[Bug target/100874] New: [12 Regression] slight missed optimization with min<a,CST>-CST on aarch64

[Bug target/100874] New: [12 Regression] slight missed optimization with min<a,CST>-CST on aarch64

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100874

            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 = a - 4; if (a < 4) return x; return 0; }
int f2(int a) { int t = 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 improved)
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

[Bug target/100874] [12 Regression] slight missed optimization with min<a,CST>-CST on aarch64

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100874

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |12.0

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note clang even does produce the same code gen for all three:
https://godbolt.org/z/hq357jdMd

Also I put how we could fix this in
https://gcc.gnu.org/pipermail/gcc-patches/2021-June/571699.html

[Bug target/100874] [12 Regression] slight missed optimization with min<a,CST>-CST on aarch64

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100874

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |acoplan at gcc dot gnu.org

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 100873 has been marked as a duplicate of this bug. ***

[Bug target/100874] [12 Regression] slight missed optimization with min<a,CST>-CST on aarch64

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100874

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note here is a 4th variant of the function:
int f1a(int a) { int x = a - 4; return  (a < 4) ? x : 0; }

[Bug target/100874] [12 Regression] slight missed optimization with min<a,CST>-CST on aarch64 (subs_compare_2.c)

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100874

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Richard Sandiford <rsandifo@gcc.gnu.org>:

https://gcc.gnu.org/g:8e84b2b37a541b27feea69769fc314d534464ebd

commit r12-7249-g8e84b2b37a541b27feea69769fc314d534464ebd
Author: Richard Sandiford <richard.sandiford@arm.com>
Date:   Tue Feb 15 18:09:35 2022 +0000

    aarch64: Fix subs_compare_2.c regression [PR100874]

    subs_compare_2.c tests that we can use a SUBS+CSEL sequence for:

    unsigned int
    foo (unsigned int a, unsigned int b)
    {
      unsigned int x = a - 4;
      if (a < 4)
        return x;
      else
        return 0;
    }

    As Andrew notes in the PR, this is effectively MIN (x, 4) - 4,
    and it is now recognised as such by phiopt.  Previously it was
    if-converted in RTL instead.

    I tried to look for ways to generalise this to other situations
    and to other ?:-style operations, not just max and min.  However,
    for general ?: we tend to push an outer â- CSTâ into the arms of
    the ?: -- at least if one of them simplifies -- so I didn't find
    any useful abstraction.

    This patch therefore adds a pattern specifically for
    max/min(a,cst)-cst.  I'm not thrilled at having to do this,
    but it seems like the least worst fix in the circumstances.
    Also, max(a,cst)-cst for unsigned a is a useful saturating
    subtraction idiom and so is arguably worth its own code
    for that reason.

    gcc/
            PR target/100874
            * config/aarch64/aarch64-protos.h (aarch64_maxmin_plus_const):
            Declare.
            * config/aarch64/aarch64.cc (aarch64_maxmin_plus_const): New
function.
            * config/aarch64/aarch64.md (*aarch64_minmax_plus): New pattern.

    gcc/testsuite/
            * gcc.target/aarch64/max_plus_1.c: New test.
            * gcc.target/aarch64/max_plus_2.c: Likewise.
            * gcc.target/aarch64/max_plus_3.c: Likewise.
            * gcc.target/aarch64/max_plus_4.c: Likewise.
            * gcc.target/aarch64/max_plus_5.c: Likewise.
            * gcc.target/aarch64/max_plus_6.c: Likewise.
            * gcc.target/aarch64/max_plus_7.c: Likewise.
            * gcc.target/aarch64/min_plus_1.c: Likewise.
            * gcc.target/aarch64/min_plus_2.c: Likewise.
            * gcc.target/aarch64/min_plus_3.c: Likewise.
            * gcc.target/aarch64/min_plus_4.c: Likewise.
            * gcc.target/aarch64/min_plus_5.c: Likewise.
            * gcc.target/aarch64/min_plus_6.c: Likewise.
            * gcc.target/aarch64/min_plus_7.c: Likewise.

[Bug target/100874] [12 Regression] slight missed optimization with min<a,CST>-CST on aarch64 (subs_compare_2.c)

[rsandifo at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100874

rsandifo at gcc dot gnu.org <rsandifo at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rsandifo at gcc dot gnu.org
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |FIXED

--- Comment #5 from rsandifo at gcc dot gnu.org <rsandifo at gcc dot gnu.org> ---
Fixed.