[Bug c++/56420] New: Arithmetic error in computation with compile time unsigned __int128 constant

[st at quanttec dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56420



             Bug #: 56420

           Summary: Arithmetic error in computation with compile time

                    unsigned __int128 constant

    Classification: Unclassified

           Product: gcc

           Version: unknown

            Status: UNCONFIRMED

          Severity: normal

          Priority: P3

         Component: c++

        AssignedTo: unassig...@gcc.gnu.org

        ReportedBy: s...@quanttec.com





With GCC 4.8 (x86-64, snapshot 20130203 on Linux) the following simple test

case demonstrates an arithmetic bug for code involving a static unsigned

__int128 constant. This seems to be a regression from GCC 4.7.





#include <iostream>



typedef unsigned __int128 uint128_t;



static const uint128_t c = ((uint128_t)-1ULL << 64) + (1ULL << 63);



int main() {

    // use volatile to prevent optimizer from optimizing the test

    volatile uint128_t a = (uint128_t)1 << 127;

    volatile uint128_t b = 1;



    volatile uint128_t cc = c;



    uint128_t result1 = a - b*c;

    uint128_t result2 = a - b*cc;



    if (result1 != result2) {

        std::cout << "Error: results differ.\n";

        return 1;

    } else {

        std::cout << "No error.\n";

        return 0;

    }

}