[LLVMbugs] [Bug 8670] Optimization causes incorrect output.

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=8670

Chris Lattner <clattner at apple.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |INVALID

--- Comment #1 from Chris Lattner <clattner at apple.com> 2010-11-22 23:19:20 CST ---
It's not obvious, but this is because your program has undefined behavior. 
This line:


    struct IntContainer *a =
          (void *)((char *)(list->next) -
                   ((char *)&a->link - (char *)(a)));

contains a dereference of a->link before a is set.  Since a contains an
uninitialized variable, a->link and a are not guaranteed to have related
values.

This is more complex but equivalent to:

int a;
print(a-a)

which is not guaranteed to print zero.

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