Re: [boost] [iterators] Proof-of-concept for a sentinel iteratoradapter

Subject: Re: [boost] [iterators] Proof-of-concept for a sentinel iteratoradapter
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2009-05-13 12:50:06

AMDG

Kevin Sopp wrote:
> Interesting, I wrote a modified version where you can enter the string
> length and the sentinel version is consistently slower (using
> gcc-4.4.0 with -O3).
>
>   
>   {
>     boost::timer t;
>     for( unsigned i = 0; i < N; ++i )
>     {
>         m1 += test_strlen( text );
>     }
>
>     std::cout << "strlen: " << t.elapsed() << " " << m1 << std::endl;
>   }
>   

I'm not absolutely sure about this, but from the assembler it
looks like the call to strlen is being pulled out of the loop.

    movl    24(%esp), %eax
    movb    $0, (%eax,%edx)
    call    _clock
    movl    %eax, 20(%esp)
    xorl    %eax, %eax
    orl    $-1, %ecx
    movl    24(%esp), %edi
    repne scasb      //  This looks like strlen
    notl    %ecx
    movl    24(%esp), %eax
    leal    -1(%eax,%ecx), %esi
    xorl    %edi, %edi
    movl    $0, 28(%esp)
    .p2align 2,,3
L11:               // and the loop is here
    cmpl    %esi, 24(%esp)
    je    L32
    movl    24(%esp), %edx
    xorl    %eax, %eax
    .p2align 2,,3
L10:
    movsbl    (%edx),%ebx
    movl    %eax, %ecx
    sall    $6, %ecx
    leal    -1640531527(%ebx,%ecx), %ebx
    movl    %eax, %ecx
    shrl    $2, %ecx
    leal    (%ebx,%ecx), %ecx
    xorl    %ecx, %eax
    incl    %edx
    cmpl    %esi, %edx
    jne    L10
    addl    %eax, 28(%esp)
    incl    %edi
    cmpl    $1000000, %edi
    jne    L11

In Christ,
Steven Watanabe