[Bug tree-optimization/83518] New: Missing optimization: useless instructions should be dropped

[zamazan4ik at tut dot by]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83518

            Bug ID: 83518
           Summary: Missing optimization: useless instructions should be
                    dropped
           Product: gcc
           Version: tree-ssa
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zamazan4ik at tut dot by
  Target Milestone: ---

gcc (trunk) with '-O3 -std=c++17' for this code:

unsigned test()
{
    int arr[] = {5,4,3,2,1};
    int sum = 0;

    for(int i = 0;i < 5;++i)
    {
        for(int j = 0; j < 5; ++j)
        {
            int t = arr[i];
            arr[i] = arr[j];
            arr[j] = t;
        }
    }

    for(int i = 0; i < 5; ++i)
    {
        sum += arr[i];
    }

    return sum;
}


generates it:

test():
  movdqa xmm0, XMMWORD PTR .LC0[rip]
  movaps XMMWORD PTR [rsp-40], xmm0
  mov rax, QWORD PTR [rsp-32]
  mov DWORD PTR [rsp-32], 1
  mov QWORD PTR [rsp-40], rax
  mov DWORD PTR [rsp-28], 5
  movdqa xmm0, XMMWORD PTR [rsp-40]
  movdqa xmm1, xmm0
  psrldq xmm1, 8
  paddd xmm0, xmm1
  movdqa xmm1, xmm0
  psrldq xmm1, 4
  paddd xmm0, xmm1
  movd eax, xmm0
  add eax, 4
  ret
.LC0:
  .long 5
  .long 4
  .long 3
  .long 2


clang (trunk) with '-O3 -std=c++17':

test(): # @test()
  mov eax, 15
  ret