Question

I’ve got the following code:

#include <iostream>  using namespace std;  int main(int argc, char *argv[]) {     string a = 'a';     for(unsigned int i=a.length()-1; i+1 >= 1; --i)     {         if(i >= a.length())         {             cerr << (signed int)i << '?' << endl;             return 0;         }     } } 

If I compile in MSVC with full optimizations, the output I get is ‘-1?’. If I compile in Debug mode (no optimizations), I get no output (expected.)

I thought the standard guaranteed that unsigned integers overflowed in a predictable way, so that when i = (unsigned int)(-1), i+1 = 0, and the loop condition i + 1 >= 1 fails. Instead, the test is somehow passing. Is this a compiler bug, or am I doing something undefined somewhere?

Answer 1

I remember having this problem in 2001. I’m amazed it’s still there. Yes, this is a compiler bug.

The optimiser is seeing

i + 1 >= 1; 

Theoretically, we can optimise this by putting all of the constants on the same side:

i >= (1-1); 

Because i is unsigned, it will always be greater than or equal to zero.

See this newsgroup discussion here.