( ESNUG 386 Item 1 ) --------------------------------------------- [01/16/02]
From: "Mike Montana" <montana@synopsys.com>
Subject: Buffer/Inverter PhysOpt 2001.08 & 2001.08-SP1 Bug Creates Bad Logic
Hello John,
One of the ACs in San Diego told me about a situation where PhysOpt can put
"extra inversions" in your netlist and create bad logic. This problem can
occur if your library has certain cell that can act as *both* a buffer and
an inverter. Below is an example of this type of cell followed by an
example of a simple buffer cell.
_________________
| CELL A |
| +---------| out_bar (pin 1)
| |\ | |\ |
in |--| >o-+---| >o--| out (pin 2)
| |/ |/ |
|_________________|
_________________
| CELL B |
| |\ |\ |
in |--| >o-----| >o--| out (pin 1)
| |/ |/ |
|_________________|
PhysOpt can swap between single output cells and the multi-output cells to
attempt buffering and/or sizing. If the two sets of cells have the pins
declared in a different order, then PhysOpt can get confused as to the
proper output pin connections. Consider the example above. CELL A pin 1 is
the inverted output while CELL B pin 1 is the non-inverted output. During
buffering and/or sizing, the swap between the two cells is done such that
net connected to pin 1 of CELL A is reconnected to pin 1 of CELL B. This
is incorrect and results in extra inversions and bad logic.
Please note the following regarding this bug:
1) It only exists in versions 2001.08 and 2001.08-SP1
2) It only occurs in Physical Compiler, not Design Compiler
3) It only occurs for those libs that have the cell pins declared in a
*different* order, (i.e. pin 1 is the non-inverting output on simple
buffers/inverters but is the inverting output on the multi-output
buffers/inverters. This problem will NOT occur if the pins are
defined in the same order on these cells.
There are two temporary workarounds to this problem.
1) use version 2000.11-SP1
2) use version 2001.08 and place a dont_use attribute on all of the
multi-output library cells which can act as both a buffer and an
inverter before running physopt. It is OK if the pre-physopt
netlist contains some of these cells.
For example:
set_dont_use vendor_lib/inv_buf*
physopt
This bug will be fixed in a patch release scheduled for EST availability the
first part of February 2002. This bug was documented in Solv-It article
Physical_Synthesis-325 but I also wanted to alert your readers to it, too.
- Mike Montana
Synopsys, Inc. Dallas, TX
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