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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/100874] New: [12 Regression] slight missed optimization
 with min<a,CST>-CST on aarch64
Date: Wed, 02 Jun 2021 09:32:35 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100874

            Bug ID: 100874
           Summary: [12 Regression] slight missed optimization with
                    min<a,CST>-CST on aarch64
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---
            Target: aarch64-*-*

Take:
int f(int a) { return a < 4 ? a - 4 : 0; }
int f1(int a) { int x =3D a - 4; if (a < 4) return x; return 0; }
int f2(int a) { int t =3D a < 4 ? a : 4; return t - 4; }

In GCC 11, we produce:
f(int):
        mov     w1, 4
        cmp     w0, w1
        csel    w0, w0, w1, le
        sub     w0, w0, #4
        ret
f1(int):
        subs    w0, w0, 4
        csel    w0, w0, wzr, lt
        ret
f2(int):
        mov     w1, 4
        cmp     w0, w1
        csel    w0, w0, w1, le
        sub     w0, w0, #4
        ret

On the trunk all three give the same code gen (due to PHI-OPT being improve=
d)
but of what f and f2 used to give.
All three should produce what f1 had produed instead.
This is gcc.target/aarch64/subs_compare_2.c=