Re: atomic in i386 Current after CLANG 6 upgrade

Tijl Coosemans <tijl_at_FreeBSD.org> writes:

> On Mon, 15 Jan 2018 11:43:44 +0100 Luca Pizzamiglio <pizzamig_at_freebsd.org> wrote:
>
>> I've already received a couple of messages from pkg-fallout about build
>> failure on head-i386-default [1] [2] both pointing to the same errors,
>> about missing intrinsic symbols related to __atomic_*
>> 
>> The clang documentation about C11 atomic builtins [3] stats that __atomic_*
>> are GCC extension and Clang provides them.
>> 
>> It seems to me that this specific GCC-compatible builtin are enabled on
>> amd64, but not on i386.
>> Is there a way to enable GCC compatible __atomic_ builtin also on i386?
>> Or should I provide patches to adopt _c11_atomic_* instead of __atomic_*
>> for every ports that need it ?
>> 
>> [1]
>> http://beefy11.nyi.freebsd.org/data/head-i386-default/p458948_s327953/logs/librdkafka-0.11.3.log
>> [2]
>> http://beefy11.nyi.freebsd.org/data/head-i386-default/p458948_s327953/logs/stress-ng-0.09.09.log
>> [3] https://clang.llvm.org/docs/LanguageExtensions.html#langext-c11-atomic
>
> 8 byte atomics requires at least i586.  So either find a way to disable
> the use of these atomics in these ports or add something like this to
> the port Makefile.
>
> .if ${ARCH} == i386 && ! ${MACHINE_CPU:Mi586}
> CFLAGS+=	-march=i586
> .endif

It wouldn't help (see below). Clang 6 accidentally made __atomic* work
enough to satisfy configure check but not for the port to build. I guess,
it also confuses configure in net/librdkafka and net-mgmt/netdata.

$ cat a.c
#include <stdint.h>

typedef struct {
  uint64_t val64;
} atomic_t;

int main()
{
  uint64_t foo;
  atomic_t bar;
#ifdef ATOMIC_STRUCT
  __atomic_fetch_add(&bar.val64, 1, __ATOMIC_RELAXED);
#else
  __atomic_fetch_add(&foo, 1, __ATOMIC_RELAXED);
#endif
  return 0;
}

$ cc -m32 -march=i586 a.c
$ clang50 -m32 -march=i586 a.c
/tmp/a-560ad1.o: In function `main':
a.c:(.text+0x46): undefined reference to `__atomic_fetch_add_8'
clang-5.0: error: linker command failed with exit code 1 (use -v to see invocation)

$ cc -m32 -DATOMIC_STRUCT -march=i586 a.c
/usr/bin/ld: error: undefined symbol: __atomic_fetch_add_8
>>> referenced by a.c
>>>               /tmp/a-ad8c36.o:(main)
cc: error: linker command failed with exit code 1 (use -v to see invocation)
$ clang50 -m32 -DATOMIC_STRUCT -march=i586 a.c
/tmp/a-0fbfd0.o: In function `main':
a.c:(.text+0x46): undefined reference to `__atomic_fetch_add_8'
clang-5.0: error: linker command failed with exit code 1 (use -v to see invocation)