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From: "vineetg at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/111467] New: REE failing to eliminate
 redundant extension due to multiple reaching def(s)
Date: Mon, 18 Sep 2023 22:32:28 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D111467

            Bug ID: 111467
           Summary: REE failing to eliminate redundant extension due to
                    multiple reaching def(s)
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: vineetg at gcc dot gnu.org
          Reporter: vineetg at gcc dot gnu.org
                CC: jeffreyalaw at gmail dot com, jivanhakobyan9 at gmail d=
ot com,
                    kito at gcc dot gnu.org, palmer at gcc dot gnu.org
  Target Milestone: ---

For the trivial test case below (credit goes to Palmer for mentioning this
almost 2 years ago).

int
foo6(int a, int b)
{
  return a > b ? a : b;
}

-O2 -march=3Drv64gc

foo6:
        mv      a5,a1
        bge     a1,a0,.L5
        mv      a5,a0
.L5:
        sext.w  a0,a5
        ret

REE fails to eliminate the sign extension due to multiple reaching definiti=
ons
constraint.

I don't know how involved or runtime cost relaxing the constraint is, so
opening this PR to investigate.

FWIW a zba build generates a max insn, eliminating the sext.w, but the vani=
lla
case shows where things can possibly be improved.=