, &__dso_handle), and
__dso_handle comes from crtbegin, which a module does not link:

  hello++3.cxx:119: undefined reference to `__dso_handle'

It is reachable today with CONFIG_PIC, where a module is linked as an
executable and the symbol has to resolve.  Without it the link is
relocatable, the symbol stays undefined and nothing complains until
something makes it resolve.

-fno-use-cxa-atexit registers the destructors with atexit() instead, which
puts them in .fini_array.  That is also where libelf_uninit() looks for them
when the module is unloaded, so the flag that makes the link work is also
the flag that makes the destructors run.

The option goes wherever CXXELFFLAGS is defined, which is the architecture
Toolchain.defs and the boards that reassign it.  The toolchains that are not
GCC or Clang are left alone: ceva, tricore, z16 and the z80 family.

The CMake build sets it once, next to where the architecture elf.cmake is
included.  A generator expression keeps it off the C compiles, because the
option is valid for C++ alone and GCC warns about it otherwise, and the
compiler id gates it so that a toolchain which is neither GCC nor Clang does
not see it.  It cannot go in the toolchain file itself: CMake reads that file
again inside try_compile, in a project that has not included the NuttX
extensions, so the call is an unknown command there.

Reproduced with apps/examples/elf on mps3-an547:picostest with CONFIG_PIC
enabled: hello++3 fails to link before and links after.

Assisted-by: Claude Opus 5 (1M context) <noreply@anthropic.com>
Signed-off-by: Marco Casaroli <marco.casaroli@gmail.com>

Author: casaroli

CMakeLists.txt

@@ -516,6 +516,20 @@ if(EXISTS ${NUTTX_DIR}/arch/${CONFIG_ARCH}/src/cmake/elf.cmake)
   include(${NUTTX_DIR}/arch/${CONFIG_ARCH}/src/cmake/elf.cmake)
 endif()
 
+# The default registers each static object's destructor with __cxa_atexit(dtor,
+# obj, &__dso_handle), and __dso_handle comes from crtbegin, which a module does
+# not link.  Turning it off also puts the destructors in .fini_array, which is
+# where the loader looks for them when the module is unloaded.
+#
+# GCC and Clang alone take the option, which is why the architectures built by
+# another compiler do not carry it in their Toolchain.defs either.  ARMClang
+# reports ARMClang, so it keeps it.  The generator expression keeps the option
+# off the C compiles, which it is not valid for.
+
+if(CMAKE_CXX_COMPILER_ID MATCHES "GNU|Clang")
+  nuttx_elf_compile_options($<$<COMPILE_LANGUAGE:CXX>:-fno-use-cxa-atexit>)
+endif()
+
 include(platform)
 
 if(CONFIG_ARCH_TOOLCHAIN_TASKING)

arch/arm/src/common/Toolchain.defs

@@ -633,6 +633,10 @@ CELFFLAGS = $(filter-out --fixed-r9,$(CFLAGS)) -fvisibility=hidden \
 CXXELFFLAGS = $(filter-out --fixed-r9,$(CXXFLAGS)) -fvisibility=hidden \
               -mlong-calls
 
+# __dso_handle comes from crtbegin, which a module does not link
+
+CXXELFFLAGS += -fno-use-cxa-atexit
+
 ifeq ($(CONFIG_PIC),y)
   # ARCHCFLAGS, not CFLAGS: board Make.defs reassign CFLAGS with ':='
   # after including this file, which would discard the flag.

arch/arm64/src/Toolchain.defs

@@ -292,6 +292,10 @@ LDMODULEFLAGS = -r -T $(call CONVERT_PATH,$(TOPDIR)/libs/libc/elf/gnu-elf.ld)
 CELFFLAGS = $(CFLAGS) -fvisibility=hidden # --target1-abs
 CXXELFFLAGS = $(CXXFLAGS) -fvisibility=hidden # --target1-abs
 
+# __dso_handle comes from crtbegin, which a module does not link
+
+CXXELFFLAGS += -fno-use-cxa-atexit
+
 LDELFFLAGS = -r -e _start
 ifneq ($(CONFIG_BUILD_KERNEL),y)
   # Flat build and protected elf entry point use crt0,

arch/avr/src/avr/Toolchain.defs

@@ -219,5 +219,9 @@ LDMODULEFLAGS = -r -T $(call CONVERT_PATH,$(TOPDIR)/libs/libc/elf/gnu-elf.ld)
 CELFFLAGS = $(CFLAGS) -fvisibility=hidden
 CXXELFFLAGS = $(CXXFLAGS) -fvisibility=hidden
 
+# __dso_handle comes from crtbegin, which a module does not link
+
+CXXELFFLAGS += -fno-use-cxa-atexit
+
 LDELFFLAGS = -r -e _start
 LDELFFLAGS += -T $(call CONVERT_PATH,$(TOPDIR)$(DELIM)libs$(DELIM)libc$(DELIM)elf$(DELIM)gnu-elf.ld)

arch/avr/src/avr32/Toolchain.defs

@@ -115,5 +115,9 @@ LDMODULEFLAGS = -r -T $(call CONVERT_PATH,$(TOPDIR)/libs/libc/elf/gnu-elf.ld)
 CELFFLAGS = $(CFLAGS) -fvisibility=hidden
 CXXELFFLAGS = $(CXXFLAGS) -fvisibility=hidden
 
+# __dso_handle comes from crtbegin, which a module does not link
+
+CXXELFFLAGS += -fno-use-cxa-atexit
+
 LDELFFLAGS = -r -e _start
 LDELFFLAGS += -T $(call CONVERT_PATH,$(TOPDIR)$(DELIM)libs$(DELIM)libc$(DELIM)elf$(DELIM)gnu-elf.ld)

arch/mips/src/mips32/Toolchain.defs

@@ -332,5 +332,9 @@ LDMODULEFLAGS = -EL -r -T $(call CONVERT_PATH,$(TOPDIR)/libs/libc/elf/gnu-elf.ld
 CELFFLAGS = $(CFLAGS) -fvisibility=hidden
 CXXELFFLAGS = $(CXXFLAGS) -fvisibility=hidden
 
+# __dso_handle comes from crtbegin, which a module does not link
+
+CXXELFFLAGS += -fno-use-cxa-atexit
+
 LDELFFLAGS = -r -e _start
 LDELFFLAGS += -T $(call CONVERT_PATH,$(TOPDIR)$(DELIM)libs$(DELIM)libc$(DELIM)elf$(DELIM)gnu-elf.ld)

arch/misoc/src/lm32/Toolchain.defs

@@ -146,5 +146,9 @@ LDMODULEFLAGS = -r -T $(call CONVERT_PATH,$(TOPDIR)/libs/libc/elf/gnu-elf.ld)
 CELFFLAGS = $(CFLAGS) -fvisibility=hidden
 CXXELFFLAGS = $(CXXFLAGS) -fvisibility=hidden
 
+# __dso_handle comes from crtbegin, which a module does not link
+
+CXXELFFLAGS += -fno-use-cxa-atexit
+
 LDELFFLAGS = -r -e _start
 LDELFFLAGS += -T $(call CONVERT_PATH,$(TOPDIR)$(DELIM)libs$(DELIM)libc$(DELIM)elf$(DELIM)gnu-elf.ld)

arch/misoc/src/minerva/Toolchain.defs

@@ -94,5 +94,9 @@ LDMODULEFLAGS = -r -T $(call CONVERT_PATH,$(TOPDIR)/libs/libc/elf/gnu-elf.ld)
 CELFFLAGS = $(CFLAGS) -fvisibility=hidden
 CXXELFFLAGS = $(CXXFLAGS) -fvisibility=hidden
 
+# __dso_handle comes from crtbegin, which a module does not link
+
+CXXELFFLAGS += -fno-use-cxa-atexit
+
 LDELFFLAGS = -r -e _start
 LDELFFLAGS += -T $(call CONVERT_PATH,$(TOPDIR)$(DELIM)libs$(DELIM)libc$(DELIM)elf$(DELIM)gnu-elf.ld)

arch/or1k/src/mor1kx/Toolchain.defs

@@ -132,5 +132,9 @@ LDMODULEFLAGS = -r -T $(call CONVERT_PATH,$(TOPDIR)/libs/libc/elf/gnu-elf.ld)
 CELFFLAGS = $(CFLAGS) -fvisibility=hidden
 CXXELFFLAGS = $(CXXFLAGS) -fvisibility=hidden
 
+# __dso_handle comes from crtbegin, which a module does not link
+
+CXXELFFLAGS += -fno-use-cxa-atexit
+
 LDELFFLAGS = -r -e _start
 LDELFFLAGS += -T $(call CONVERT_PATH,$(TOPDIR)$(DELIM)libs$(DELIM)libc$(DELIM)elf$(DELIM)gnu-elf.ld)

arch/risc-v/src/common/Toolchain.defs

@@ -449,6 +449,10 @@ LDMODULEFLAGS = -r -T $(call CONVERT_PATH,$(TOPDIR)/libs/libc/elf/gnu-elf.ld)
 CELFFLAGS = $(CFLAGS) -fvisibility=hidden
 CXXELFFLAGS = $(CXXFLAGS) -fvisibility=hidden
 
+# __dso_handle comes from crtbegin, which a module does not link
+
+CXXELFFLAGS += -fno-use-cxa-atexit
+
 LDELFFLAGS = -e _start
 
 ifeq ($(CONFIG_BINFMT_ELF_RELOCATABLE),y)