(Optimization?) Bug regarding GCC std::thread

问题

While testing some functionality with std::thread, a friend encountered a problem with GCC and we thought it's worth asking if this is a GCC bug or perhaps there's something wrong with this code (the code prints (for example) "7 8 9 10 1 2 3", but we expect every integer in [1,10] to be printed):

#include <algorithm>
#include <iostream>
#include <iterator>
#include <thread>

int main() {
    int arr[10];
    std::iota(std::begin(arr), std::end(arr), 1);
    using itr_t = decltype(std::begin(arr));

    // the function that will display each element
    auto f = [] (itr_t first, itr_t last) {
        while (first != last) std::cout<<*(first++)<<' ';};

    // we have 3 threads so we need to figure out the ranges for each thread to show
    int increment = std::distance(std::begin(arr), std::end(arr)) / 3;
    auto first    = std::begin(arr);
    auto to       = first + increment;
    auto last     = std::end(arr);
    std::thread threads[3] = {
        std::thread{f, first, to},
        std::thread{f, (first = to), (to += increment)},
        std::thread{f, (first = to), last} // go to last here to account for odd array sizes
    };
    for (auto&& t : threads) t.join();
}

The following alternate code works:

int main()
{
    std::array<int, 10> a;
    std::iota(a.begin(), a.end(), 1);
    using iter_t = std::array<int, 10>::iterator;
    auto dist = std::distance( a.begin(), a.end() )/3;
    auto first = a.begin(), to = first + dist, last = a.end();
    std::function<void(iter_t, iter_t)> f =
        []( iter_t first, iter_t last ) {
            while ( first != last ) { std::cout << *(first++) << ' '; }
        };
    std::thread threads[] {
            std::thread { f,  first, to },
            std::thread { f, to, to + dist },
            std::thread { f, to + dist, last }
    };
    std::for_each(
        std::begin(threads),std::end(threads),
        std::mem_fn(&std::thread::join));
    return 0;
}

We thought maybe its got something to do with the unsequenced evaluation of function's arity or its just the way std::thread is supposed to work when copying non-std::ref-qualified arguments. We then tested the first code with Clang and it works (and so started to suspect a GCC bug).

Compiler used: GCC 4.7, Clang 3.2.1

EDIT: The GCC code gives the wrong output with the first version of the code, but with the second version it gives the correct output.

回答1:

From this modified program:

#include <algorithm>
#include <iostream>
#include <iterator>
#include <thread>
#include <sstream>

int main()
{
    int arr[10];
    std::iota(std::begin(arr), std::end(arr), 1);
    using itr_t = decltype(std::begin(arr));

    // the function that will display each element
    auto f = [] (itr_t first, itr_t last) {
        std::stringstream ss;
        ss << "**Pointer:" << first  << " | " << last << std::endl;
        std::cout << ss.str();
        while (first != last) std::cout<<*(first++)<<' ';};

    // we have 3 threads so we need to figure out the ranges for each thread to show
    int increment = std::distance(std::begin(arr), std::end(arr)) / 3;
    auto first    = std::begin(arr);
    auto to       = first + increment;
    auto last     = std::end(arr);
    std::thread threads[3] = {
        std::thread{f, first, to},
#ifndef FIX
        std::thread{f, (first = to), (to += increment)},
        std::thread{f, (first = to), last} // go to last here to account for odd array sizes
#else
        std::thread{f,  to,  to+increment},
        std::thread{f,  to+increment, last} // go to last here to account for odd array sizes
#endif
    };
    for (auto&& t : threads) {
        t.join();
    }
}

I add the prints of the first and last pointer for lambda function f, and find this interesting results (when FIX is undefined):

**Pointer:0x28abd8 | 0x28abe4
1 2 3 **Pointer:0x28abf0 | 0x28abf0
**Pointer:0x28abf0 | 0x28ac00
7 8 9 10

Then I add some code for the #ELSE case for the #ifndef FIX. It works well.

- Update: This conclusion, the original post below, is wrong. My fault. See Josh's comment below -

I believe the 2nd line std::thread{f, (first = to), (to += increment)}, of threads[] contains a bug: The assignment inside the two pairs of parenthesis, can be evaluated in any order, by the parser. Yet the assignment order of 1st, 2nd and 3rd argument of the constructor needs to keep the order as given.

--- Update: corrected ---

Thus the above debug printing results suggest that GCC4.8.2 (my version) is still buggy (not to say GCC4.7), but GCC 4.9.2 fixes this bug, as reported by Maxim Yegorushkin (see comment above).

来源：https://stackoverflow.com/questions/25115189/optimization-bug-regarding-gcc-stdthread

标签

multithreading

c++11

clang

gcc4.7