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flattening synthetic operators

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Hi, I have seen that flattening synthetic operators right after elaboration time, before any synthesis gives more efficient implementation (smaller area, faster circuit and less CPU runtime).

I want to know whether flattening operators before any synthesis has a disadvantage?


Originally posted in cdnusers.org by sporadic crash
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  • Hi Grasshopper, I was quite busy with some other things. I am also using the latest 7.2-xxx versions here in house.

    I have more findings in this subject. Flatting helps if not all ports of inferred synthetic operator are used. This is reasonable, in that case RC can optimize out the parts of the operator which are connected to unused ports.

    Otherwise, it is better to keep the synthetic operators until the end of synthesis as you said. Sharing, CSA speculation or other transformations want to see unflattened synthetic operators, ie. keep them "virgin" until the end. The attributes you mentioned increase CPU runtime, which is also addressed in manuals.

    But synthetic operators must be flattened in the final stage, because in most case not all the bits in an arithmetic operation are used. Flattening and 2nd incremental synthesis saves silicon.

    It is also better to keep the synthetic operators unflattened even if retiming is enabled. In this case RTL Compiler is so intelligent that it moves FFs into the synthetic operators "thru the membrane", without destroying their structure. I must make experiments to see whether the attributes you mention really downsize/upsize datapaths with such structures. This is of course time-consuming to test..

    But I must admit, RTL Compiler is a very intelligent tool. This is not an ad. As a user I am really impressed of the performance of the tool.


    Originally posted in cdnusers.org by sporadic crash
Reply

  • Hi Grasshopper, I was quite busy with some other things. I am also using the latest 7.2-xxx versions here in house.

    I have more findings in this subject. Flatting helps if not all ports of inferred synthetic operator are used. This is reasonable, in that case RC can optimize out the parts of the operator which are connected to unused ports.

    Otherwise, it is better to keep the synthetic operators until the end of synthesis as you said. Sharing, CSA speculation or other transformations want to see unflattened synthetic operators, ie. keep them "virgin" until the end. The attributes you mentioned increase CPU runtime, which is also addressed in manuals.

    But synthetic operators must be flattened in the final stage, because in most case not all the bits in an arithmetic operation are used. Flattening and 2nd incremental synthesis saves silicon.

    It is also better to keep the synthetic operators unflattened even if retiming is enabled. In this case RTL Compiler is so intelligent that it moves FFs into the synthetic operators "thru the membrane", without destroying their structure. I must make experiments to see whether the attributes you mention really downsize/upsize datapaths with such structures. This is of course time-consuming to test..

    But I must admit, RTL Compiler is a very intelligent tool. This is not an ad. As a user I am really impressed of the performance of the tool.


    Originally posted in cdnusers.org by sporadic crash
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