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)
