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From: "olegendo at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug rtl-optimization/64309] New: if (1 & (1 << n)) not simplified to if (n == 0) Date: Mon, 15 Dec 2014 02:49:00 -0000 [thread overview] Message-ID: <bug-64309-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64309 Bug ID: 64309 Summary: if (1 & (1 << n)) not simplified to if (n == 0) Product: gcc Version: 5.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: rtl-optimization Assignee: unassigned at gcc dot gnu.org Reporter: olegendo at gcc dot gnu.org Target: sh*-*-* The SH specific test case gcc/testsuite/gcc.target/sh/pr51244-8.c has been failing for a while now. Originally I've added some combine patterns to handle the special case if (1 & (1 << n)) -> if (n == 0) I could fix this by modifying the SH combine patterns so that they match again. However, it would fail to handle all the other cases such as if ((1 << 1) & (1 << n)) -> if (n == 1) if ((1 << 2) & (1 << n)) -> if (n == 2) if ((3 << 0) & (3 << n)) -> if (n == 0) etc I think it should be handled in a target independent way either at the tree level or by combine, but I'm not sure what is easier/makes more sense. Taking it a bit further... On SH it's better to avoid dynamic shifts (e.g. on SH2 dynamic shifts are library calls). Thus, in cases of something like int test (int n) { return ((1 << 1) & (1 << n)); } it's often better to do a comparison and a cstore instead. Here again, I could add SH specific patterns, but maybe it makes more sense to do it in a target independent way at tree level or in combine. RTX costs can/should be used to check whether a 'shift,and' is more expensive than a 'cmp,cstore'.
next reply other threads:[~2014-12-15 2:49 UTC|newest] Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top 2014-12-15 2:49 olegendo at gcc dot gnu.org [this message] 2014-12-15 11:45 ` [Bug middle-end/64309] " mpolacek at gcc dot gnu.org 2014-12-15 13:23 ` mpolacek at gcc dot gnu.org 2014-12-15 19:08 ` olegendo at gcc dot gnu.org 2014-12-15 19:22 ` mpolacek at gcc dot gnu.org 2014-12-15 20:19 ` glisse at gcc dot gnu.org 2014-12-15 20:41 ` olegendo at gcc dot gnu.org 2014-12-15 20:46 ` jakub at gcc dot gnu.org 2014-12-15 20:46 ` mpolacek at gcc dot gnu.org 2014-12-15 20:49 ` mpolacek at gcc dot gnu.org 2014-12-15 21:06 ` glisse at gcc dot gnu.org 2014-12-15 21:11 ` glisse at gcc dot gnu.org 2014-12-16 8:54 ` rguenther at suse dot de 2014-12-16 18:29 ` mpolacek at gcc dot gnu.org 2014-12-16 18:30 ` mpolacek at gcc dot gnu.org
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