[Bug tree-optimization/35269] New: missed optimization of std::vector access.

[pluto at agmk dot net]

please consider following testcase:

#include <vector>
void f( int );
typedef std::vector< int > V;
void g( V const& v, V::size_type n )
{
        for ( V::size_type i = 0; i < n; i++ )
                f( v[ i ] );
}
void h( V const& v, V::size_type n )
{
        for ( V::const_iterator i = v.begin(), e = i + n; i != e; ++i )
                f( *i );
}

g() and h() traverse vector in the same way and imho should
produces the same assembly code but compiler producess
unoptimal code for g().

_Z1gRKSt6vectorIiSaIiEEm:
        pushq   %r12
        testq   %rsi, %rsi
        movq    %rdi, %r12
        pushq   %rbp
        movq    %rsi, %rbp
        pushq   %rbx
        je      .L12
        xorl    %ebx, %ebx

.L11:   movq    (%r12), %rax         <=== h() is smarter in this point.
        movl    (%rax,%rbx,4), %edi  <===

        addq    $1, %rbx
        call    _Z1fi
        cmpq    %rbp, %rbx
        jne     .L11

        popq    %rbx
        popq    %rbp
        popq    %r12
        ret

_Z1hRKSt6vectorIiSaIiEEm:
        pushq   %rbp
        pushq   %rbx
        subq    $8, %rsp
        movq    (%rdi), %rbx
        leaq    (%rbx,%rsi,4), %rbp
        cmpq    %rbp, %rbx
        je      .L4

.L5:    movl    (%rbx), %edi
        addq    $4, %rbx
        call    _Z1fi
        cmpq    %rbx, %rbp
        jne     .L5

        addq    $8, %rsp
        popq    %rbx
        popq    %rbp
        ret


-- 
           Summary: missed optimization of std::vector access.
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: pluto at agmk dot net
GCC target triplet: x86_64-gnu-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35269