Verilator divides its code into two categories. That which is executed every cycle ("fast" code) and that which is executed less frequently ("slow" code). The OPT_FAST macro of the Verilator generated Makefile specifies optimizations to be applied to the "fast" code. Conversely the OPT_SLOW macro specifies optimizations to be applied to the "slow" code. For convenience the macro OPT can be used to specify optimizations that will be applied to both categories of code.

The separation allows focusing of optimization effort for large designs, where compile times are significant. Just specifying OPT_FAST gains most of the model performance benefit, without the overhead of optimizing the "slow" code.

Table 7.2 shows the effect of using the GNU C++ compiler's highest level of optimization (-O3) with OPT_FAST, OPT_SLOW and OPT. These can be passed as macros to the Makefile in the example for this application note:

make verilate COMMAND_FILE=cf-optimized-8.scr NUM_RUNS=1000 OPT_FAST="-O3"
	

In the example used in this application note (which is not huge), none of the model build times are unreasonable. As can be seen OPT_SLOW profiling has no significant effect in this example.