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 rguent...@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?
