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Topic: About LOCAL string (Read 10948 times)
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stbfish
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LOCAL szDir[124]:BYTE
how to initialize it to all 0? like this szBuffer db 124 dup(0)
when i define a LOCAL Var, what is the default value inside?
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BlackVortex
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Well, default value is zero !
Right ?
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sinsi
Member
Gender: 
Posts: 1758
RIP Bodie 1999-2011
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Because locals are on the stack they could be any value. You will need to zero it yourself - RtlZeroMemory I think will work here.
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Light travels faster than sound, that's why some people seem bright until you hear them.
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ecube
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sinsi is right, and this is important to remember as it could have unexpected results for your program. I always rtlzeromemory my local buffers before using it, unless I Use GlobalAlloc or similar myproc proc
local mybuf[1024]:BYTE
invoke RtlZeroMemory,addr mybuf,1024
;now save to use
ret
myproc endp a global buffer will always be initalized to zero though e.g mygbuf byte 1024 dup(?) also when it comes to buffer sizes I usually try and stick with 1 evenly divisible by 4,since the 32bit processor reads everything in dwords, you could also use align 4, align 8 etc before it... to speed things up. align 8 myebuf byte 16 dup(?) |
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NervGaz
Member
Posts: 25
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RtlZeroMemory should work fine... But IIRC it's a lot faster just using a "rep stosb/w/d" to do it... mind you I haven't really been coding anything in asm for a good few years so I think someone else might be better suited to give and example that'll work straight away... I'll give it a shot tho... xor eax,eax
lea edi,szDir
mov ecx,31 ;as it's a quarter of the buffers size
rep stosd
hopefully I got that right |
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ecube
Guest
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RtlZeroMemory should work fine... But IIRC it's a lot faster just using a "rep stosb/w/d" to do it... mind you I haven't really been coding anything in asm for a good few years so I think someone else might be better suited to give and example that'll work straight away... I'll give it a shot tho... xor eax,eax
lea edi,szDir
mov ecx,31 ;as it's a quarter of the buffers size
rep stosd
hopefully I got that right I disagree, back when I started the speed test thread on Zero memory functions http://www.masm32.com/board/index.php?topic=6576.0 , it was determined RtlZeroMemory was the fastest on buffers 1024 bit+ and masmlib memfill was faster on smaller buffers. I think RtlZeroMemory uses rep in its code anyway. |
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NervGaz
Member
Posts: 25
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Again it's been ages since I coded in asm, actually apart from a few filemaker scripts
(i know not even really coding) it's been ages since I coded anything... But yeah I
remember a similar conversation about 7 or 6 years ago on a forum... Only reason i
mentioned stosb/w/d was that the buffer in his example was 124 bytes long...
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hutch--
Administrator
Member
Posts: 12013
Mnemonic Driven API Grinder
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A couple of things, in most instances when you allocate a buffer on the stack for text, you only ever need to write ONE ZERO at the start, not zero fill the entire buffer. As long as you don't try and write past the end of the allocated space, normal zero terminated string procedures only need to know the start address of the buffer.
If you really must zero fill the buffer, DON'T use REP STOSD as its slow on the first 500 bytes or so, do it in the normal manner using a DWORD sized register with zero in it.
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NervGaz
Member
Posts: 25
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Goes to show how much I remember... heh...
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Slugsnack
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RtlZeroMemory should work fine... But IIRC it's a lot faster just using a "rep stosb/w/d" to do it... mind you I haven't really been coding anything in asm for a good few years so I think someone else might be better suited to give and example that'll work straight away... I'll give it a shot tho... xor eax,eax
lea edi,szDir
mov ecx,31 ;as it's a quarter of the buffers size
rep stosd
hopefully I got that right I disagree, back when I started the speed test thread on Zero memory functions http://www.masm32.com/board/index.php?topic=6576.0 , it was determined RtlZeroMemory was the fastest on buffers 1024 bit+ and masmlib memfill was faster on smaller buffers. I think RtlZeroMemory uses rep in its code anyway. Source for RtlZeroMemory, taken from debugging in VC++ ( nice comments Microsoft ! ) : memset proc \
dst:ptr byte, \
value:byte, \
count:dword
OPTION PROLOGUE:NONE, EPILOGUE:NONE
.FPO ( 0, 3, 0, 0, 0, 0 )
mov edx,[esp + 0ch] ; edx = "count"
mov ecx,[esp + 4] ; ecx points to "dst"
test edx,edx ; 0?
jz short toend ; if so, nothing to do
xor eax,eax
mov al,[esp + 8] ; the byte "value" to be stored
; Special case large block zeroing using SSE2 support
test al,al ; memset using zero initializer?
jne dword_align
cmp edx,0100h ; block size exceeds size threshold?
jb dword_align
cmp DWORD PTR __sse2_available,0 ; SSE2 supported?
je dword_align
jmp _VEC_memzero ; use fast zero SSE2 implementation
; no return
; Align address on dword boundary
dword_align:
push edi ; preserve edi
mov edi,ecx ; edi = dest pointer
cmp edx,4 ; if it's less then 4 bytes
jb tail ; tail needs edi and edx to be initialized
neg ecx
and ecx,3 ; ecx = # bytes before dword boundary
jz short dwords ; jump if address already aligned
sub edx,ecx ; edx = adjusted count (for later)
adjust_loop:
mov [edi],al
add edi,1
sub ecx,1
jnz adjust_loop
dwords:
; set all 4 bytes of eax to [value]
mov ecx,eax ; ecx=0/0/0/value
shl eax,8 ; eax=0/0/value/0
add eax,ecx ; eax=0/0val/val
mov ecx,eax ; ecx=0/0/val/val
shl eax,10h ; eax=val/val/0/0
add eax,ecx ; eax = all 4 bytes = [value]
; Set dword-sized blocks
mov ecx,edx ; move original count to ecx
and edx,3 ; prepare in edx byte count (for tail loop)
shr ecx,2 ; adjust ecx to be dword count
jz tail ; jump if it was less then 4 bytes
rep stosd
main_loop_tail:
test edx,edx ; if there is no tail bytes,
jz finish ; we finish, and it's time to leave
; Set remaining bytes
tail:
mov [edi],al ; set remaining bytes
add edi,1
sub edx,1 ; if there is some more bytes
jnz tail ; continue to fill them
; Done
finish:
mov eax,[esp + 8] ; return dest pointer
pop edi ; restore edi
ret
toend:
mov eax,[esp + 4] ; return dest pointer
ret
memset endp
end |
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ecube
Guest
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Nice Slugsnack  I wonder if lingo or the other speed guru's could speed that up. I'm suprised they used sse2, considering how a lot of other WinAPI are slow. |
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Slugsnack
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Nice Slugsnack I wonder if lingo or the other speed guru's could speed that up. I'm suprised they used sse2, considering how a lot of other WinAPI are slow. Huhhh I don't see any SSE2 instructions lol. Only regular x86 ones |
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sinsi
Member
Gender:
Posts: 1758
RIP Bodie 1999-2011
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Well, you missed the interesting bit jmp _VEC_memzero ; use fast zero SSE2 implementation |
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Light travels faster than sound, that's why some people seem bright until you hear them.
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ecube
Guest
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hehe woops |
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hutch--
Administrator
Member
Posts: 12013
Mnemonic Driven API Grinder
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Something that needs to be kept in mind with memory copy algos is if both locations, (source and destination) are already in processor cache. If they are the DWORD sized copy is reasonably fast but if the target and source are not in cache, you get cache pollution problems. REP MOVSD and the SSE/MMX version if written correctly read through cache but write back directly to memory which is usually faster when source and destination are not both in cache.
You test this by allocating two memory blocks that are large enough not to fit into cache then copy data from one to the other.
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