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From: "amacleod at redhat dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug tree-optimization/108360] [13 Regression] Dead Code Elimination Regression at -Os since r13-2048-g418b71c0d535bf Date: Tue, 10 Jan 2023 19:18:29 +0000 [thread overview] Message-ID: <bug-108360-4-QgVS0OQMJn@http.gcc.gnu.org/bugzilla/> (raw) In-Reply-To: <bug-108360-4@http.gcc.gnu.org/bugzilla/> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108360 --- Comment #4 from Andrew Macleod <amacleod at redhat dot com> --- The IL is different in VRP2 between GCC12 and GCC13. IN GCC 12 I see: <bb 2> [local count: 1073741824]: b.2_1 = b; _2 = b.2_1 <= 0; h.0_20 = (unsigned short) _2; _21 = h.0_20 + 65535; _22 = (short int) _21; _3 = _22 >= 0; _4 = (char) _3; f = _4; f.5_5 = (unsigned char) _3; _6 = f.5_5 << 4; e = _6; h_23 = (short int) _6; if (_6 == 0) goto <bb 5>; [50.00%] else goto <bb 3>; [50.00%] <bb 3> [local count: 805306368]: _8 = b.2_1 == h_23; Global ranges for bb2 are: _4 : char [0, 1] f.5_5 : unsigned char [0, 1] _6 : unsigned char [0, 0][16, 16] h.0_20 : unsigned short [0, 1] _21 : unsigned short [0, 0][+INF, +INF] _22 : short int [-1, 0] Looking at what ranger calculates for the edge 2->3 based on _6 having to be 0: 2->3 (F) b.2_1 : short int [-INF, 0] 2->3 (F) _2 : _Bool [1, 1] 2->3 (F) _3 : _Bool [1, 1] 2->3 (F) _4 : char [1, 1] 2->3 (F) f.5_5 : unsigned char [1, 1] 2->3 (F) _6 : unsigned char [16, 16] 2->3 (F) h.0_20 : unsigned short [1, 1] 2->3 (F) _21 : unsigned short [0, 0] 2->3 (F) _22 : short int [0, 0] 2->3 (F) h_23 : short int [16, 16] It can determine that h_23 is [16,16] and therefore _8 is always false. and when you use _8 = [0, 0] the condition leading to the call can never be true so it's eliminated. The code sequence at -Os is quite different coming into VRP2 in GCC13. It involves a PHI node and the condition uses _21 == -1 instead of _6 == 0. This cahnges what we can evaluate going out, and when we get to valuation of _8, it sees: _8 = b.2_1 == iftmp.8_28; and iftmp.8_28 is evaluated as [0, 0][16, 16][512, 512] Because 0 hasnt been eliminated, we cant fold the condition. We do still get this at -O2... just not at -Os.
next prev parent reply other threads:[~2023-01-10 19:18 UTC|newest] Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top 2023-01-10 13:10 [Bug tree-optimization/108360] New: Dead Code Elimination Regression at -Os (trunk vs. 12.2.0) yann at ywg dot ch 2023-01-10 14:48 ` [Bug tree-optimization/108360] [13 Regression] Dead Code Elimination Regression at -Os since r13-2048-g418b71c0d535bf marxin at gcc dot gnu.org 2023-01-10 17:00 ` jakub at gcc dot gnu.org 2023-01-10 17:32 ` jakub at gcc dot gnu.org 2023-01-10 17:45 ` jakub at gcc dot gnu.org 2023-01-10 19:18 ` amacleod at redhat dot com [this message] 2023-01-10 19:26 ` amacleod at redhat dot com 2023-01-10 19:28 ` amacleod at redhat dot com 2023-03-27 11:37 ` rguenth at gcc dot gnu.org 2023-04-08 14:37 ` law at gcc dot gnu.org 2023-04-26 6:57 ` [Bug tree-optimization/108360] [13/14 " rguenth at gcc dot gnu.org 2023-07-27 9:25 ` rguenth at gcc dot gnu.org 2023-08-07 8:53 ` pinskia at gcc dot gnu.org 2023-08-09 1:25 ` pinskia at gcc dot gnu.org 2024-05-21 9:13 ` [Bug tree-optimization/108360] [13/14/15 " jakub at gcc dot gnu.org
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