Studio 12.5 miscompiles std::atomic<int>::fetch_add

koval

Studio 12.5 C++ compiler generates incorrect code for fetch_add method in std::atomic

The problem occurs at optimization level -xO3 and higher

#include <atomic>std::atomic<int> counter = ATOMIC_VAR_INIT(0);int main(){    if (counter.fetch_add(1, std::memory_order_release) == 0)        counter.fetch_sub(1, std::memory_order_release);    return counter;}

The if generates following assembly:

{      514  } movl $1,%eax{      514  } lock xaddl %eax,counter{           } movl %eax,-12(%ebp) / sym=.CV0{      514  } jne .L77000030.57{      524  } .L77000028.58:{      524  } movl $-1,%eax{      524  } lock xaddl %eax,counter{      396  } .L77000030.57:{      396  } movl counter,%eax

The jump (514) suggests that the optimizer assumes xaddl setting flags for the original value (i.e. result of function fetch_add) which is wrong - xaddl sets the flags to represent result of addition, i.e. the new value

The bug causes QMutex class of Qt 5.9 to get into infinite loop, there is code which tests result of fetch_add against 0 like in my example.

Fedor-Oracle

> xaddl sets the flags to represent result of addition, i.e. the new value

Indeed, it should be doing something like:

    xaddl %eax, counter

    testl %eax, %eax

so current 12.5 behavior is surely a bug.

Studio 12.6 seems to be doing it right, so the bug must be fixed, though I cant tell exactly which bug fix it was.

You might want to switch to 12.6, as it has quite a share of C++11-related bug fixes.

regards,

  Fedor.

Steve.Clamage-Oracle

Your problem looks like bug 24312660, which was fixed in Studio 12.6.

If you have a Studio service contract, you could request a fix in a patch via  your support channel.

But as Fedor has already noted, upgrading to Studio 12.6 is likely a better solution.

koval

I am giving a try to 12.6, seems like this bug disappeared indeed

Alexgreen-Oracle

The CR which fixed that problem is - 25726453