( ESNUG 313 Item 8 ) ---------------------------------------------- [3/10/99]
From: [ Kenny from South Park ]
Subject: Bug Alert Concerning The hdlin_use_cin Design Compiler Switch
John,
Way back in ESNUG 264 we learned about a neat switch in Synopsys for
optomizing logic. The switch was "hdlin_use_cin = true" which allows you
to use the carry in input of an adder if you are doing an "a + b + 1"
operation. (Why Synopsys doesn't do this automatically I have no idea....)
This saves you an entire adder in your design, and can really speed up
your arithmatic.
However, recently a bug has surfaced. If you have any logic associated
with the "a" and "b" inputs which moves bits in the buss around (like a
simple shift to multiply by 2) then that logic gets "eaten" by the adder,
and you wind up with something that doesn't add up. I've actually gone
to the point of removing this variable from all of my .synopsys_dc.setup
files because of this.
- [ Kenny from South Park ]
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