https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103724
Frank Heckenbach <f.heckenb...@fh-soft.de> changed:
What |Removed |Added
----------------------------------------------------------------------------
Summary|warning |invalid warning: iteration
| |7 invokes undefined
| |behavior
--- Comment #1 from Frank Heckenbach <f.heckenb...@fh-soft.de> ---
% cat test.c
int d = 0, b = 8, a[8][8];
int main ()
{
for (int c = 0; c < 11; c++)
{
d = a[c >= b ? c - b : c][c + 1 >= b ? c + 1 - b : c + 1];
if (b && c)
d = c;
}
}
% gcc -O3 test.c
test.c: In function 'main':
test.c:7:32: warning: iteration 7 invokes undefined behavior
[-Waggressive-loop-optimizations]
7 | d = a[c >= b ? c - b : c][c + 1 >= b ? c + 1 - b : c + 1];
| ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
test.c:5:3: note: within this loop
5 | for (int c = 0; c < 11; c++)
| ^~~
Many things are strange about this warning:
- First, I think it's invalid -- the "?:" ensures the array indexes are in
bounds, all variables are initialized,so I don't see what would be UB.
- It claims UB occurs on iteration 7, but the warning disappears when I change
the loop condition to "c < 10" (which would also include iteration 7)
- The "if" statement with both conditions and the assignment is necessary to
trigger the warning, although it has no actual effect.
- The warning seems to apply to the 2nd array index, but already disappears
when I turn the first index into "0" without changing the 2nd one.
- It disappears when I make all variables local or all global.
