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bitRAKE



Joined: 21 Jul 2003
Posts: 4042
Location: vpcmpistri
bitRAKE 07 Jul 2009, 20:35
Can you give a code snippet, revolution? I'm having trouble imagining how that would be optimal. I did find this recently - many awesome case studies.
Post 07 Jul 2009, 20:35
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 20340
Location: In your JS exploiting you and your system
revolution 08 Jul 2009, 01:13
Text is basically a big-endian storage format. The first character is the most significant for sorting purposes.
Code:
string_a: db 'Hello',0
string_b: db 'world',0

movbe eax,dword[string_a]
movbe ebx,dword[string_b]
cmp eax,ebx
ja swapStrings
jb alreadyInOrder
je goToNextDword
Post 08 Jul 2009, 01:13
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r22



Joined: 27 Dec 2004
Posts: 805
r22 08 Jul 2009, 20:49
Code:
movbe eax,dword[string_a]
movbe ebx,dword[string_b]

Is more compact, but is it faster than...
Code:
mov eax,dword[string_a]
mov ebx,dword[string_b]
bswap eax
bswap ebx


In any case the string compare is a good example of usage for the MOVBE REG,MEM (although it wouldn't be useful for unicode).

Can anyone think of a similar snippet/example/algo for MOVBE MEM,REG? The equivalent MOV / BSWAP / MOV would seem to benefit from this new opcode but I can't think of a situation where it would be done in iteration and where SIMD wouldn't be better served for the task.

Does Intel have a rationalization for MOVBE? Maybe it was purely for the sake of completeness or did they find a bottle neck in a large algorithm that the Intel Compiler couldn't handle well.

At this point in the x86 instruction set it's funny that BSWAP is getting a "service pack" Smile
Post 08 Jul 2009, 20:49
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