why am I not getting an “used uninitialized” warning from gcc in this trivial example? [duplicate]

Question

This question already has an answer here:

• Why is there not any warning on a declaration without initialization in a for loop? 1 answer

Once again a stupid uninitialized variable error in How to fix this segmentation error in a sequence inverting program?.

So I was going to repeat the "please use -Wall flags" comment, but when I tested the code against warnings, I found no warnings reported to my great surprise.

So I trimmed it down to this below (this code makes no sense for execution purposes but it illustrates what I want to show):

#include <stdio.h>

int main()
{

  int i,len=12;

  /* printf("%d\n",i); */

  while(i!=len-1)
  {

    i++;
    len--;
  }

 return 0;
 }

when compiling it using gcc 4.7.3 and 6.2.1 using

gcc -Wall -Wextra -pedantic

I get no warnings, whereas i is blatantly not initialized before using in the while loop.

Now if I uncomment the printf statement I get:

warning: 'i' is used uninitialized in this function [-Wuninitialized]

So why is the warning issued when passing i to printf but not in the while test?

(It's different of gcc failing to warn of uninitialized variable because in my case, there's are no branches)

(Sounds like a bug, but it's so trivial that I wonder if I'm not missing something huge.)

Answer 1

It's hard to say it's a bug, because gcc mixes up code for optimization and for creating warnings, which is even documented for this particular warning:

-Wuninitialized
Warn if an automatic variable is used without first being initialized or if a variable may be clobbered by a setjmp call. [...]
Because these warnings depend on optimization, the exact variables or elements for which there are warnings depends on the precise optimization options and version of GCC used.

(from GCC docs Options to Request or Suppress Warnings, emphasis mine)

You found an IMHO very silly case here. Try -O1 and you'll get an unexpected warning:

warn.c: In function ‘main’:
warn.c:13:6: warning: ‘i’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     i++;
      ^

So, gcc still misses the first uninitialized use, but finds the second one! Try -O0 or -O2 and the warning is again gone...

You could still try to file a bug about this. Note clang gets it right:

warn.c:10:9: warning: variable 'i' is uninitialized when used here
      [-Wuninitialized]
  while(i!=len-1)
        ^
warn.c:6:8: note: initialize the variable 'i' to silence this warning
  int i,len=12;
       ^
        = 0
1 warning generated.