Re: -latomics vs riscv64

[Aurelien Jarno <aurelien@aurel32.net>]

On 2022-01-31 11:36, Jessica Clarke wrote:
> On 31 Jan 2022, at 08:37, Mathieu Malaterre <malat@debian.org> wrote:
> > 
> > [cc me please]
> > 
> > Dear riscv64 porters,
> > 
> > I am trying to get JPEG-XL to compile on riscv64. After reading of:
> > 
> > * https://github.com/riscv-collab/riscv-gcc/issues/12
> > 
> > I am under the impression that the black magic to get -latomic passed
> > on the linker line is handled somewhere in gcc spec file.
> > 
> > This also seems to be the case, per my reading of the log file at:
> > 
> > * https://buildd.debian.org/status/fetch.php?pkg=jpeg-xl&arch=riscv64&ver=0.7.0%7Egit20220120.0647da4%2Bds-3&stamp=1643404075&raw=0
> > 
> > The following c++11 code (1):
> > 
> > ```
> > #include <atomic>
> > #include <cstdint>
> > std::atomic<uint64_t> num (0);
> > int main() {
> >  uint64_t i = num.load(std::memory_order_relaxed);
> >  return 0;
> > }
> > ```
> > 
> > seems to compile and link nicely without the need for an explicit
> > `-latomic`. However somewhere later during the compilation, I receive
> > an error that:
> > 
> > /usr/bin/ld: libjxl_dec.so.0.7.0: undefined reference to `__atomic_fetch_and_1'
> > /usr/bin/ld: libjxl_dec.so.0.7.0: undefined reference to `__atomic_fetch_or_1'
> > 
> > So now it seems that I need an explicit `-latomic`
> > 
> > Could someone please share his knowledge on -latomic on riscv64 Debian
> > arch ? Please note that the above c++11 code rightfully fails to
> > compile on armel/mipsel and powerpc which allow the buildsystem to
> > detect the need for an explicit `-latomic` being added by cmake.
> 
> RISC-V only has 32-bit and 64-bit atomic instructions. GCC is supposed
> to convert smaller atomics to those larger ones via masking and
> shifting like LLVM, but it’s a known bug that it does not. This means
> anything that wants to use atomics on 1-byte or 2-byte types needs
> -latomic, but not 4-byte or 8-byte (though it does no harm).

It actually needs -latomic or -pthread. Unfortunately those days many
projects uses -lpthread instead of -pthread, just like cmake in that
case. -pthread is the correct way. The trick is to update the
CMakeLists.txt file to add:

  set(THREADS_PREFER_PTHREAD_FLAG ON)

before the call to find_package(Threads REQUIRED)

Regards
Aurelien

-- 
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
aurelien@aurel32.net                 http://www.aurel32.net