OpenMP tasking issues with Intel compilers (and clang) - clang

Question

The code below shows problems with with OpenMP tasking in ICL 2021.6.0 and in ICX 2022.1.0 (Clang based)
Firstly, I am wondering if I am doing something fundamentally wrong in my OpenMP code and it is just showing up differently when compiled by different compilers.
Assuming the code is valid OpenMP...
When the function fails_intel_icl() runs under ICL, the task execution is just wrong. Some task are run twice, some not at all. Compiled by ICX/Clang it executes as I expect.
When crash_icx_2022() is compiled under ICX it just crashes at runtime. I am testing using Visual Studio 20222/Debug/x64 and latest OneAPI Base and HPC installation.
Examples of incorrect runtime behaviour of the function fails_intel_icl() when compiled with ICL is as follows
Thread:12 launching task for 0,1 <--- you will note the task for pair 0,1 never runs.
Thread:12 launching task for 0,2
Thread:9 Executing task with pair 0,2
....
#include <iostream>
#include <vector>
#include <omp.h>
std::vector<std::pair<int, std::vector<int>>> data;
void setup()
{
std::vector<int> tmp({ 1,2,3,4,5 });
for (int i = 0; i < 5; i++)
{
data.push_back({ i,tmp });
}
}
void DoTask(int a, int b)
{
{
#pragma omp critical
std::cout << "Thread:" << omp_get_thread_num() << " Executing task with pair " << a << ',' << b << std::endl;
}
}
// runs correctly under icl, but crashes at runtime with icx and clang
void crash_icx_2022()
{
# pragma omp parallel
{
# pragma omp single
{
for (auto iter = data.begin(); iter != data.end(); ++iter)
{
const auto& a = iter->first;
const auto& b = iter->second;
for (const auto& aa : b)
{
if (aa != a)
{
{
#pragma omp critical
std::cout << "Thread:" << omp_get_thread_num() << " launching task for " << ' ' << a << ',' << aa << std::endl;
}
# pragma omp task
{
DoTask(a, aa);
}
}
}
}
}
}
}
// this compiles and runs incorrectly under icl but runs correctly with icx or clang
void fails_intel_icl()
{
# pragma omp parallel
{
# pragma omp single
{
for (auto iter = data.begin(); iter != data.end(); ++iter)
{
const auto a = iter->first;
const auto b = iter->second;
for (const auto aa : b)
{
if (aa != a)
{
{
#pragma omp critical
std::cout << "Thread:" << omp_get_thread_num() << " launching task for " << ' ' << a << ',' << aa << std::endl;
}
# pragma omp task
{
DoTask(a, aa);
}
}
}
}
}
}
}
void testTaskingBug()
{
setup();
std::cout << "\nStarting test using copies\n" << std::endl;
fails_intel_icl();
std::cout << "\nStarting test using references" << std::endl;
crash_icx_2022();
}
int main()
{
testTaskingBug();
return 0;
}
The following C++17 code will not compile under clang. Not sure if the error is real.
void clang_wont_compile()
{
# pragma omp parallel
{
# pragma omp single
{
for (const auto& [a, b] : data)
{
for (const auto& aa : b)
{
if (aa != a)
{
# pragma omp task
DoTask(a, aa);
}
}
}
}
}
}

Answer 1

thanks for pointing this out. It does look like it should be valid OMP code. Maybe something on the backend with the task + critical which is throwing off the compiler and/or if it was not allowed per the spec but doesn’t seem to be the case.
Double checking with some OpenMP folks to see if we have a bug on this (or a better explanation as to the behavior).

Answer 2

So after more investigation I seem to have answers
the OpenMP code is valid and all variations of the functions should
run correctly
icl (intel classic) and icx (clang based) have some bugs as of the versions I have tested with
A newer clang compiler I able to test with (14.0.6) has
resolved the issues and executes correctly.