Illegal context for vector clause in simple OpenACC kernel - openacc

Question

I'm trying to compile a simple OpenACC benchmark:
void foo(const float * restrict a, int a_stride, float * restrict c, int c_stride) {
#pragma acc parallel copyin(a[0:a_stride*256]) copyout(c[0:c_stride*256])
#pragma acc loop vector(128)
{
for (int i = 0; i < 256; ++i) {
float sum = 0;
for (int j = 0; j < 256; ++j) {
sum += *(a + a_stride * i + j);
}
*(c + c_stride * i) = sum;
}
}
}
with Nvidia HPC SDK 21.5 and run into an error
$ nvc++ -S tmp.cc -Wall -Wextra -O2 -acc -acclibs -Minfo=all -g -gpu=cc80
NVC++-S-0155-Illegal context for gang(num:) or worker(num:) or vector(length:) (tmp.cc: 7)
NVC++/x86-64 Linux 21.5-0: compilation completed with severe errors
Any idea what may cause this? From what I can tell my syntax for vector(128) is legal.

Answer 1

It's illegal OpenACC syntax to use "vector(value)" with a parallel construct. You need to use a "vector_length" clause on the parallel directive to define the vector length. The reason is because "parallel" defines a single compute region to be offloaded and hence all vector loops in this region need to have the same vector length.
You can use "vector(value)" only with a "kernels" construct since the compiler can then split the region into multiple kernels each having a different vector length.
Option 1:
% cat test.c
void foo(const float * restrict a, int a_stride, float * restrict c, int c_stride) {
#pragma acc parallel vector_length(128) copyin(a[0:a_stride*256]) copyout(c[0:c_stride*256])
#pragma acc loop vector
{
for (int i = 0; i < 256; ++i) {
float sum = 0;
for (int j = 0; j < 256; ++j) {
sum += *(a + a_stride * i + j);
}
*(c + c_stride * i) = sum;
}
}
}
% nvc -acc -c test.c -Minfo=accel
foo:
4, Generating copyout(c[:c_stride*256]) [if not already present]
Generating copyin(a[:a_stride*256]) [if not already present]
Generating Tesla code
5, #pragma acc loop vector(128) /* threadIdx.x */
7, #pragma acc loop seq
5, Loop is parallelizable
7, Loop is parallelizable
Option 2:
% cat test.c
void foo(const float * restrict a, int a_stride, float * restrict c, int c_stride) {
#pragma acc kernels copyin(a[0:a_stride*256]) copyout(c[0:c_stride*256])
#pragma acc loop independent vector(128)
{
for (int i = 0; i < 256; ++i) {
float sum = 0;
for (int j = 0; j < 256; ++j) {
sum += *(a + a_stride * i + j);
}
*(c + c_stride * i) = sum;
}
}
}
% nvc -acc -c test.c -Minfo=accel
foo:
4, Generating copyout(c[:c_stride*256]) [if not already present]
Generating copyin(a[:a_stride*256]) [if not already present]
5, Loop is parallelizable
Generating Tesla code
5, #pragma acc loop gang, vector(128) /* blockIdx.x threadIdx.x */
7, #pragma acc loop seq
7, Loop is parallelizable