Undefined symbol when -polly-parallel is enabled

[llvmbot]

Bugzilla Link	38493
Version	unspecified
OS	Windows NT
Reporter	LLVM Bugzilla Contributor
CC	@Meinersbur

Extended Description

When automatic parallelization using polly is applied, a linker outputs an error: undefined symbol: GOMP_*.

-------- test.c --------

include

include

define N 300

define M 4

double a[N][M]; double b[N][M]; double c[N][M];

void foo(int m) { int i,j; for (j=0; j<N; ++j) { for (i=0; i<m; ++i) { a[j][i] += b[j][i]*c[j][i]; } } }

double suma(void) { int i,j; double s; s = 0.0; for (j=0; j<N; ++j) { for (i=0; i<M; ++i) { s += a[j][i]; } } return s; }

define ANS (double)300.0000

define GOSA (double)0.0

int main(void) { int i,j; double s; for (j=0; j<N; ++j) { for (i=0; i<M; ++i) { a[j][i] = 0.0; b[j][i] = 0.5; c[j][i] = 0.5; } } foo(M); s = suma(); if (fabs(s - ANS) <= GOSA) { printf("ok\n"); } else { printf("ng: %lf != %lf\n", s, ANS); } }

$ clang --version clang version 8.0.0 (trunk 339303) Target: x86_64-unknown-linux-gnu

$ clang -O3 -mllvm -polly -mllvm -polly-parallel test.c ld: error: undefined symbol: GOMP_parallel_loop_runtime_start

referenced by 3-P-275_01.c /home/takahiro/tmp/3-P-275_01-0c2466.o:(foo)

ld: error: undefined symbol: GOMP_parallel_end

referenced by 3-P-275_01.c /home/takahiro/tmp/3-P-275_01-0c2466.o:(foo)

ld: error: undefined symbol: GOMP_loop_runtime_next

referenced by 3-P-275_01.c /home/takahiro/tmp/3-P-275_01-0c2466.o:(foo_polly_subfn)

ld: error: undefined symbol: GOMP_loop_runtime_next

referenced by 3-P-275_01.c /home/takahiro/tmp/3-P-275_01-0c2466.o:(foo_polly_subfn)

ld: error: undefined symbol: GOMP_loop_end_nowait

referenced by 3-P-275_01.c /home/takahiro/tmp/3-P-275_01-0c2466.o:(foo_polly_subfn)

But, If I added -fopenmp option to the above options, the linker didn't output the error and compilation succeeded. So, when -polly-parallel option is specified, should I need to specify -fopenmp option? Or, should clang pass "-lomp" to the linker implicitly?

[Meinersbur]

The only possible fix would be to add a compiler flags such as -fpolly-parallel to clang that would add -mllvm -polly -mllvm -polly-parallel as well as an OpenMP runtime.

[Meinersbur]

clang, the driver component of the frontend, does not know that Polly is generating calls to an external library. Hence, some command line argument has to be passed to tell the driver to pass the OpenMP runtime library to the linker. -fopenmp, -lgomp or -fomp should do.