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From: "guillaume dot melquiond at ens-lyon dot fr" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug c/30364] New: Wrong variable ranges due to constant folding Date: Thu, 04 Jan 2007 09:16:00 -0000 [thread overview] Message-ID: <bug-30364-7904@http.gcc.gnu.org/bugzilla/> (raw) Testcase: (compiled with -O2 at least) int f(int a, int b) { if (a > 0x7FFFFFF0) return 0; if (b > 0x7FFFFFF0) return 0; int c = (a - 20) + (b - 20); return c > 0x7FFFFFF0; } GCC 4.1.2 and 4.3.0 (snapshot from 2006-12-17) optimizes the whole function to a single "return 0;". This would be correct if the function was actually written with "c = a + b - 40" under a non-overflow assumption. GCC could indeed deduce that c is no bigger than 0x7FFFFFFF - 40. But as the function was originally written, this property does not hold any longer. For example, a = 0x7FFFFFF0 and b = 41 will not cause any overflow during computations, and the last conditional shall hence evaluate to true. The problem is that GCC performs VRP with C language semantic (undefined behavior on overflow) on code that is no longer the input as written by the user; so this semantic is not valid at that point. The user input should not have undergone a transformation based on associativity. Tested with Debian packages. GCC 3.3.6, 3.4.6, and 4.0.4 generate correct code. GCC 4.1.2 and 4.3.0 generates wrong code. As the expression "a + b - 40" is generated early, I suppose any GCC with VRP would produce wrong code for this testcase. -- Summary: Wrong variable ranges due to constant folding Product: gcc Version: 4.1.2 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: guillaume dot melquiond at ens-lyon dot fr http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30364
next reply other threads:[~2007-01-04 9:16 UTC|newest] Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top 2007-01-04 9:16 guillaume dot melquiond at ens-lyon dot fr [this message] 2007-01-04 9:30 ` [Bug middle-end/30364] [4.1/4.2/4.3 Regression] " pinskia at gcc dot gnu dot org 2007-01-04 10:15 ` guillaume dot melquiond at ens-lyon dot fr 2007-01-04 10:23 ` pinskia at gcc dot gnu dot org 2007-01-04 10:26 ` pinskia at gcc dot gnu dot org 2007-01-04 10:40 ` pinskia at gcc dot gnu dot org 2007-01-04 11:25 ` guillaume dot melquiond at ens-lyon dot fr 2007-01-05 19:06 ` jakub at gcc dot gnu dot org 2007-01-05 19:58 ` pinskia at gcc dot gnu dot org 2007-01-05 20:32 ` pinskia at gcc dot gnu dot org 2007-01-05 21:06 ` gdr at integrable-solutions dot net 2007-01-05 21:09 ` gdr at integrable-solutions dot net 2007-02-05 5:50 ` mmitchel at gcc dot gnu dot org 2007-02-14 9:03 ` mmitchel at gcc dot gnu dot org 2007-02-21 14:24 ` rguenth at gcc dot gnu dot org 2007-02-28 21:57 ` rguenth at gcc dot gnu dot org 2007-02-28 23:03 ` [Bug middle-end/30364] [4.1/4.2 " rguenth at gcc dot gnu dot org 2007-03-05 13:15 ` rguenth at gcc dot gnu dot org 2007-03-05 13:17 ` [Bug middle-end/30364] [4.1 " rguenth at gcc dot gnu dot org 2007-03-15 18:09 ` rguenth at gcc dot gnu dot org 2007-03-15 18:10 ` rguenth at gcc dot gnu dot org 2007-08-24 16:33 ` jakub at gcc dot gnu dot org 2007-08-24 16:40 ` rguenth at gcc dot gnu dot org
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