[llvm-bugs] [Bug 28249] New: Missing warning about uninitialized var when going through a pointer

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=28249

            Bug ID: 28249
           Summary: Missing warning about uninitialized var when going
                    through a pointer
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: ch3root at openwall.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Source code:

----------------------------------------------------------------------
#include <stdio.h>

int main()
{
  int i;
  printf("%d\n", *(int *)&i);
}
----------------------------------------------------------------------

Results:

----------------------------------------------------------------------
$ clang -std=c11 -Weverything -O3 test.c && ./a.out
-1069783944
----------------------------------------------------------------------

clang version: clang version 3.9.0 (trunk 271312)

For comparison:

----------------------------------------------------------------------
$ gcc -std=c11 -pedantic -Wall -Wextra -O3 test.c && ./a.out
test.c: In function ‘main’:
test.c:6:3: warning: ‘i’ is used uninitialized in this function
[-Wuninitialized]
   printf("%d\n", *(int *)&i);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~
0
----------------------------------------------------------------------

gcc version: gcc (GCC) 7.0.0 20160616 (experimental)

The optimizer sees that an undef value is used:

----------------------------------------------------------------------
*** IR Dump After SROA ***
; Function Attrs: nounwind uwtable
define i32 @main() #0 {
entry:
  %call = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([4 x i8], [4
x i8]* @.str, i32 0, i32 0), i32 undef)
  ret i32 0
}
----------------------------------------------------------------------

but nevertheless a warning is not emitted.

The code in the testcase is useless but the same happens when a representation
of an uninitialized variable is accessed, i.e. there is no warning for `*(char
*)&i`.

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