omp with gcc and intel compiler - c++

According to this question, the use of threadprivate with openmp is
problematic. Here is a minimum (non-)working example of the problem:
#include"omp.h"
#include<iostream>
extern const int a;
#pragma omp threadprivate(a)
const int a=2;
void my_call(){
std::cout<<a<<std::endl;
};
int main(){
#pragma omp parallel for
for(unsigned int i=0;i<8;i++){
my_call();
}
}
This codes compiles with intel 15.0.2.164 but not with gcc 4.9.2-10.
gcc says:
g++ -std=c++11 -O3 -fopenmp -O3 -fopenmp test.cpp -o test
test.cpp:5:29: error: ‘a’ declared ‘threadprivate’ after first use
#pragma omp threadprivate(a)
I would be very happy to find a way to compile it with gcc.
Note: I know that global variables are a nightmare, but this example is the
coming from a code I haven't written and that I need to use... It's >11000
lines and I don't want to rewrite everything.

Related

OpenACC nvlink undefined reference to class

I am new to OpenACC and I am writing a new program from scratch (I have a fairly good idea what loops will be computationally costly from working in a similar problem before). I am getting an "Undefined reference" from nvlink. From my research, I found this is because no device code is being generated for the class I created. However, I don't understand why this is happening and how to fix it.
Below I send a MWE from my code.
include/vec1.h
#ifndef VEC1_H
#define VEC1_H
class Vec1{
public:
double data[1];
#pragma acc routine seq
Vec1();
#pragma acc routine seq
Vec1(double x);
#pragma acc routine seq
Vec1 operator* (double x);
};
#endif
src/vec1.cpp
#include "vec1.h"
Vec1::Vec1(){
data[0] = .0;
}
Vec1::Vec1(double x){
data[0] = x;
}
Vec1 Vec1::operator*(double c){
Vec1 r = Vec1(0.);
r.data[0] = c*data[0];
return r;
}
vec1_test_gpu.cpp
#include "vec1.h"
#define NUM_VECTORS 1000000
int main(){
Vec1 vec1_array[NUM_VECTORS];
for(int iv=0; iv<NUM_VECTORS; ++iv){
vec1_array[iv] = Vec1(iv);
}
#pragma acc data copyin(vec1_array)
#pragma acc parallel loop
for(int iv=0; iv<NUM_VECTORS; ++iv){
vec1_array[iv] = vec1_array[iv]*2;
}
return 0;
}
I compile them in the following way
$ nvc++ src/vec1.cpp -c -I./include -O3 -march=native -ta=nvidia:cuda11.2 -fPIC
$ nvc++ -shared -o libvec1.so vec1.o
$ nvc++ vec1_test_gpu.cpp -I./include -O3 -march=native -ta=nvidia:cuda11.2 -L./ -lvec1
The error message appears just after the last command and reads nvlink error : Undefined reference to '_ZN4Vec1mlEd' in '/tmp/nvc++jOtCBiT_m38d.o'
The problem here is that you're trying to call a device routine, "Vec1::operator*", that's contained in a shared object from a kernel in the main program. nvc++'s OpenACC implementation uses CUDA to target NVIDIA devices. Since CUDA doesn't have a dynamic linker for device code, at least not yet, this isn't supported.
You'll need to either link this statically, or move the "parallel loop" into the shared object.
Note that the "-ta" flag has been deprecated. Please consider using "-acc -gpu=cuda11.2" instead.

Why is there uninitialized value in std::normal_distribution

Although the example in std::normal_distribution reference compiled fine, I got a warning on uninitialized value in a similar code below.
#include <random>
void foo(double);
int main()
{
std::default_random_engine generator;
std::normal_distribution<double> norm_dist(0,1);
for(int i=0; i<3; i++)
foo(norm_dist(generator));
return 0;
}
When compiled with "g++ -std=c++11 -Wall -ffast-math -O -c foo.cpp" using gcc5.3.1 on my CentOS7.2, I got:
foo.cpp:8:30: warning:
‘norm_dist.std::normal_distribution::_M_saved’ may be used
uninitialized in this function [-Wmaybe-uninitialized]
foo(norm_dist(generator));
I thought STL had been well tested, but why does the compiler warn? Is the problem in g++, STL, or my code?

OpenMP 'unused variable' compiling error

i tried to write a c++ programm using openmp for parallelization. Unfortunately i get a compiling error which i dont understand. I have listed the g++ command, the problematic lines of code and the error message. If i missed to give important information please let me know.
g++ -o Pogramm -Wall -fopenmp Programm.cpp
#pragma omp parallel
int id,nths,tnbr;
id=omp_get_thread_num();
nths=omp_get_num_thread();
Tree.cpp:52:7: warning: unused variable ‘id’ [-Wunused-variable]
error: ‘id’ was not declared in this scope id=omp_get_thread_num();
Can someone tell me why 'id' ist not undeclared?
According to your code, the scope of the parallel region, which is the scope where you define id, only includes the subsequent line, i.e. the line where you define id. Therefore, when you use the id variable outside, you get an undefined variable error. Besides, you are also getting an unused id variable warning because you are not using it in the parallel region (where you could use it).
Most probably you just forgot to add the curly braces to enlarge the scope to be parallelized alltogether, i.e.
#pragma omp parallel
{
int id,nths,tnbr;
id=omp_get_thread_num();
nths=omp_get_num_thread();
...
}
A minimal working example is:
#include<iostream>
#include<omp.h>
using namespace std;
int main() {
#pragma omp parallel
{
int id,nths,tnbr;
id=omp_get_thread_num();
nths=omp_get_num_threads();
cout << "id, nths: " << id << nths << endl;
}
return 0;
}
This can be successfully compiled, e.g. using g++ v. 4.8.5
g++ main.cpp -fopenmp -Wall

Why do I get undefined behavior when using OpenMP's firstprivate with std::vector on Intel compiler?

I have a problem when using OpenMP in combination with firstprivate and std::vector on the Intel c++ compiler. Take the following three functions:
#include <omp.h>
void pass_vector_by_value(std::vector<double> p) {
#pragma omp parallel
{
//do sth
}
}
void pass_vector_by_value_and_use_firstprivate(std::vector<double> p) {
#pragma omp parallel firstprivate(p)
{
//do sth
}
}
void create_vector_locally_and_use_firstprivate() {
std::vector<double> p(3, 7);
#pragma omp parallel firstprivate(p)
{
//do sth
}
}
The code compiles without warnings doing:
icc filename.cpp -openmp -Wall -pedantic
(icc version 14.0.1 (gcc version 4.7.0 compatibility))
or:
g++ filename.cpp -fopenmp -Wall -pedantic
(gcc version 4.7.2 20130108 [gcc-4_7-branch revision 195012] (SUSE Linux))
but after compiling with icc I am getting runtime errors such as:
*** Error in `./a.out': munmap_chunk(): invalid pointer: 0x00007fff31bcc980 ***
when calling the second function (pass_vector_by_value_and_use_firstprivate)
So the error only occurs when the firstprivate clause is used (which should invoke the copy constructor) and the vector is passed by value to the function (which should invoke the copy constructor as well). When either not passing the vector but creating it locally in the function or not using firstprivate there is no error! On gcc I do not get any errors.
I am wondering if the code somehow produces undefined behavior or if this is a bug in icc ?
I get the same problem with ICC but not GCC. Looks like a bug. Here is a workaround
void pass_vector_by_value2(std::vector<double> p) {
#pragma omp parallel
{
std::vector<double> p_private = p;
//do sth with p_private
}
}
On the other hand, in general, I don't pass non-POD by value to functions anyway. I would use a reference but if you do that you get the error
error: ‘p’ has reference type for ‘firstprivate’
The solution to that is the code I posted above anyway. Pass it by value or by reference and then define a private copy inside the parallel region as I did in the code above.

OpenMP with restrict pointers fails with ICC while GCC/G++ succeeds

I implemented a simple matrix vector multiplication for sparse matrices in CRS using an implicit openMP directive in the multiplication loop.
The complete code is in GitHub: https://github.com/torbjoernk/openMP-Examples/blob/icc_gcc_problem/matxvec_sparse/matxvec_sparse.cpp
Note: It's ugly ;-)
To control the private and shared memory I'm using restrict pointers. Compiling it with GCC 4.6.3 on 64bit Linux works fine (besides two warnings about %u and unsigned int in a printf command, but that's not the point).
However, compiling it with ICC 12.1.0 on 64bit Linux failes with the error:
matxvec_sparse.cpp(79): error: "default_n_row" must be specified in a variable list at enclosing OpenMP parallel pragma
#pragma omp parallel \
^
with the definition of the variable and pointer in question
int default_n_row = 4;
int *n_row = &default_n_row;
and the openMP directive defined as
#pragma omp parallel \
default(none) \
shared(n_row, aval, acolind, arowpt, vval, yval) \
private(x, y)
{
#pragma omp for \
schedule(static)
for ( x = 0; x < *n_row; x++ ) {
yval[x] = 0;
for ( y = arowpt[x]; y < arowpt[x+1]; y++ ) {
yval[x] += aval[y] * vval[ acolind[y] ];
}
}
} /* end PARALLEL */
Compiled with g++:
c++ -fopenmp -O0 -g -std=c++0x -Wall -o matxvec_sparse matxvec_sparse.cpp
Compiled with icc:
icc -openmp -O0 -g -std=c++0x -Wall -restrict -o matxvec_sparse matxvec_sparse.cpp
Is it an error in usage of GCC/ICC?
Is this a design issue in my code causing undefined behaviour?
If so, which line(s) is/are causing it?
Is it just inconsistency between ICC and GCC?
If so, what would be a good way to achieve compiler independence and compatibility?
Huh. Looking at the code, it's clear what icpc thinks the problem is, but I'm not sure without going through the specification which compiler is doing the right thing here, g++ or icpc.
The issue isn't the restrict keyword; if you take all those out and lose the -restrict option to icpc, the problem remains. The issue is that you've got in that parallel section default(none) shared(n_row...), but n_row is, at the start of the program, a pointer to default_n_row. And icpc is requiring that default_n_row also be shared (or, at least, something) in that omp parallel section.