问题
While looking for ways to speed up my simulation, I came across the --force-lto option.
I've heard about LTO (Link Time Optimization) before, so that made me wonder why isn't --force-lto the default while building gem5?
Would that make a simulation go much faster than a gem5.fast build compared to a gem5.opt build?
回答1:
In gem5 fe15312aae8007967812350f8cdac9ad766dcff7 (2019), the gem5.fast build already enables LTO by default, so you generally never want to use that option explicitly, but rather want just gem5.opt.
That option exists because the more common gem5.opt build also uses partial linking, which in some versions of GCC was incompatible with LTO.
Therefore, as its the name suggests, --force-lto forces the use of LTO together with partial linking, which might not be stable. That's why I recommend that you use gem5.fast rather than touching --force-lto.
The goal of partial linking is presumably to speed up the link step, which can easily be the bottleneck in a "change on file, rebuild, relink, test" loop, although in my experiments it is not clear that it is efficient at doing that. Today it might just be a relic from the past.
To try to speed up linking, I recommend that you try scons --gold-linker instead, which uses the GOLD linker instead of ld. Note that this option was more noticeably effective for gem5.debug however.
I have found that gem5.fast is generally 20% faster than gem5.opt for Atomic CPUs.
来源：https://stackoverflow.com/questions/58133850/does-the-force-lto-gem5-scons-build-option-speed-up-simulation-significantly-a