https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107839
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> --- We see <bb 2> [local count: 3508266]: if (c_4(D) != 0) goto <bb 3>; [33.00%] else goto <bb 10>; [67.00%] <bb 10> [local count: 2350538]: goto <bb 4>; [100.00%] <bb 4> [local count: 3508266]: # v_10 = PHI <v_5(D)(10), v_8(3)> _3 = (unsigned int) v_10; _12 = _3 * 2; _1 = (int) _12; <bb 5> [local count: 354334800]: if (c_4(D) != 0) goto <bb 7>; [66.33%] else goto <bb 8>; [33.67%] it's loop invariant motion that hoists the v + v compute out of the loop and thus outside of its controlling condition. You can see it's careful to not introduce undefined overflow that is possibly conditionally executed only but it fails to consider the case of 'v' being conditionally uninitialized. It's very difficult to do the right thing here - it might be tempting to hoist the compute as if (c) tem = v+v; while (1) if (c) f(tem); but apart from the technical problems in invariant motion this would cause it does introduce another variable that's only conditionally initialized and thus might be prone to false positive diagnostics. Not to mention the hoisted if (c) branch having a cost. Maybe the simplest thing would be to never hoist v + v, or only hoist it when the controlling branch is not loop invariant. The original testcase is probably more "sensible", does it still have a loop invariant controlling condition and a loop invariant computation under that control?