would the workings of strlen() be something like this?
int strlen(const char * str)
{
int size = 0;
while (!!*str)
size++;
return size;
}any feedback would be appreciated. also i have learned that !! will convert a value to an actual bool eg. !!104 = 1
Jump to PostNo
while( *str++ ) size++;
Jump to PostThat's basically the idea. On Intel based PCs the compiler might actually implement it by using assembly instructions such as the scansb instruction, which greatly speeds up the operation because there is no software loop involved.
No while( *str++ ) size++;
ah yes I forgot to increment the pointer as well. so just drop the double ! and the incrementation would be the basic workings right?
int strlen(const char * str)
{
int size = 0;
while (*str++)
size++;
return size;
}That's basically the idea. On Intel based PCs the compiler might actually implement it by using assembly instructions such as the scansb instruction, which greatly speeds up the operation because there is no software loop involved.
I'm using MSVC++ 2005 is there a way to see the code for this function in assembly to see if it does convert it to a scansb instruction?
I think AD is referring to scasb, not scansb? I get way more hits on scasb. Frankly, I had never heard of either of them, but you could fit what I know about Assembly in a test tube and still have room for several cc's of blood. Here's a thread that might be of interest:
Google "assembly strlen scasb" and some good hits come up.
Thanks Vernon for the link it was very insightful. Also I found out how to output the code in assembly to check it and my code is much longer. I think I might have to play with my optimization settings. here it is just FYI
// driver.cpp
int strlen(const char * str)
{
int size = 0;
while (*str++)
size++;
return size;
}and
//driver.asm
; Listing generated by Microsoft (R) Optimizing Compiler Version 14.00.50727.762
TITLE c:\Documents and Settings\Nathan\My Documents\Visual Studio 2005\Projects\daniweb\daniweb\Driver.cpp
.686P
.XMM
include listing.inc
.model flat
INCLUDELIB MSVCRTD
INCLUDELIB OLDNAMES
PUBLIC ?strlen@@YAHPBD@Z ; strlen
EXTRN __RTC_Shutdown:PROC
EXTRN __RTC_InitBase:PROC
; COMDAT rtc$TMZ
; File c:\documents and settings\nathan\my documents\visual studio 2005\projects\daniweb\daniweb\driver.cpp
rtc$TMZ SEGMENT
__RTC_Shutdown.rtc$TMZ DD FLAT:__RTC_Shutdown
rtc$TMZ ENDS
; COMDAT rtc$IMZ
rtc$IMZ SEGMENT
__RTC_InitBase.rtc$IMZ DD FLAT:__RTC_InitBase
; Function compile flags: /Odtp /RTCsu /ZI
rtc$IMZ ENDS
; COMDAT ?strlen@@YAHPBD@Z
_TEXT SEGMENT
tv65 = -205 ; size = 1
_size$ = -8 ; size = 4
_str$ = 8 ; size = 4
?strlen@@YAHPBD@Z PROC ; strlen, COMDAT
; 11 : {
push ebp
mov ebp, esp
sub esp, 208 ; 000000d0H
push ebx
push esi
push edi
lea edi, DWORD PTR [ebp-208]
mov ecx, 52 ; 00000034H
mov eax, -858993460 ; ccccccccH
rep stosd
; 12 : int size = 0;
mov DWORD PTR _size$[ebp], 0
$LN2@strlen:
; 13 : while (*str++)
mov eax, DWORD PTR _str$[ebp]
mov cl, BYTE PTR [eax]
mov BYTE PTR tv65[ebp], cl
mov edx, DWORD PTR _str$[ebp]
add edx, 1
mov DWORD PTR _str$[ebp], edx
movsx eax, BYTE PTR tv65[ebp]
test eax, eax
je SHORT $LN1@strlen
; 14 : size++;
mov eax, DWORD PTR _size$[ebp]
add eax, 1
mov DWORD PTR _size$[ebp], eax
jmp SHORT $LN2@strlen
$LN1@strlen:
; 15 : return size;
mov eax, DWORD PTR _size$[ebp]
; 16 : }
pop edi
pop esi
pop ebx
mov esp, ebp
pop ebp
ret 0
?strlen@@YAHPBD@Z ENDP ; strlen
_TEXT ENDS
ENDafter optimization the code for my strlen function is now
?strlen@@YAHPBD@Z PROC ; strlen
; 12 : int size = 0;
xor eax, eax
$LL2@strlen:
; 13 : while (*str++)
; 14 : size++;
add eax, 1
cmp BYTE PTR $SG-5[eax], 0
jne SHORT $LL2@strlen
; 15 : return size;
; 16 : }
ret 0
?strlen@@YAHPBD@Z ENDP ; strlenI just found out that VC++ 2008 Express doesn't produce the assembly code that older M$ compilers did. Your function is the same as produced by VC++ 2008.
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