strlen() speed test

[Nollie]

I need to write a couple of my own string manipulation routines (e.g.
a strcpy() alternative that returns the number of chars copied). I've
started with one of the simpler functions, strlen(). I've written a
very simple version call StringLength(), but it performs significantly
slower than its CRT counterparts.

Here's my implementation:
inline unsigned int StringLength( const char *pszString )
{
unsigned int cch = 0;
while ( *pszString++ )
cch++;
return cch;
}

I've compared my version to strlen() and _mbslen() using my own timer
function, which is a wrapper around the Windows
QueryPerformanc eCounter() API.

My only guess is that the CRT versions use the processor registers for
its counters and my version uses RAM. (You can't make use of the
"register" keyword with the VC++ compiler.)

How can I make my string manipulation functions compete speedwise with
the CRT functions?

[Peter Smithson]

The CRT source is available. I can't remember where you get it but it's
on my PC and I put a breakpoint in strlen. Here's the source. It's in
assembler and reads 4 bytes at a time (if it's word aligned) -
CODESEG

public strlen

strlen proc

.FPO ( 0, 1, 0, 0, 0, 0 )

string equ [esp + 4]

mov ecx,string ; ecx -> string
test ecx,3 ; test if string is aligned on
32 bits
je short main_loop

str_misaligned:
; simple byte loop until string is aligned
mov al,byte ptr [ecx]
inc ecx
test al,al
je short byte_3
test ecx,3
jne short str_misaligned

add eax,dword ptr 0 ; 5 byte nop to align label
below

align 16 ; should be redundant

main_loop:
mov eax,dword ptr [ecx] ; read 4 bytes
mov edx,7efefeffh
add edx,eax
xor eax,-1
xor eax,edx
add ecx,4
test eax,81010100h
je short main_loop
; found zero byte in the loop
mov eax,[ecx - 4]
test al,al ; is it byte 0
je short byte_0
test ah,ah ; is it byte 1
je short byte_1
test eax,00ff0000h ; is it byte 2
je short byte_2
test eax,0ff000000h ; is it byte 3
je short byte_3
jmp short main_loop ; taken if bits 24-30 are clear
and bit
; 31 is set

byte_3:
lea eax,[ecx - 1]
mov ecx,string
sub eax,ecx
ret
byte_2:
lea eax,[ecx - 2]
mov ecx,string
sub eax,ecx
ret
byte_1:
lea eax,[ecx - 3]
mov ecx,string
sub eax,ecx
ret
byte_0:
lea eax,[ecx - 4]
mov ecx,string
sub eax,ecx
ret

strlen endp

end

[Thomas Matthews]

Peter Smithson wrote:

The CRT source is available. I can't remember where you get it but it's
on my PC and I put a breakpoint in strlen. Here's the source. It's in
assembler and reads 4 bytes at a time (if it's word aligned) -
CODESEG

public strlen

strlen proc

.FPO ( 0, 1, 0, 0, 0, 0 )

string equ [esp + 4]

mov ecx,string ; ecx -> string
test ecx,3 ; test if string is aligned on

This doesn't work on my embedded ARM7TDMI platform.
Care to post a solution in:
ARM Assembly
Z80 Assembly
8051 Assembly
Sparc Assembly
.. etc.

In otherwords, assembly is off-topic in this newsgroup.
However, the algorithm for assembly, when coded in C++
is on topic. One can have good luck at forcing the
compiler to generate desired code by writing simple
C++ that imitates the assembly language statements.
--
Thomas Matthews

C++ newsgroup welcome message:
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Other sites:
http://www.josuttis.com -- C++ STL Library book
http://www.sgi.com/tech/stl -- Standard Template Library

[Peter Smithson]

Thomas Matthews wrote:

In otherwords, assembly is off-topic in this newsgroup.
However, the algorithm for assembly, when coded in C++
is on topic. One can have good luck at forcing the
compiler to generate desired code by writing simple
C++ that imitates the assembly language statements.

OK - I'll not do it again.

In my defence the poster is asking how he can write code as fast as the
CRT code that comes with the compiler he's using with (I know, OT). By
looking at the assembly you can see how to improve the C++ algorithim.
Would it have been on topic if I'd said "I've looked at the assembly to
the routines you've been asking about .." and then described the
algorithim without using C++ or assembler but just words?

This reminds me of how in the UK the actual voice of IRA leaders were
not allowed on the TV in case they corrupt us. So we'd see news clips
of them with dubed over voices!