[Mesa-dev] Fix __atomic* builtins detection for Clang

[Jonathan Gray]

On Sat, May 13, 2017 at 09:01:49AM +0200, Jan Beich wrote:
> Jonathan Gray <jsg at jsg.id.au> writes:
> 
> > On Sat, May 13, 2017 at 02:22:30AM +0200, Jan Beich wrote:
> >
> >> "int" isn't large enough and lack of builtins only manifests at link time.
> >> This was breaking build on FreeBSD 11.0 i386 with Clang 3.8.0.
> >> 
> >> glsl/.libs/libstandalone.a(libmesautil_la-disk_cache.o): In function `disk_cache_remove':
> >> disk_cache.c:(.text+0x763): undefined reference to `__atomic_fetch_add_8'
> >> glsl/.libs/libstandalone.a(libmesautil_la-disk_cache.o): In function `cache_put':
> >> disk_cache.c:(.text+0xabc): undefined reference to `__atomic_fetch_add_8'
> >> disk_cache.c:(.text+0xec1): undefined reference to `__atomic_fetch_add_8'
> >> c++: error: linker command failed with exit code 1 (use -v to see invocation)
> >
> > You should be building with -march=i586 on i386 for CX8.
> 
> -march=i586 doesn't help. Dimitry, could this be a Clang bug?

As it is accepted by clang 4.0 on amd64 but not with -m32 looks like.

> 
> $ cat a.c
> #include <stdint.h>
> int main() {
>     uint64_t n;
>     return __atomic_load_n(&n, __ATOMIC_ACQUIRE);
> }
> 
> $ clang40 -m32 -march=i586 a.c
> /tmp/a-d984ec.o: In function `main':
> a.c:(.text+0x21): undefined reference to `__atomic_load_8'
> clang-4.0: error: linker command failed with exit code 1 (use -v to see invocation)
> 
> full build log: http://sprunge.us/LJdh
> 
> >
> > The diff is wrong as it will break other architectures, ie powerpc.
> 
> And it's not broken already? -latomic isn't passed anywhere.

It is broken for different reasons on platforms that aren't linux.

It used to be you could disable the building the shader cache and not
build the i965 driver to avoid 64 bit atomics in Mesa.  The situation
may have gotten worse in recent releases, not sure.  If you are using
a compiler with __sync but not __atomic builtins it seems to have
gotten worse.

> 
> >
> > commit a6a38a038bd62e6d9558905f00bef81b5e7e6fcc
> > Author: Grazvydas Ignotas <notasas at gmail.com>
> > Date:   Fri Mar 31 01:26:25 2017 +0300
> >
> >     util/u_atomic: provide 64bit atomics where they're missing
> >
> > attempts to handle powerpc like situations, though not sync
> > builtins being missing entirely.
> >
> >> ---
> >>  configure.ac | 5 +++--
> >>  1 file changed, 3 insertions(+), 2 deletions(-)
> >> 
> >> diff --git a/configure.ac b/configure.ac
> >> index 957d15df8c..5b11941a0c 100644
> >> --- a/configure.ac
> >> +++ b/configure.ac
> >> @@ -403,9 +403,10 @@ AM_CONDITIONAL([SSE41_SUPPORTED], [test x$SSE41_SUPPORTED = x1])
> >>  AC_SUBST([SSE41_CFLAGS], $SSE41_CFLAGS)
> >>  
> >>  dnl Check for new-style atomic builtins
> >> -AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
> >> +AC_LINK_IFELSE([AC_LANG_SOURCE([[
> >> +#include <stdint.h>
> >>  int main() {
> >> -    int n;
> >> +    uint64_t n;
> >>      return __atomic_load_n(&n, __ATOMIC_ACQUIRE);
> >>  }]])], GCC_ATOMIC_BUILTINS_SUPPORTED=1)
> >>  if test "x$GCC_ATOMIC_BUILTINS_SUPPORTED" = x1; then
> >> _______________________________________________
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> >> mesa-dev at lists.freedesktop.org
> >> https://lists.freedesktop.org/mailman/listinfo/mesa-dev