g++ “warning: iteration … invokes undefined behavior” for Seemingly Unrelated Variable

问题

Consider the following code in strange.cpp:

#include <vector> 


using namespace std;


int i = 0;


int *bar()
{   
    ++i;
    return &i; 
}   


int main()
{   
    for(size_t j = 0; j < 99999999999; ++j) // (*)
    {   
        const auto p = bar();
        if(!p) // (**)
            return -1; 
    }   
}

Compiling this with g++ gives a warning:

$ g++ --std=c++11 -O3 strange.cpp 
strange.cpp: In function ‘int main()’:
strange.cpp:12:12: warning: iteration 4294967296ul invokes undefined behavior [-Waggressive-loop-optimizations]
         ++i;
            ^
strange.cpp:19:9: note: containing loop
         for(size_t j = 0; j < 99999999999; ++j) // (*)
         ^

I don't understand why the increment invokes undefined behavior. Moreover, there are two changes, each of which makes the warning disappear:

changing the line (*) to for(int j...
changing the line (**) to if(!*p)

What is the meaning of this warning, and why are the changes relevant to it?

Note

$ g++ --version
g++ (Ubuntu 4.8.4-2ubuntu1~14.04) 4.8.4

回答1:

The increment is undefined because once i reaches std::numeric_limits<int>::max() (2³¹ - 1 on a 32-bit, LP64 or LLP64 platform), incrementing it will overflow, which is undefined behavior for signed integral types.

gcc is warning on iteration 4294967296ul (2³²) rather than iteration 2147483646u (2³¹) as you might expect, because it doesn't know the initial value of i; some other code might have run before main to set i to something other than 0. But once main is entered, no other code can run to alter i, and so once 2³² iterations have completed it will have at some point reached 2³¹ - 1 and overflowed.

"fixes" it by turning the controlling condition of the loop into a tautologically true expression; this makes the loop an infinite loop, since the if inside the loop will never execute, as &i cannot be a null pointer. Infinite loops can be optimized away, so gcc eliminates the body of the loop and the integer overflow of i does not occur.
"fixes" it by allowing gcc an out from the undefined behavior of integer overflow. The only way to prevent integer overflow is for i to have an initial value that is negative, such that at some point i reaches zero. This is possible (see above), and the only alternative is undefined behavior, so it must happen. So i reaches zero, the if inside the loop executes, and main returns -1.

来源：https://stackoverflow.com/questions/35299041/g-warning-iteration-invokes-undefined-behavior-for-seemingly-unrelated

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c++

g++

compiler-warnings