ModelSim SE User's Manual - Electrical and Computer Engineering

UM-128 5 - Verilog simulation
ModelSim SE User’s Manual
In these cases, you should optimize the entire design with vopt.
Several switches to vlog can be used to further increase optimizations on gate-level designs.
The +nocheck arguments are described in the Command Reference under the vlog
command (CR-358).
You can use the write cell_report command (CR-421) and the -debugCellOpt argument to
the vlog command (CR-358) to obtain information about which cells have and have not been
optimized. write cell_report produces a text file that lists all modules. Modules with
"(cell)" following their names are optimized cells. For example,
Module: top
Architecture: fast
Module: bottom (cell)
Architecture: fast
In this case, both top and bottom were compiled with -fast, but top was not optimized and
bottom was.
The -debugCellOpt argument is used with -fast when compiling the cell library. Using this
argument produces output in the Transcript pane that identifies why certain cells were not
optimized.
Event order and optimized designs
As mentioned earlier in the chapter, the Verilog language does not require that the
simulator execute simultaneous events in any particular order. Optimizations performed by
vopt may expose event order dependencies that cause a design to behave differently than
when run unoptimized. Event order dependencies are considered errors and should be
corrected (see "Event ordering in Verilog designs" (UM-132) for details).
Timing checks in optimized designs
Timing checks are performed whether you optimize the design or not. In general you'll see
the same results in either case. However, in a cell where there are both interconnect delays
and conditional timing checks, you might see different timing check results.
Without vopt the conditional checks are evaluated with non-delayed values, complying
with the original IEEE Std 1364-1995 specification. With vopt the conditional checks will
be evaluated with delayed values, complying with the new IEEE Std 1364-2001
specification.