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From: "rguenth at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug tree-optimization/37542] [4.4 Regression] PRE doesn't simplify during phi-translation Date: Sat, 01 Nov 2008 17:12:00 -0000 [thread overview] Message-ID: <20081101171129.835.qmail@sourceware.org> (raw) In-Reply-To: <bug-37542-10053@http.gcc.gnu.org/bugzilla/> ------- Comment #4 from rguenth at gcc dot gnu dot org 2008-11-01 17:11 ------- We need to be careful with simplifying to SSA_NAMEs as the following example shows: int foo (int i, int b) { int mask; int result; if (b) mask = -1; else mask = 0; result = i + 1; result = result & mask; return result; } we have a phi-translation for result & mask that is 0 or result dependent on the path through the CFG. But we cannot insert this as a PHI as one argument (result with value i + 1) is not available there. The PRE of 4.3 inserts i + 1, re-generating the expression recursively and exposing a code hoisting opportunity: <bb 2>: if (b_2(D) != 0) goto <bb 5>; else goto <bb 3>; <bb 5>: pretmp.6_10 = i_5(D) + 1; pretmp.6_11 = pretmp.6_10; goto <bb 4>; <bb 3>: pretmp.6_13 = i_5(D) + 1; <bb 4>: # prephitmp.7_14 = PHI <pretmp.6_10(5), pretmp.6_13(3)> # prephitmp.7_12 = PHI <pretmp.6_11(5), 0(3)> # mask_1 = PHI <-1(5), 0(3)> result_6 = prephitmp.7_14; result_7 = prephitmp.7_12; return result_7; I don't think we want to do this now (without code hoisting implemented), but for cases where result_6 is available we surely want it. I'm trying to find a way to detect whether it is safe to phi-translate to result_6. -- rguenth at gcc dot gnu dot org changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |dberlin at gcc dot gnu dot | |org http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37542
next prev parent reply other threads:[~2008-11-01 17:12 UTC|newest] Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top 2008-09-16 15:23 [Bug tree-optimization/37542] New: " rguenth at gcc dot gnu dot org 2008-09-16 15:23 ` [Bug tree-optimization/37542] " rguenth at gcc dot gnu dot org 2008-09-16 15:35 ` [Bug tree-optimization/37542] [4.4 Regression] " rguenth at gcc dot gnu dot org 2008-09-22 12:21 ` rguenth at gcc dot gnu dot org 2008-10-22 3:47 ` mmitchel at gcc dot gnu dot org 2008-11-01 17:12 ` rguenth at gcc dot gnu dot org [this message] 2008-11-01 17:13 ` rguenth at gcc dot gnu dot org 2008-11-02 15:28 ` rguenth at gcc dot gnu dot org 2008-11-02 15:29 ` rguenth at gcc dot gnu dot org
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