https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86880
Bug ID: 86880 Summary: Incorrect mersenne_twister_engine equality comparison between rotated states Product: gcc Version: 9.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: libstdc++ Assignee: unassigned at gcc dot gnu.org Reporter: hstong at ca dot ibm.com Target Milestone: --- mersenne_twister_engines whose textual representation should be equal should also compare equal. The following program has two engines with initial states |2 1 2 and |3 1 2. The engine with state |3 1 2 transitions to states 0 |1 2, 0 2 |2, |0 2 1, and 2 |2 1. The textual representation of states |2 1 2 and 2 |2 1 are both specified to be "2 1 2". That is, adjusted by rotation, the states are the same. The engines are compared while in those states. The comparison fails with libstdc++. Online compiler: https://wandbox.org/permlink/RqdbiJZk6gVN0ici Note: libstdc++ does not produce "2 1 2" as the textual representation (as reported by PR 60441). === SOURCE (<stdin>): #include <random> #ifdef NDEBUG #undef NDEBUG #endif #include <cassert> #ifdef TRACE #include <iostream> #undef TRACE #define TRACE( X ) do { X } while(0) #else #define TRACE( X ) // #endif int main(void) { typedef unsigned int EngResType; constexpr size_t word_size = 3u; constexpr size_t state_size = 3u; constexpr size_t shift_size = 1u; constexpr size_t tempering_l = word_size; using namespace std; using EngineType = std::mersenne_twister_engine< EngResType, word_size, state_size, shift_size, 0u, 0x0, 0u, 0x0, 0u, 0x0, 0u, 0x0, tempering_l, 0>; EngineType mteA(0b010), mteB(0b011); TRACE( cout << "Engine A: " << mteA << endl; cout << "Engine B: " << mteB << "\n" << endl; ); for (int i = 0; i < 4; ++i) { mteB(); TRACE( cout << "Engine B: " << mteB << endl; ); } TRACE( cout << "\nEngine A: " << mteA << endl; cout << "Engine B: " << mteB << endl; ); assert(mteA == mteB); } === COMPILER INVOCATION: g++ -std=c++17 -pedantic-errors -Wall -Wextra -Werror -xc++ - -o ./a.out === RUN SCRIPT: ./a.out === EXPECTED RUN OUTPUT: (Exits with rc=0). === ACTUAL RUN OUTPUT: a.out: <stdin>:47: int main(): Assertion `mteA == mteB' failed. === COMPILER VERSION INFO: Using built-in specs. COLLECT_GCC=/opt/wandbox/gcc-head/bin/g++ COLLECT_LTO_WRAPPER=/opt/wandbox/gcc-head/libexec/gcc/x86_64-pc-linux-gnu/9.0.0/lto-wrapper Target: x86_64-pc-linux-gnu Configured with: ../source/configure --prefix=/opt/wandbox/gcc-head --enable-languages=c,c++ --disable-multilib --without-ppl --without-cloog-ppl --enable-checking=release --disable-nls --enable-lto LDFLAGS=-Wl,-rpath,/opt/wandbox/gcc-head/lib,-rpath,/opt/wandbox/gcc-head/lib64,-rpath,/opt/wandbox/gcc-head/lib32 Thread model: posix gcc version 9.0.0 20180805 (experimental) (GCC)