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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug tree-optimization/107839] spurious "may be used uninitialized" warning while all uses are under "if (c)" Date: Thu, 24 Nov 2022 10:12:30 +0000 [thread overview] Message-ID: <bug-107839-4-MS51D0v6I5@http.gcc.gnu.org/bugzilla/> (raw) In-Reply-To: <bug-107839-4@http.gcc.gnu.org/bugzilla/> 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?
next prev parent reply other threads:[~2022-11-24 10:12 UTC|newest] Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top 2022-11-23 16:38 [Bug tree-optimization/107839] New: " vincent-gcc at vinc17 dot net 2022-11-23 21:00 ` [Bug tree-optimization/107839] " rguenth at gcc dot gnu.org 2022-11-24 10:12 ` rguenth at gcc dot gnu.org [this message] 2022-11-24 17:09 ` vincent-gcc at vinc17 dot net 2022-12-05 9:32 ` cvs-commit at gcc dot gnu.org 2022-12-05 9:47 ` rguenth at gcc dot gnu.org 2022-12-12 11:20 ` cvs-commit at gcc dot gnu.org 2024-01-20 17:22 ` pinskia at gcc dot gnu.org
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