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From: Andy on 23 Sep 2009 16:51 On Sep 23, 10:06 am, gabor <ga...(a)alacron.com> wrote: > It turns out that even with simple binary encoding, the values > -1 to 2 (or any set of four contiguous integer values) can > always be decoded using only bits 1 and 0 as these are > different for each value. In the case of -1 to 2 they are > 11 00 01 10, and so even though bit 2 also changes it is > not necessary to use bit 2 for decoding IF the optimising > agent is smart enough to detect that bit 2 can be described > as a function of bits 1 and 0 and is therefore redundant. You know that, and I know that, but I have yet to see a synthesis tool that knows that and takes advantage of it. Make an enumerated type with values minus_one, zero, one and two, and it might, but only because it has no clue that "one" in english is 1. Andy
From: glen herrmannsfeldt on 23 Sep 2009 17:31
Andy <jonesandy(a)comcast.net> wrote: (snip) < You know that, and I know that, but I have yet to see a synthesis tool < that knows that and takes advantage of it. It is probably right to say that the synthesis tool doesn't know that. But following synthesis on most systems is a redundant logic removal pass which will likely figure out many cases. More often, non-redundant logic is removed due to errors in the input. Signals may be constant for testing purposes, or otherwise accidentally, and be removed. With a little luck, the whole design is removed! < Make an enumerated type with values minus_one, zero, one and two, and < it might, but only because it has no clue that "one" in english is 1. If you assign it to a two bit bus it will know. (I think in verilog, not VHDL, but I don't think that matters for this question.) -- glen |