g++ compiler: optimization flag adds warning message

Question

I noticed this interesting behaviour of the g++ compiler, if I add a -O3 flag to the compiler, I get

otsu.cpp:220: warning: ‘x’ may be used uninitialized in this function

However, when I do not use optimization and instead use a debug flag -g I got no warnings at all. Now, I trust the compiler more when the -g flag is on; however, I'm wondering if this is well defined behaviour that should be expected?

For clarity, the code that causes this is something along these lines:

int x; //uninitialized


getAValueForX( &x ); // function makes use of x,
                     // but x is unitialized

where

 void getAValueForX( int *x )
 {
     *x = 4;
 }

or something along those lines, obviously more complex.

Answer 1

That's expected. The optimizations cause a specific code analysis to run and that's how gcc finds the un-initialized variables. It's in the manual page:

. . . these warnings depend on optimization

http://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html

Answer 2

This is actually very common with gcc. And yes, this is to be expected.

As far as I understood, in order to optimize, the compiler generates a lot of metrics and transform the code (or, more precisely, the representation it has of the code) in a way that allows the detection of uninitialized or unused variables for examples (there are a few other warnings like that, I don't remember the list).

Doing the same thing without optimization would require doing, and then scrapping away all this analysis. This would slow down compilation significantly for no good purpose (especially as, in debug, the compiler is not suppose to rearrange the code).