[Bug tree-optimization/109443] New: missed optimization of std::vector access (Related to issue 35269)

["hiraditya at msn dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109443

            Bug ID: 109443
           Summary: missed optimization of std::vector access (Related to
                    issue 35269)
           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: ---

here is slightly modified code example from issue #35269. Both accesses are
similar bug different code is generated. the function `h` has better codegen
than `g` for some reason.


$ g++ -O3 -std=c++20 -fno-exceptions

void f(int);

void g(std::vector<int> v)
{
    for (std::vector<int>::size_type i = 0; i < v.size(); i++)
        f( v[ i ] );
}

void h(std::vector<int> v)
{
    for (std::vector<int>::const_iterator i = v.begin(); i != v.end(); ++i)
        f( *i );
}


g(std::vector<int, std::allocator<int> >):
        mov     rdx, QWORD PTR [rdi]
        cmp     QWORD PTR [rdi+8], rdx
        je      .L6
        push    rbp
        mov     rbp, rdi
        push    rbx
        xor     ebx, ebx
        sub     rsp, 8
.L3:
        mov     edi, DWORD PTR [rdx+rbx*4]
        add     rbx, 1
        call    f(int)
        mov     rdx, QWORD PTR [rbp+0]
        mov     rax, QWORD PTR [rbp+8]
        sub     rax, rdx
        sar     rax, 2
        cmp     rbx, rax
        jb      .L3
        add     rsp, 8
        pop     rbx
        pop     rbp
        ret
.L6:
        ret



h(std::vector<int, std::allocator<int> >):
        push    rbp
        push    rbx
        sub     rsp, 8
        mov     rbx, QWORD PTR [rdi]
        cmp     rbx, QWORD PTR [rdi+8]
        je      .L10
        mov     rbp, rdi
.L12:
        mov     edi, DWORD PTR [rbx]
        add     rbx, 4
        call    f(int)
        cmp     QWORD PTR [rbp+8], rbx
        jne     .L12
.L10:
        add     rsp, 8
        pop     rbx
        pop     rbp
        ret