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From: "hjl.tools at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/54829] bad optimization: sub followed by cmp w/ zero (ARM)
Date: Sat, 06 Oct 2012 13:31:00 -0000	[thread overview]
Message-ID: <bug-54829-4-BCtwe4Bxe8@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-54829-4@http.gcc.gnu.org/bugzilla/>


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54829

H.J. Lu <hjl.tools at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|bad optimization: sub       |bad optimization: sub
                   |followed by cmp w/ zero     |followed by cmp w/ zero
                   |(x86 & ARM)                 |(ARM)

--- Comment #2 from H.J. Lu <hjl.tools at gmail dot com> 2012-10-06 13:31:00 UTC ---
i386.md has

(define_insn "*cmp<mode>_minus_1"
  [(set (reg FLAGS_REG)
        (compare
          (minus:SWI (match_operand:SWI 0 "nonimmediate_operand" "<r>m,<r>")
                     (match_operand:SWI 1 "<general_operand>" "<r><i>,<r>m"))
          (const_int 0)))]
  "ix86_match_ccmode (insn, CCGOCmode)"
  "cmp{<imodesuffix>}\t{%1, %0|%0, %1}"
  [(set_attr "type" "icmp")
   (set_attr "mode" "<MODE>")])

Since cmp will also set OF and CF flags, it can't be used for GT and LE
which checks OF.


  parent reply	other threads:[~2012-10-06 13:31 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-10-05 23:16 [Bug target/54829] New: bad optimization: sub followed by cmp w/ zero (x86 & ARM) daniel.santos at pobox dot com
2012-10-05 23:40 ` [Bug target/54829] " pinskia at gcc dot gnu.org
2012-10-06 13:31 ` hjl.tools at gmail dot com [this message]
2012-10-06 15:56 ` daniel.santos at pobox dot com
2012-10-06 15:57 ` daniel.santos at pobox dot com
2012-10-13 16:05 ` rearnsha at gcc dot gnu.org
2012-10-13 16:18 ` rearnsha at gcc dot gnu.org
2012-11-15 21:56 ` daniel.santos at pobox dot com
2013-08-05 21:04 ` rearnsha at gcc dot gnu.org
2015-02-15  6:17 ` daniel.santos at pobox dot com

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