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From: "gil.hur at sf dot snu.ac.kr" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/65752] Too strong optimizations int -> pointer casts
Date: Tue, 19 May 2015 15:12:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65752

--- Comment #22 from Chung-Kil Hur <gil.hur at sf dot snu.ac.kr> ---
(In reply to Chung-Kil Hur from comment #21)
> (In reply to Marek Polacek from comment #20)
> > (In reply to Chung-Kil Hur from comment #19)
> > > (In reply to rguenther@suse.de from comment #18)
> > > > On Tue, 19 May 2015, gil.hur at sf dot snu.ac.kr wrote:
> > > > 
> > > > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65752
> > > > > 
> > > > > --- Comment #17 from Chung-Kil Hur <gil.hur at sf dot snu.ac.kr> ---
> > > > > Hi Richard,
> > > > > 
> > > > > I modified the example further.
> > > > > 
> > > > > #include <stdio.h>
> > > > > 
> > > > > int main() {
> > > > >   int x = 0;
> > > > >   uintptr_t xp = (uintptr_t) &x;
> > > > >   uintptr_t i, j;
> > > > > 
> > > > >   for (i = 0; i < xp; i++) { }
> > > > >   j = i;
> > > > >   /* The following "if" statement is never executed because j == xp */
> > > > >   if (j != xp) { 
> > > > >     printf("hello\n");
> > > > >     j = xp; 
> > > > >   }
> > > > 
> > > > Here j is always xp and thus ...
> > > > 
> > > 
> > > Why is "j" always "xp"?
> > > Since "hello" is not printed, "j = xp;" is not executed.
> > 
> > Because that "if (j != xp)" guarantees it.
> 
> OK. here is another modification.
> 
> #include <stdio.h>
> 
> int main() {
>   int x = 0;
>   uintptr_t xp = (uintptr_t) &x;
>   uintptr_t i, j;
> 
>   for (i = 0; i < xp; i++) { }
>   j = i;
> 
>   *(int*)j = 15;
> 
>   /* The following "if" statement is never executed because j == xp */
>   if (j != xp) { 
>     printf("hello\n");
>     j = xp; 
>   }
> 
>   *(int*)((xp+i)-j) = 15;
> 
>   printf("%d\n", x);
> }
> 
> This program just prints "0".
> 
> So we know that "*(int*)j = 15;" is not executed and thus "j == xp" is not
> true.
> 
> Then, can the following statement change "j" even if the printf is not
> executed?
> 
> if (j != xp) {
>    printf("hello\n");
>    j = xp;
> }
> 
> If not, how can "j == xp" suddenly hold?

One more thing.

If you remove the if-statement, then it prints "15" with GCC -O2.

Since "hello" is not printed, I think the if-statement is the same as no-op.
Thus, removing the if-statement should not change the behavior of the program
according to ISO C11.

But, they print different values.

Can you explain this?