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