[Bug target/54829] bad optimization: sub followed by cmp w/ zero (ARM)

[hjl.tools at gmail dot com]

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.