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Azu



Joined: 16 Dec 2008
Posts: 1159
Azu
What's the point of lzcnt when there is already bsr?

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Post 24 Sep 2009, 13:09
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Tomasz Grysztar



Joined: 16 Jun 2003
Posts: 7802
Location: Kraków, Poland
Tomasz Grysztar
They behave differently, when the value consists of all zeros.
BSR searches for the highest bit that is set to 1, and when there's no such bit found, it returns with ZF cleared, and destination not modified.
On the other hand, LZCNT counts the amount of zeros from the highest bit, and when the value consists of all zeros, it simply returns the amount equal to the size (in bits) of the source operand.
Post 24 Sep 2009, 13:49
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17716
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revolution
Tomasz Grysztar wrote:
BSR searches for the highest bit that is set to 1, and when there's no such bit found, it returns with ZF cleared, and destination not modified.
Minor correction: Although what you say may be true in practice, the docs state that the dest is undefined in that situation. So it is probably best not to rely upon any unmodified behaviour.
Post 24 Sep 2009, 13:54
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MazeGen



Joined: 06 Oct 2003
Posts: 977
Location: Czechoslovakia
MazeGen
And don't forget that LZCNT is AMD-specific and AFAIK Intel is not going to implement it. Let me know if I'm wrong.
Post 24 Sep 2009, 14:08
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rugxulo



Joined: 09 Aug 2005
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Location: Usono (aka, USA)
rugxulo
MazeGen wrote:
And don't forget that LZCNT is AMD-specific and AFAIK Intel is not going to implement it. Let me know if I'm wrong.


Why, though? Patents? Pride? I can't see the harm in both implementing such instructions, but I don't know what goes on behind the scenes.
Post 27 Sep 2009, 01:41
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MazeGen



Joined: 06 Oct 2003
Posts: 977
Location: Czechoslovakia
MazeGen
I'm not sure. Competition and different roadmaps perhaps. And important players' opinion matters I think. If developers of Visual C++ tell Intel "hey, there's LZCNT which could be useful", they can force them to implement it.

BTW, there are Intel forums where you could ask. The Intel guys are pretty responsive.
Post 27 Sep 2009, 12:16
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