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From: "ktkachov at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug rtl-optimization/67635] [SH] ifcvt missed optimization Date: Mon, 19 Oct 2015 16:06:00 -0000 [thread overview] Message-ID: <bug-67635-4-hpp1RKJyg6@http.gcc.gnu.org/bugzilla/> (raw) In-Reply-To: <bug-67635-4@http.gcc.gnu.org/bugzilla/> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67635 ktkachov at gcc dot gnu.org changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |ktkachov at gcc dot gnu.org --- Comment #2 from ktkachov at gcc dot gnu.org --- I had a look at: unsigned int test_03 (unsigned int x) { return x > 0 ? x + 1 : x; } The cse1 pass transforms this into something not quite comfortable for ifcvt. Basically, it transforms the RTL equivalent of: compare (x, 0); if (x > 0) temp := x + 1; else temp := x; return temp; into: compare (x, 0); if (x > 0) temp := x + 1; else temp := 0; return temp; which is a valid transformation to make as far as cse is concerned. Unfortunately, when ifcvt comes along, it's asked to optimise a conditional select between '0' and 'x + 1', which is not something it can do with an addc-style pattern. I initially thought of looking into the condition to see if it contains x (so that we can deduce that the 'else' arm could actually be replaced with x, undoing the cse in that case), but the condition rtl contains a comparison with the T-register, rather than the original comparison with 0. Perhaps we can consider teaching cse to not transform these kinds of expressions (c ? x : x + a) if the target has a store_flag/addcc instruction of the appropriate mode? But I'm not familiar with the cse code, so I don't know how easy/clean that is
next prev parent reply other threads:[~2015-10-19 16:06 UTC|newest] Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top 2015-09-19 4:09 [Bug rtl-optimization/67635] New: " olegendo at gcc dot gnu.org 2015-09-30 12:49 ` [Bug rtl-optimization/67635] " olegendo at gcc dot gnu.org 2015-10-19 16:06 ` ktkachov at gcc dot gnu.org [this message] 2015-10-21 13:51 ` olegendo at gcc dot gnu.org 2023-08-08 7:02 ` pinskia at gcc dot gnu.org
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