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From: "amker at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug tree-optimization/43174] Teaching SCEV about ADDR_EXPR causes regression Date: Mon, 17 Aug 2015 09:25:00 -0000 [thread overview] Message-ID: <bug-43174-4-zMlXE2j5vD@http.gcc.gnu.org/bugzilla/> (raw) In-Reply-To: <bug-43174-4@http.gcc.gnu.org/bugzilla/> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=43174 --- Comment #3 from amker at gcc dot gnu.org --- Note for the three levels of loop example, GCC chooses one IV for both j and k loops, thus generates pretty clean output on x86_64 with O2. For the simple example, now gcc can eliminate comparison iv with the address candidate, and generates below codes: <bb 2>: ivtmp.18_14 = (unsigned long) &a; _10 = &a + 60516; _28 = (unsigned long) _10; goto <bb 7>; <bb 3>: <bb 4>: # s_18 = PHI <s_9(3), s_19(7)> # ivtmp.9_12 = PHI <ivtmp.9_1(3), ivtmp.9_4(7)> _22 = (void *) ivtmp.9_12; _8 = MEM[base: _22, offset: 0B]; s_9 = _8 + s_18; ivtmp.9_1 = ivtmp.9_12 + 4; if (ivtmp.9_1 != _27) goto <bb 3>; else goto <bb 5>; <bb 5>: # s_17 = PHI <s_9(4)> ivtmp.18_15 = ivtmp.18_5 + 492; if (ivtmp.18_15 != _28) goto <bb 6>; else goto <bb 8>; <bb 6>: <bb 7>: # s_19 = PHI <s_17(6), 0(2)> # ivtmp.18_5 = PHI <ivtmp.18_15(6), ivtmp.18_14(2)> ivtmp.9_4 = ivtmp.18_5; _29 = ivtmp.18_5 + 492; _27 = _29; goto <bb 4>; <bb 8>: # s_16 = PHI <s_17(5)> return s_16; With this, following gimple optimizers are able to CSE the opportunity of "ivtmp.18_5 + 492". As a result, optimal code is generated as in optimized dump: <bb 2>: ivtmp.18_14 = (unsigned long) &a; _28 = (unsigned long) &MEM[(void *)&a + 60516B]; goto <bb 5>; <bb 3>: # s_18 = PHI <s_9(3), s_19(5)> # ivtmp.9_12 = PHI <ivtmp.9_1(3), ivtmp.18_5(5)> _22 = (void *) ivtmp.9_12; _8 = MEM[base: _22, offset: 0B]; s_9 = _8 + s_18; ivtmp.9_1 = ivtmp.9_12 + 4; if (ivtmp.9_1 != _29) goto <bb 3>; else goto <bb 4>; <bb 4>: if (_28 != _29) goto <bb 5>; else goto <bb 6>; <bb 5>: # s_19 = PHI <s_9(4), 0(2)> # ivtmp.18_5 = PHI <_29(4), ivtmp.18_14(2)> _29 = ivtmp.18_5 + 492; goto <bb 3>; <bb 6>: return s_9; This in effect is the transformation you wanted in this PR, but I doubt if GCC can do this if it can't eliminate the inner loop's comparison using ivtmp.9_1 at the first place. On the other hand, for cases that we can use IV's final value, it maybe likely for GCC to eliminate the comparison IV.
next parent reply other threads:[~2015-08-17 9:25 UTC|newest] Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top [not found] <bug-43174-4@http.gcc.gnu.org/bugzilla/> 2015-08-17 9:25 ` amker at gcc dot gnu.org [this message] 2015-08-17 9:26 ` amker at gcc dot gnu.org 2010-02-25 14:43 [Bug tree-optimization/43174] New: " amonakov at gcc dot gnu dot org 2010-02-25 15:26 ` [Bug tree-optimization/43174] " steven at gcc dot gnu dot org 2010-03-01 17:44 ` amonakov at gcc dot gnu dot org 2010-03-03 9:21 ` rakdver at kam dot mff dot cuni dot cz
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