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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/101501] [11/12 Regression] wrong code at -O3 on x86_64-linux-gnu Date: Thu, 22 Jul 2021 08:07:45 +0000 [thread overview] Message-ID: <bug-101501-4-zKHeQZF102@http.gcc.gnu.org/bugzilla/> (raw) In-Reply-To: <bug-101501-4@http.gcc.gnu.org/bugzilla/> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101501 --- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> --- # of iterations (a_lsm.5_10 * 52) / 52, bounded by 63 is obviously wrong with a_lsm.5_10 being a 'char', that boils down to zero iterations when computed at runtime. The loop body looks like <bb 3> [local count: 1073741824]: # a_3 = PHI <a_5(2), a_6(5)> d: a_6 = a_3 + 255; c_7 = a_3 * 52; if (c_7 != 0) goto <bb 5>; [89.00%] else goto <bb 4>; [11.00%] <bb 5> [local count: 955630224]: goto <bb 3>; [100.00%] and SCEV computes (scalar = c_7) (scalar_evolution = {a_5 * 52, +, 204}_1)) which looks correct. niter compute does static bool number_of_iterations_ne (class loop *loop, tree type, affine_iv *iv, tree final, class tree_niter_desc *niter, bool exit_must_be_taken, bounds *bnds) { ... /* Compute no-overflow information for the control iv. This can be proven when below two conditions are satisfied: 1) IV evaluates toward FINAL at beginning, i.e: base <= FINAL ; step > 0 base >= FINAL ; step < 0 2) |FINAL - base| is an exact multiple of step. ... if (!niter->control.no_overflow && (integer_onep (s) || multiple_of_p (type, c, s))) { and there we have the issue of multiple_of_p not handling overflow (there's a duplicate bug, PR100499 about this issue). The 1) condition ends up as unsigned-val >= 0 which is trivially true.
next prev parent reply other threads:[~2021-07-22 8:07 UTC|newest] Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-07-18 22:50 [Bug tree-optimization/101501] New: " qrzhang at gatech dot edu 2021-07-18 22:51 ` [Bug tree-optimization/101501] " qrzhang at gatech dot edu 2021-07-18 23:01 ` [Bug tree-optimization/101501] [11/12 Regression] " pinskia at gcc dot gnu.org 2021-07-19 3:40 ` glisse at gcc dot gnu.org 2021-07-19 6:39 ` rguenth at gcc dot gnu.org 2021-07-22 7:19 ` rguenth at gcc dot gnu.org 2021-07-22 8:07 ` rguenth at gcc dot gnu.org [this message] 2021-07-22 8:09 ` rguenth at gcc dot gnu.org
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