[llvm] [GitHub] Remove redundant cache key prefix (PR #76914)

[Craig Hesling via llvm-commits]

linux4life798 wrote:

> On a side note, I think we should look into how many cache hits we get in a build and see if we can improve it, I don't think we have good enough hits since the builds still takes quite a while. In our internal jobs we run ccache -s after each build to see how many hits and misses we have, maybe we can do something similar with sscache? Just writing it here to see if someone wants to pick that task up :)

Yeah, I've been doing a bit of active research on this. It seems to me that we need to ensure that their are periodic builds on main (the default branch), since caches will be restored from the default branch (main) cache if none exist in the branch being built (might be a PR branch). Additionally, we could consider disable cache saving on non-main branches (like pull requests) to ensure that the main branch cache doesn't get evicted and can continue to grow towards that 10GB cache quota. I saw on the Discord that there were a lot of opinions and options, but I think we could probably just get this to work reasonably well within GitHub.

If it wasn't already known, a warm cache can bring a build time down from roughly 1.33 hours to 9 mins (libclang build).

https://github.com/llvm/llvm-project/pull/76914