https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109701
Bug ID: 109701
Summary: I have a MWE where an omp reduction breaks if I add
the option for GPU offloading (even if it isn't used).
Product: gcc
Version: 12.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: fortran
Assignee: unassigned at gcc dot gnu.org
Reporter: thomas.meltzer1 at gmail dot com
Target Milestone: ---
Created attachment 54972
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=54972=edit
source code to reproduce bug
I think I have identified a bug when using gfortran and openMP. I have tested
on gfortran versions:
* gfortran 10.3.0
* gfortran 11.3.0
* gfortran 12.2.0
I have posted a question on stackoverflow:
https://stackoverflow.com/questions/76119137/potential-gfortran-or-openmp-bug-when-using-omp-if-and-reduction
Here is mwe:
---
program test
use omp_lib
implicit none
integer, parameter :: N=3
integer:: i, j
real :: a(N,N), b(N,N), max_diff
logical:: is_GPU
is_GPU = .false.
#ifdef USEGPU
is_GPU = .true.
#endif
!$omp target data if(is_GPU) map(to:a, b)
!$omp target teams if(is_GPU)
!$omp distribute parallel do simd collapse(2)
do j = 1, N
do i = 1, N
a(i, j) = i*j
b(i, j) = i*j*0.9
end do
end do
!$omp end target teams
max_diff = 0.0
!$omp target teams if(is_GPU) !< comment this
!$omp distribute parallel do simd reduction(max:max_diff) collapse(2)
do j = 1, N
do i = 1, N
max_diff = max(max_diff, abs(b(i, j) - a(i, j)))
end do
end do
!$omp end target teams !< comment this
write (*,'("max_diff = ", F6.3)') max_diff
!$omp end target data
end program
---
Here is the command to compile and run:
gfortran -cpp -fopenmp mwe.f90 && OMP_NUM_THREADS=2 ./a.out
I have also tried with extra flags (-Wall -Wextra) and there are no reported
warnings.
Expected output is:
max_diff = 0.900
but with gfortran I get:
max_diff = 0.000
It works with nvfortran 22.5-0 (from nvhpc toolkit) but not for gfortran.
Command for nvfortran is:
nvfortran -cpp -mp=multicore mwe.f90 && OMP_NUM_THREADS=2 ./a.out
I want to keep portability so that openMP handles whether I build with GPU or
not. I am aware I can workaround it without openMP "if" statements and instead
use pre-processor directives.
If I comment out the lines marked with (!< comment this) and remove
"distribute" from the line "!$omp distribute parallel do simd
reduction(max:max_diff) collapse(2)" then the code runs as expected.
Am I mis-using the openMP if statements or doing something else which is not
portable or is this a bug?
Please let me know if you need any further information.
gfortran -v 11.3.0 output:
Using built-in specs.
COLLECT_GCC=gfortran
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/11/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
11.3.0-1ubuntu1~22.04' --with-bugurl=file:///usr/share/doc/gcc-11/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,m2 --prefix=/usr
--with-gcc-major-version-only --program-suffix=-11
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --enable-bootstrap --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib
--enable-libphobos-checking=release --with-target-system-zlib=auto
--enable-objc-gc=auto --enable-multiarch --disable-werror --enable-cet
--with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32
--enable-multilib --with-tune=generic
--enable-offload-targets=nvptx-none=/build/gcc-11-xKiWfi/gcc-11-11.3.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-11-xKiWfi/gcc-11-11.3.0/debian/tmp-gcn/usr
--without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
--with-build-config=bootstrap-lto-lean --enable-link-serialization=2
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 11.3.0 (Ubuntu 11.3.0-1ubuntu1~22.04)
gfortran -v 12.2.0 output:
Reading specs from
/software/spack/opt/spack/linux-ubuntu22.04-skylake/gcc-11.3.0/gcc-12.2.0-7szeaw2tk7ndv3brjeitsqmi3r6cz2sx/lib/gcc/x86_64-pc-linux-gnu/12.2.0/specs
COLLECT_GCC=gfortran
COLLECT_LTO_WRAPPER=/software/spack/opt/spack/linux-ubuntu22.04-skylake/gcc-11.3.0/gcc-12.2.0-7szeaw2tk7ndv3brjeitsqmi3r6cz2sx/libexec/gcc/x86_64-pc-linux-gnu/12.2.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with:
