[Bug tree-optimization/114145] New: Missed optimization of loop deletion

[Bug tree-optimization/114145] New: Missed optimization of loop deletion

[652023330028 at smail dot nju.edu.cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114145

            Bug ID: 114145
           Summary: Missed optimization of loop deletion
           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 noticed that in the code below, looping is not necessary, but gcc
seems to have missed this optimization.

https://godbolt.org/z/sYqzh8M3c
int a, b;
void func(int c){
    for(int i=0;i<700;i++){
        b=c;
        c=a;
    }
}

GCC -O3:
func(int):
        mov     edx, DWORD PTR a[rip]
        mov     eax, 700
        jmp     .L2
.L3:
        sub     eax, 3
        mov     edi, edx
.L2:
        cmp     eax, 1
        jne     .L3
        mov     DWORD PTR b[rip], edi
        ret

Expected code (Clang):
func(int):                               # @func(int)
        mov     eax, dword ptr [rip + a]
        mov     dword ptr [rip + b], eax
        ret

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

[Bug tree-optimization/114145] Missed optimization of loop deletion

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114145

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2024-02-28
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Store motion turns this into

  <bb 2> [local count: 10737416]:
  b_lsm.3_3 = _15(D);
  c_7 = a;

  <bb 3> [local count: 1063004408]:
  # c_12 = PHI <c_7(5), c_4(D)(2)>
  # i_13 = PHI <i_8(5), 0(2)>
  b_lsm.3_2 = c_12;
  i_8 = i_13 + 1;
  if (i_8 != 700)
    goto <bb 5>; [98.99%]
  else
    goto <bb 4>; [1.01%]

  <bb 5> [local count: 1052266995]:
  goto <bb 3>; [100.00%]

  <bb 4> [local count: 10737416]:
  # b_lsm.3_11 = PHI <b_lsm.3_2(3)>
  b = b_lsm.3_11;
  return;

where ultimatively final value replacement fails because SCEV fails here:

(analyze_scalar_evolution
  (loop_nb = 1)
  (scalar = c_7)
(get_scalar_evolution  
  (scalar = c_7)
  (scalar_evolution = ))
) 
(instantiate_scev 
  (instantiate_below = 2 -> 3)
  (evolution_loop = 1)
  (chrec = c_7)
  (res = c_7))
  (evolution_function = scev_not_known))

indeed there's no way to express the evolution of this induction variable
which has just two values.  Might be a simple thing to special case
in final value replacement though.  There we see just

  <bb 3> [local count: 1063004408]:
  # c_12 = PHI <c_7(5), c_4(D)(2)>
...
  <bb 4> [local count: 10737416]:
  # c_9 = PHI <c_12(3)>

so the final value is niter == 0 ? c_4(D) : c_7.