How to speed up the Rust compiler one last time
> Contrary to what you might expect, instruction counts have proven much better than wall times when it comes to detecting performance changes on CI, because instruction counts are much less variable than wall times (e.g. ±0.1% vs ±3%; the former is highly useful, the latter is barely useful). Using instruction counts to compare the performance of two entirely different programs (e.g. GCC vs clang) would be foolish, but it’s reasonable to use them to compare the performance of two almost-identical programs (e.g. rustc before PR #12345 and rustc after PR #12345). It’s rare for instruction count changes to not match wall time changes in that situation. If the parallel version of the rustc front-end ever becomes the default, it will be interesting to see if instruction counts continue to be effective in this manner.This is a supremely surprising conclusion, especially in 2020. Is instruction count really still tied to wall clock count? I would have thought that some instructions could be slower than others (especially on x86) so that using more faster individual instructions could be faster than 1 slower instruction. Similarly, cache effects & data dependencies can result in more instructions being faster than fewer instructions.I think what the author is trying to say is that when evaluating micro-optimizations, cycle counts are pretty valuable still because you're making a small intentional change & evaluating its impact & usually the correlation holds. The dashboard clearly still measures wall-clock since just comparing instruction count over time would be misleading.I'm curious if the Rust team has evaluated stabilizer to be more robust about the optimizations they choose: https://emeryberger.com/research/stabilizer/