Nvc++ -O2: Segfault for *= in large std::complex<double> array

[simon.kollecker]

Hello everyone,

if I compile the following code with nvc++ segfaulttest.cpp -O2 -o segfaulttest and execute it, it first prints the address of the pointer (as expected) and then gives a segmentation fault.

#include <complex>
#include <iostream>

int main(){
    int size = 150000000;

    auto pointer = (std::complex<double> *) malloc(size * sizeof(std::complex<double>));

    // Make sure that allocation worked
    if (pointer)
    {
        std::cout << pointer << std::endl;
    }

    for (int ind =0; ind < size; ++ind)
    {
        pointer[ind] *= 5.;
    }
    
    free(pointer);
    return 0;
}

Any of the following actions resolves the issue:

• change nvc++ to g++
• reduce the optimization level to -O0 or -O1 (the problem occurs at -O2 and -O3)
• reduce the parameter size to less than about 130 million
• print the index every now and then, e.g. by adding if (ind % 1000000 == 0) std::cout << ind << std::endl; to the for loop
• write out the *=operator as pointer[ind] = pointer[ind] * 5.;, this was particularly surprising for me.

I tried it out on an Ubuntu 22.04 with HPC SDK version 22.9 and on a WSL2 Ubuntu 20.04 with HPC SDK version 22.5. Both show the same behavior.

Kind regards,
Simon

[MatColgrove]

Hi Simon,

It looks like the array is right at the 2GB boundary so will need 64-bit offsets. Hence try changing the “ind” variable to be a long instead of an int so the compiler knows to use 64-bits. You may also want to use the “-mcmodel=medium” flag if you go bigger.

While I’m not 100% sure why the segv doesn’t occur at lower opts, but my guess is that we’re doing more alignment. A segv won’t occur until after crossing a page boundary, so the alignment may be changing how the pages are used.

-Mat

[simon.kollecker]

Hi Mat, thank you very much for your reply! That solved the issue.