Author: Eugene Nalimov
Date: 19:23:32 01/03/99
Go up one level in this thread
On January 03, 1999 at 19:10:48, James Robertson wrote:
>On January 03, 1999 at 19:06:03, Eugene Nalimov wrote:
>
>>On January 03, 1999 at 18:40:08, James Robertson wrote:
>>
>>>[snip]
>>>
>>>>Actually, you don't need BSF/BSR instructions here. You can
>>>>use exactly the same trick that Robert Hyatt used in Crafty
>>>>source - use the fact that A&(A-1) clears least significant
>>>>bit. In Crafty 16.3 source there is in-line assembly stuff
>>>>for VC++ as well as C function. Function that counts bits is
>>>>just 2 loops, first handles lower half of 64-bit bitboard,
>>>>and second handles upper half:
>>>>
>>>> mov ecx, low_part_of_the_bitboard
>>>> xor eax, eax
>>>> test ecx, ecx
>>>> jz end_of_loop1
>>>>loop1:
>>>> lea edx, [ecx-1]
>>>> inc eax
>>>> and ecx, edx
>>>> jnz loop1
>>>>end_of_loop1:
>>>> mov ecx,high_part_of_the_bitboard
>>>> test ecx, ecx
>>>> jz end_of_loop2
>>>>loop2:
>>>> lea edx, [ecx-1]
>>>> inc eax
>>>> and ecx, edx
>>>> jnz loop2
>>>>end_of_loop:
>>>> ret
>>>>
>>>>I will not be surpised if that function will be faster even
>>>>on x86 processors that have fast BSF/BSR. Even if it's
>>>>slightly slower, it has the benefit that it's portable -
>>>>it will work reasonable well on *any* x86 compatible, be it
>>>>AMD, Cyrix, IDT, etc.
>>>>
>>>>Eugene
>>>
>>>I have several questions about x86 processors:
>>>Is
>>> xor eax,eax
>>>faster than
>>> mov eax,0
>>>
>>>Also, is
>>> inc eax
>>>faster than
>>> add eax,1
>>>?
>>>
>>>Thanks!
>>>
>>>James
>>
>>No and yes. If you just count # of ticks (or of ROPs),
>>they will be exactly the same. But those instructions
>>are shorter - xor uses 2 bytes, move uses 5 bytes; inc
>>is 1 byte, add is 3 bytes. Smaller programs usually
>>run faster - they occupy less space in cache, so
>>traffic to slow main memory is smaller.
>>
>>Eugene
>
>Thanks! Also, I just noticed a command I am unfamiliar with:
>lea
>What does it do?
>
>Thanks!
>James
Load effective address. It evaluates address of its'
second operand (exactly as 'move' does), and loads that
address into destination register (move loads *contents*
of memory at that address). You often can use that
instruction to perform several operations at once - e.g.
lea eax, 8[ebx+ecx*8]
roughly equivalent (it doesn't set flags) to
mov eax, ecx
shl eax, 3
add eax, ebx
add eax, 8
Eugene
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