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That pipelining is going to be external to multiplier. With inference you can not decide internal pipe which could fail timing
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You have to let the synthesizer/fitter do register retiming, and the synthesisor WILL infer internal pipe stages.
There is a bug for stratix 4 that means that for larger multipliers (larger than 18x18, so multiple DSPs required) the output register is not inferred properly, and so an LPM mult block is required to use all the internal pipe stages, but the same bug is not present on Stratix V or Arria 10.
This was an issue I came up against and confirmed it with Altera support.
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