cross-compile error: undefined reference to `__atomic_load_8'

[Lapshin]

Bug report

Bug description:

Should not configure script check if -latomic is needed?

getting build error when cross-compile:

arm-linux-gnueabi-gcc -L/opt/zlib-arm-linux-gnueabi/lib -L/opt/xz-arm-linux-gnueabi/lib -L/opt/readline-arm-linux-gnueabi/lib -L/opt/libffi-arm-linux-gnueabi/lib -L/opt/libtirpc-arm-linux-gnueabi/lib -L/opt/libuuid-arm-linux-gnueabi/lib -L/opt/zlib-arm-linux-gnueabi/lib -L/opt/xz-arm-linux-gnueabi/lib -L/opt/readline-arm-linux-gnueabi/lib -L/opt/libffi-arm-linux-gnueabi/lib -L/opt/libtirpc-arm-linux-gnueabi/lib -L/opt/libuuid-arm-linux-gnueabi/lib   -Xlinker -export-dynamic -o Programs/_testembed Programs/_testembed.o -L. -lpython3.13 -lpthread -ldl  -lpthread -lutil                          -lm 
/root/x-tools/arm-linux-gnueabi/lib/gcc/arm-linux-gnueabi/13.2.0/../../../../arm-linux-gnueabi/bin/ld: ./libpython3.13.so: undefined reference to `__atomic_load_8'
/root/x-tools/arm-linux-gnueabi/lib/gcc/arm-linux-gnueabi/13.2.0/../../../../arm-linux-gnueabi/bin/ld: ./libpython3.13.so: undefined reference to `__atomic_store_8'
/root/x-tools/arm-linux-gnueabi/lib/gcc/arm-linux-gnueabi/13.2.0/../../../../arm-linux-gnueabi/bin/ld: ./libpython3.13.so: undefined reference to `__atomic_fetch_add_8'
/root/x-tools/arm-linux-gnueabi/lib/gcc/arm-linux-gnueabi/13.2.0/../../../../arm-linux-gnueabi/bin/ld: ./libpython3.13.so: undefined reference to `__atomic_compare_exchange_8'
collect2: error: ld returned 1 exit status

configure cmd line:

./configure --prefix=/opt/python-arm-linux-gnueabi-3.13.0 --host=arm-linux-gnueabi --target=arm-linux-gnueabi --build=x86_64-linux-gnu --disable-ipv6 ac_cv_file__dev_ptmx=no ac_cv_file__dev_ptc=no  --enable-shared --with-openssl=/opt/openssl-arm-linux-gnueabi --with-build-python=/root/.pyenv/versions/3.13.0/bin/python

workaround

export LDFLAGS="-latomic $LDFLAGS"

CPython versions tested on:

3.13

Operating systems tested on:

Linux

Linked PRs

• gh-125269: Use AC_LINK_IF_ELSE to detect if -latomic is needed #125416
• [3.13] gh-125269: Use AC_LINK_IF_ELSE to detect if -latomic is needed (GH-125416) #125493

[colesbury]

The configure script has a check for -latomic. I'm not sure why it's not working on your system:

cpython/configure.ac

Lines 7498 to 7545 in 01fc3b3

	AC_CACHE_CHECK([whether libatomic is needed by <pyatomic.h>],
	[ac_cv_libatomic_needed],
	[AC_RUN_IFELSE([AC_LANG_SOURCE([[
	// pyatomic.h needs uint64_t and Py_ssize_t types
	#include <stdint.h> // int64_t, intptr_t
	#ifdef HAVE_SYS_TYPES_H
	# include <sys/types.h> // ssize_t
	#endif
	// Code adapted from Include/pyport.h
	#if HAVE_SSIZE_T
	typedef ssize_t Py_ssize_t;
	#elif SIZEOF_VOID_P == SIZEOF_SIZE_T
	typedef intptr_t Py_ssize_t;
	#else
	# error "unable to define Py_ssize_t"
	#endif
	#include "pyatomic.h"
	int main()
	{
	uint64_t value;
	_Py_atomic_store_uint64(&value, 2);
	if (_Py_atomic_or_uint64(&value, 8) != 2) {
	return 1; // error
	}
	if (_Py_atomic_load_uint64(&value) != 10) {
	return 1; // error
	}
	uint8_t byte = 0xb8;
	if (_Py_atomic_or_uint8(&byte, 0x2d) != 0xb8) {
	return 1; // error
	}
	if (_Py_atomic_load_uint8(&byte) != 0xbd) {
	return 1; // error
	}
	return 0; // all good
	}
	]])],
	[ac_cv_libatomic_needed=no], dnl build succeeded
	[ac_cv_libatomic_needed=yes], dnl build failed
	[ac_cv_libatomic_needed=no]) dnl cross compilation
	])
	AS_VAR_IF([ac_cv_libatomic_needed], [yes],
	[LIBS="${LIBS} -latomic"
	LIBATOMIC=${LIBATOMIC-"-latomic"}])
	_RESTORE_VAR([CPPFLAGS])

[colesbury]

Oh, I guess we disabled it for cross compilation (#109211). cc @vstinner @erlend-aasland

[colesbury]

Could we use AC_LINK_IFELSE instead of AC_RUN_IFELSE for this check?

[vstinner]

Could we use AC_LINK_IFELSE instead of AC_RUN_IFELSE for this check?

I don't know, it's beyond my autotools skills :-) Maybe @erlend-aasland or @corona10 knows.

[vstinner]

I don't know autotools. When Python is cross-compiled, does AC_RUN_IFELSE build the program? Or does it do nothing?

[erlend-aasland]

IIRC, the LINK variant only runs the linker, so it should work on a correctly setup cross-compiling environment. The RUN variant also runs the resulting executable, which will fail in a cross-compiling dev environment.

[erlend-aasland]

Ah, the check in our configure script defaults to not requiring libatomic if we are cross compiling. Perhaps we want to change that configure branch.

[colesbury]

Ah, the check in our configure script defaults to not requiring libatomic if we are cross compiling. Perhaps we want to change that configure branch.

Looking at the past PRs:

• Sep 8, 2023: gh-109054: configure checks if libatomic is needed #109101 added the check and made -latomic the default when cross-compiling
• Sep 10, 2023: gh-109054: Don't use libatomic on cross-compilation #109211 changed the cross-compling default to not require -latomic to fix wasm32, which is cross-compiled but doesn't support -latomic.

IIRC, the LINK variant only runs the linker, so it should work on a correctly setup cross-compiling environment

I think the link time check should be sufficient for both cross-compiling and native compilation (famous last words). I'll give it a go.