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From: "652023330028 at smail dot nju.edu.cn" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/111724] New: [Regression] Missed
 optimizations probably because of too early arithmetic optimization
Date: Sun, 08 Oct 2023 07:55:46 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 14.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D111724

            Bug ID: 111724
           Summary: [Regression] Missed optimizations probably because of
                    too early arithmetic optimization
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: 652023330028 at smail dot nju.edu.cn
  Target Milestone: ---

Hello, we found some optimizations (probably because of too early arithmetic
optimization) that GCC may have missed. We would greatly appreicate if you =
can
take a look and let us know what you think.

Given the following code:
https://godbolt.org/z/8EW8fx78K
int n;
void func(int w, int a, int b){
    for(int i=3D0;i<w;i++){
        n +=3D (a+a)+(b+b);
        b +=3D b;
    }
}

`a+a` is a loop invariant. But gcc-trunk -O3:
func(int, int, int):
        push    {r4, lr}
        subs    lr, r0, #0
        ble     .L1
        movw    r4, #:lower16:.LANCHOR0
        movt    r4, #:upper16:.LANCHOR0
        mov     ip, #0
        ldr     r0, [r4]
.L3:
        adds    r3, r1, r2
        add     ip, ip, #1
        lsls    r2, r2, #1
        cmp     lr, ip
        add     r0, r0, r3, lsl #1
        bne     .L3
        str     r0, [r4]

>From original(tree):
   (void) (n =3D (a + b) * 2 + n) >>>>>;

This leads to:
  _1 =3D a_11(D) + b_18;
  _2 =3D _1 * 2;
  _4 =3D _2 + n_lsm.4_7;

So, it looks like the missed LICM is due to too early arithmetic optimizati=
on.

On gcc-6.4, it works as expected.

We found that this also affects other optimizations, such as common
subexpression elimination. We can provide examples if required.

Thank you very much for your time and effort! We look forward to hearing fr=
om
you.=