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From: "hiraditya at msn dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/109441] New: missed optimization when all
 elements of vector are known
Date: Thu, 06 Apr 2023 18:31:37 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109441

            Bug ID: 109441
           Summary: missed optimization when all elements of vector are
                    known
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hiraditya at msn dot com
  Target Milestone: ---

Reference: https://godbolt.org/z/af4x6zhz9

When all elements of vector are 0, then the compiler should be able to remo=
ve
the loop and just return 0.

Testcase:

#include<vector>
using namespace std;

using T =3D int;


T v() {
    T s;
    std::vector<T> v;
    v.resize(1000, 0);
    for (auto i =3D 0; i < v.size(); ++i) {
        s +=3D v[i];
    }

    return s;
}



$ g++ -O3 -std=3Dc++17

.LC0:
  .string "vector::_M_fill_insert"
v():
  push rbx
  pxor xmm0, xmm0
  mov edx, 1000
  xor esi, esi
  sub rsp, 48
  lea rcx, [rsp+12]
  lea rdi, [rsp+16]
  mov QWORD PTR [rsp+32], 0
  mov DWORD PTR [rsp+12], 0
  movaps XMMWORD PTR [rsp+16], xmm0
  call std::vector<int, std::allocator<int>
>::_M_fill_insert(__gnu_cxx::__normal_iterator<int*, std::vector<int,
std::allocator<int> > >, unsigned long, int const&)
  mov rdx, QWORD PTR [rsp+24]
  mov rdi, QWORD PTR [rsp+16]
  mov rax, rdx
  sub rax, rdi
  mov rsi, rax
  sar rsi, 2
  cmp rdx, rdi
  je .L99
  test rax, rax
  mov ecx, 1
  cmovne rcx, rsi
  cmp rax, 12
  jbe .L107
  mov rdx, rcx
  pxor xmm0, xmm0
  mov rax, rdi
  shr rdx, 2
  sal rdx, 4
  add rdx, rdi
.L101:
  movdqu xmm2, XMMWORD PTR [rax]
  add rax, 16
  paddd xmm0, xmm2
  cmp rdx, rax
  jne .L101
  movdqa xmm1, xmm0
  psrldq xmm1, 8
  paddd xmm0, xmm1
  movdqa xmm1, xmm0
  psrldq xmm1, 4
  paddd xmm0, xmm1
  movd ebx, xmm0
  test cl, 3
  je .L99
  and rcx, -4
  mov eax, ecx
.L100:
  lea edx, [rax+1]
  add ebx, DWORD PTR [rdi+rcx*4]
  movsx rdx, edx
  cmp rdx, rsi
  jnb .L99
  add eax, 2
  lea rcx, [0+rdx*4]
  add ebx, DWORD PTR [rdi+rdx*4]
  cdqe
  cmp rax, rsi
  jnb .L99
  add ebx, DWORD PTR [rdi+4+rcx]
.L99:
  test rdi, rdi
  je .L98
  mov rsi, QWORD PTR [rsp+32]
  sub rsi, rdi
  call operator delete(void*, unsigned long)
.L98:
  add rsp, 48
  mov eax, ebx
  pop rbx
  ret
.L107:
  xor eax, eax
  xor ecx, ecx
  jmp .L100
  mov rbx, rax
  jmp .L105
v() [clone .cold]:=