[Libre-soc-bugs] [Bug 1156] Verilator compile flag testing

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=1156

--- Comment #1 from Sadoon Albader <sadoon at albader.co> ---
Some testing was done with verilator's compile flags to measure if there is any
performance left on the table without modifying verilator's source code.

These flags were applied as "CFLAGS" in addition to "-O3" which was already
applied.

`-mcpu=native -mtune=native`

These flags were applied as verilator flags as well

`--threads-dpi all --x-initial fast`

Tests were made with `THREADS = 3` and produced the following results:

`verilator_trace` branch: 83m17s to boot linux-microwatt-5.7 (to the login
prompt)
`optimizations` branch: 83m53s to boot linux-microwatt-5.7 (to the login
prompt)

Both tests were done on an Acer Swift 5 with an Intel i7 1260P (performance
cpupower setting).

Conclusion: there is no measurable benefit to using any of these flags, and
reading through verilator's manpage; it seems there are no other flags that can
affect performance. We need to modify verilator's source code to have any speed
ups.

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