将c映射到程序集

我以为我可能有一些无意的未定义行为(使用ubsan编译删除了意外的行为)所以我试图简化程序,发现一个特定的函数似乎导致行为的差异.

现在,我将程序集映射回c以查看它出错的地方,尝试确定为什么会发生这种情况,并且有一些部分我难以映射回来.

Godbolt link

C ：

#include <atomic>
#include <cstdint>
#include <cstdlib>
#include <thread>
#include <cstdio>

enum class FooState { A,B };

struct Foo {
  std::atomic<std::int64_t> counter{0};
  std::atomic<std::int64_t> counter_a{0};
  std::atomic<std::int64_t> counter_b{0};
};

//__attribute__((noinline))
FooState to_state(const std::int64_t c) {
  return c >= 0 ? FooState::A : FooState::B;
}

static const int NUM_MODIFIES = 100;

int value_a = 0,value_b = 0;
Foo foo;
std::atomic<std::int64_t> total_sum{0};

void test_function() {
  bool done = false;
  while (!done) {
    const std::int64_t count =
        foo.counter.fetch_add(1,std::memory_order_seq_cst);
    const FooState state = to_state(count);

    int &val = FooState::A == state ? value_a : value_b;
    if (val == NUM_MODIFIES) {
      total_sum += val;
      done = true;
    }

    std::atomic<std::int64_t> &c =
        FooState::A == state ? foo.counter_a : foo.counter_b;
    c.fetch_add(1,std::memory_order_seq_cst);
  }
}

部件：

test_function():                     # @test_function()
        test    rax,rax
        setns   al
        lock
        inc     qword ptr [rip + foo]
        mov     ecx,value_a
        mov     edx,value_b
        cmovg   rdx,rcx
        cmp     dword ptr [rdx],100
        je      .LBB1_3
        mov     ecx,foo+8
        mov     edx,value_a
.LBB1_2:                                # =>This Inner Loop Header: Depth=1
        test    al,1
        mov     eax,foo+16
        cmovne  rax,rcx
        lock
        inc     qword ptr [rax]
        test    rax,rax
        setns   al
        lock
        inc     qword ptr [rip + foo]
        mov     esi,value_b
        cmovg   rsi,rdx
        cmp     dword ptr [rsi],100
        jne     .LBB1_2
.LBB1_3:
        lock
        add     qword ptr [rip + total_sum],100
        test    al,al
        mov     eax,foo+8
        mov     ecx,foo+16
        cmovne  rcx,rax
        lock
        inc     qword ptr [rcx]
        ret

我发现将to_state标记为noinline或者将done更改为全局似乎“修复”了意外行为.

我看到的意外行为是,当计数器为> = 0时,则应增加counter_a,否则应增加counter_b.从我所知,有时候这种情况并没有发生,但确切地确定何时/为何困难.

我可以使用一些帮助的组件的一部分是测试rax,rax; setns al和测试al,1部分.似乎初始测试不会确定性地设置,然后该值用于确定要递增的计数器,但也许我误解了一些东西.

下面是一个演示此问题的小例子.它通常在使用clang 3.9和-O2编译时永远挂起,否则运行完成.

#include <atomic>
#include <cstdint>
#include <cstdlib>
#include <thread>
#include <cstdio>

enum class FooState { A,B };

struct Foo {
  std::atomic<std::int64_t> counter{0};
  std::atomic<std::int64_t> counter_a{0};
  std::atomic<std::int64_t> counter_b{0};
};

//__attribute__((noinline))
FooState to_state(const std::int64_t c) {
  return c >= 0 ? FooState::A : FooState::B;
}

//__attribute__((noinline))
FooState to_state2(const std::int64_t c) {
  return c >= 0 ? FooState::A : FooState::B;
}

static const int NUM_MODIFIES = 100;

int value_a = 0,std::memory_order_seq_cst);
  }
}

int main() {
  std::thread thread = std::thread(test_function);

  for (std::size_t i = 0; i <= NUM_MODIFIES; ++i) {
    const std::int64_t count =
        foo.counter.load(std::memory_order_seq_cst);
    const FooState state = to_state2(count);

    unsigned log_count = 0;

    auto &inactive_val = FooState::A == state ? value_b : value_a;
    inactive_val = i;

    if (FooState::A == state) {
      foo.counter_b.store(0,std::memory_order_seq_cst);
      const auto accesses_to_wait_for =
          foo.counter.exchange((std::numeric_limits<std::int64_t>::min)(),std::memory_order_seq_cst);
      while (accesses_to_wait_for !=
             foo.counter_a.load(std::memory_order_seq_cst)) {
        std::this_thread::yield();

        if(++log_count <= 10) {
          std::printf("#1 wait_for=%ld,val=%ldn",accesses_to_wait_for,foo.counter_a.load(std::memory_order_seq_cst));
        }
      }
    } else {
      foo.counter_a.store(0,std::memory_order_seq_cst);

      auto temp = foo.counter.exchange(0,std::memory_order_seq_cst);
      std::int64_t accesses_to_wait_for = 0;
      while (temp != INT64_MIN) {
        ++accesses_to_wait_for;
        --temp;
      }

      while (accesses_to_wait_for !=
             foo.counter_b.load(std::memory_order_seq_cst)) {
        std::this_thread::yield();

        if (++log_count <= 10) {
          std::printf("#2 wait_for=%ld,foo.counter_b.load(std::memory_order_seq_cst));
        }
      }
    }

    std::printf("modify #%lu completen",i);
  }

  std::printf("modifies completen");

  thread.join();

  const std::size_t expected_result = NUM_MODIFIES;
  std::printf("%sn",total_sum == expected_result ? "ok" : "fail");
}