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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/49513] PRE inhibits if-conversion and vectorization Date: Thu, 23 Jun 2011 11:55:00 -0000 [thread overview] Message-ID: <bug-49513-4-DPXo2cpGVV@http.gcc.gnu.org/bugzilla/> (raw) In-Reply-To: <bug-49513-4@http.gcc.gnu.org/bugzilla/> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49513 Richard Guenther <rguenth at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Status|UNCONFIRMED |NEW Keywords| |missed-optimization Last reconfirmed| |2011.06.23 11:55:09 CC| |rguenth at gcc dot gnu.org Ever Confirmed|0 |1 Summary|introducing a product |PRE inhibits if-conversion |inhibit vectorization |and vectorization --- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-06-23 11:55:09 UTC --- That is because PRE decided to optimize the first iteration where it knows that z == 0 and thus z*s[i] and z*c[i] are 0 and thus a[i] will be 0. Which means we confuse if-conversion which in turn causes this missed vectorization. IL before if-conversion: <bb 2>: goto <bb 6>; <bb 3>: z_3 = (float) i_9; D.3280_4 = c[i_9]; D.3281_5 = D.3280_4 * z_3; D.3282_6 = s[i_9]; D.3283_7 = D.3282_6 * z_3; yy_13 = ABS_EXPR <D.3281_5>; yy_14 = ABS_EXPR <D.3283_7>; if (yy_13 < yy_14) goto <bb 5>; else goto <bb 4>; <bb 4>: <bb 5>: # yy_28 = PHI <yy_13(4), yy_14(3)> # yy_29 = PHI <yy_14(4), yy_13(3)> <bb 6>: # yy_16 = PHI <yy_28(5), 0.0(2)> # yy_15 = PHI <yy_29(5), 0.0(2)> # i_30 = PHI <i_9(5), 0(2)> # ivtmp.44_24 = PHI <ivtmp.44_10(5), 1024(2)> t_17 = yy_15 / yy_16; a[i_30] = t_17; i_9 = i_30 + 1; ivtmp.44_10 = ivtmp.44_24 - 1; if (ivtmp.44_10 != 0) goto <bb 3>; else goto <bb 7>; <bb 7>: return; The PHI nodes in bb 6 is what makes if-conversion fail (thus, the irregular loop entry which really should be peeled off). This situation commonly occurs when PRE can compute the first iterations result. Manually peeling off the iteration like the following is a workaround: void foo2() { a[0] = 0; for (int i=1; i!=1024; ++i) { float z = i; a[i] = bar(z*s[i],z*c[i]); } } Not sure if your original issue you derived this testcase from really matches the above problem though.
next prev parent reply other threads:[~2011-06-23 11:55 UTC|newest] Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top 2011-06-23 6:55 [Bug tree-optimization/49513] New: introducing a product inhibit vectorization vincenzo.innocente at cern dot ch 2011-06-23 11:55 ` rguenth at gcc dot gnu.org [this message] 2011-06-23 12:17 ` [Bug tree-optimization/49513] PRE inhibits if-conversion and vectorization vincenzo.innocente at cern dot ch 2011-06-23 12:59 ` [Bug tree-optimization/49513] DOM " rguenth at gcc dot gnu.org 2011-07-08 21:12 ` law at redhat dot com 2023-05-02 0:46 ` pinskia at gcc dot gnu.org
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