7.3.2. Choice of optimization level

The GNU C++ compiler (like other Linux C++ compilers) offers various levels of optimization from none (-O0) through to (-O3). There is a trade off to be made—more optimization means longer compile times, but faster run times.

GNU C++ also offers -Os, to optimize for space. This is equivalent to -O2, but omitting any optimizations that tend to increase the size of the program.

Table 7.3 shows the impact of the different optimization levels on the example design.

Run Description	Build Time	Run Time	Performance
`-O0`	13.98 s	25.05 s	47.10 kHz
`-O1`	21.51 s	13.13 s	89.90 kHz
`-O2`	32.77 s	12.76 s	92.46 kHz
`-O3`	35.35 s	12.39 s	95.25 kHz
`-Os`	26.23 s	12.24 s	96.41 kHz

Table 7.3. Comparison of model performance with different compiler optimization settings.

Almost all the benefit is gained from -O1, but there are incremental benefits, at the expense of greater compile times for higher levels of optimization.

Note however that the highest performance is with -Os. Code generated by Verilator (and its commercial rivals) has a classic "cache-busting" structure. On each code cycle execution starts at the top and proceeds linearly to the bottom. Anything that reduces the code size, increases the likelihood of code remaining in the cache, and so can have a very large performance benefit.

The recommendation is to use -Os as the preferred C++ compiler option.