XST是否只使用一个LUT6来执行

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- 鲍勃埃尔金德
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以下为原文

This recent thread might be helpful for you.
 
-- Bob Elkind
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鲁迪写道：
嗨，
我正在使用Virtex 5设备。
我想一次采用5位的逻辑或，另一次采用这5位的逻辑AND。
我知道在Virtex 5中，LUT6架构是这样的，我们可以在6个独立输入上执行6个布尔运算并产生一个输出，或者，我们可以使用LUT6，这样我们只使用5位（以共享方式）
产生2组独立输出，每组输出在5个输入位上进行不同的布尔运算。
所以，我相信只有一个LU6，我能够执行5位输入的逻辑OR和逻辑AND。
但这是我的问题？
我如何知道综合工具（XST）将如何解释此操作？
我知道我可以选择以我想要的方式实例化LUT6，但我想避免实例化，让工具推断出我想要的操作。
假设在我的vhdl代码中，II有类似下面一行的内容：
X
ÿ
我如何知道XST是否只使用一个LUT6来执行此操作，或者实际上它将使用两个LUT6来执行上述操作？
在我的设计中，我的资源非常有限，我必须采取所有预防措施，以确保我使用尽可能少的资源。
关于我能代表我做什么（实例化除外）的任何想法，以确保该工具仅使用一个LUT6执行上述操作？
提前致谢，
鲁迪
为什么不以对人类有意义的方式编写代码，让合成器对其进行排序？
只要您满足时序约束，工具如何选择构建逻辑就无关紧要了。
----------------------------是的，我这样做是为了谋生。



以下为原文

rudy wrote:
 Hi,
 
I am working with a Virtex 5 device.  I would like to once take the logical OR of 5 bits, and the other time, take the logical AND of these 5 bits.
I know that in Virtex 5, LUT6 architecture is such that we can perform either 6 Boolean operation on 6 independent inputs and produce one output, Or, that we can use the LUT6 such that we use only 5 bits (in a shared fashion) to produce 2 sets of independent outputs, with each being a different Boolean operation on the 5 input bits. 
So, I believe that with just one LU6, I am able to perform both logical OR and logical AND of a 5 bit input.
But here is my question?
How do I know that how the synthesis tool (XST) will interpret this operation? 
I know that I have an option of instantiating a LUT6 the way that I want, but I would like to avoid the instantiation, and let the tool infer my desired operation. 
Let's say that II have something like the following line, in my vhdl code:
 
X <= x(0) or x(1) or x(2) or x(3) or x(4) 
Y <= x(0) and x(1) and x(2) and x(3) and x(4)
 
How do I know if XST will use just one LUT6 to perform this operation, or it will in fact use two LUT6 to perform the above operation?
In my design, I am very resource limited, and I have to take all the precautions to make sure I am using as less resources as possible.
Any idea as far as what I can do on my behalf (except instantiation) to make sure that the tool will perform the above operation using only one LUT6?
 
Thanks in advance, 
Rudy 
 

Why not just write the code in a way that makes sense to a human, and let the synthesizer sort it out? As long as you meet your timing constraints, how the tools choose to build the logic is irrelevant.
----------------------------Yes, I do this for a living.