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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/107852] [12/13 Regression] Spurious warnings stringop-overflow and array-bounds copying data as bytes into vector Date: Tue, 29 Nov 2022 11:46:58 +0000 [thread overview] Message-ID: <bug-107852-4-Qn1Dbozawy@http.gcc.gnu.org/bugzilla/> (raw) In-Reply-To: <bug-107852-4@http.gcc.gnu.org/bugzilla/> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107852 Richard Biener <rguenth at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Status|UNCONFIRMED |NEW Last reconfirmed| |2022-11-29 Ever confirmed|0 |1 Keywords| |missed-optimization --- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> --- The array bound diagnostic happens on __builtin_memcpy (_48, _137, _Num.12_51); where we have _89 = operator new (4); _48 = _89 + 4; so the destination has no space left, thus _Num.12_51 must be zero but the actual guard of the memcpy is: if (_Num_50 != 0) goto <bb 11>; [33.00%] else goto <bb 12>; [67.00%] <bb 11> [local count: 72751929]: _Num.12_51 = (long unsigned int) _Num_50; __builtin_memcpy (_48, _137, _Num.12_51); so the value-range of that is known > 0. There's a missed optimization: <bb 2> [local count: 1073741824]: _5 = bytes.D.25336._M_impl.D.24643._M_start; _6 = bytes.D.25336._M_impl.D.24643._M_finish; pretmp_66 = bytes.D.25336._M_impl.D.24643._M_end_of_storage; if (_5 != _6) goto <bb 3>; [70.00%] else goto <bb 4>; [30.00%] ... <bb 6> [local count: 329045359]: # _137 = PHI <_6(4), _5(3)> _89 = operator new (4); _43 = bytes.D.25336._M_impl.D.24643._M_start; _Num_44 = _137 - _43; if (_Num_44 != 0) goto <bb 7>; [33.00%] else goto <bb 9>; [67.00%] here we could see that _137 is equal to _5 and thus _Num_44 is zero (not sure why _43 was not yet CSEd to _5 here, supposedly this is exposed too late for the last FRE?). Knowing this would make the memcpy unreachable. So there's some std::__throw_length_error unresolved still which clobbers 'bytes'. I have a patch to elide the PHI node but the issue with bytes.D.25336._M_impl.D.24643._M_start not CSEd remains, it's in this case possibly clobbered by operator new (4). There's the possible option to avoid re-loading from 'this' in the involved(?) std::vector implementation (vector.tcc around line 800 which is where eventually the operator new originates from). IIRC we removed an optimization that 'new' cannot clobber global memory because of correctness issues recently.
next prev parent reply other threads:[~2022-11-29 11:47 UTC|newest] Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top 2022-11-23 23:24 [Bug tree-optimization/107852] New: " cuzdav at gmail dot com 2022-11-23 23:31 ` [Bug tree-optimization/107852] " pinskia at gcc dot gnu.org 2022-11-24 1:12 ` pinskia at gcc dot gnu.org 2022-11-24 7:07 ` rguenth at gcc dot gnu.org 2022-11-24 7:08 ` [Bug tree-optimization/107852] [12/13 Regression] " rguenth at gcc dot gnu.org 2022-11-29 11:46 ` rguenth at gcc dot gnu.org [this message] 2022-11-29 11:47 ` [Bug libstdc++/107852] " rguenth at gcc dot gnu.org 2022-11-29 12:01 ` redi at gcc dot gnu.org 2022-11-29 12:26 ` rguenth at gcc dot gnu.org 2022-11-29 13:30 ` cvs-commit at gcc dot gnu.org 2022-11-29 13:31 ` rguenth at gcc dot gnu.org 2022-11-29 16:50 ` redi at gcc dot gnu.org 2022-11-29 17:14 ` cvs-commit at gcc dot gnu.org 2023-03-19 10:31 ` [Bug libstdc++/107852] [12 " dvirtz at gmail dot com 2023-04-20 13:57 ` cvs-commit at gcc dot gnu.org 2023-04-27 11:44 ` rguenth at gcc dot gnu.org 2023-05-01 13:12 ` pinskia at gcc dot gnu.org 2023-06-29 18:08 ` rogerio.souza at gmail dot com 2023-06-29 18:12 ` rogerio.souza at gmail dot com 2023-06-29 19:16 ` redi at gcc dot gnu.org
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