CFamily Analysis Cache Questions

[Abbas (Abbas Sabra)]

Hello @Sam_Anthonisz,

@Sam_Anthonisz

In this scenario we can see it’s because the dependencies i.e. header have changed. Can I confirm if the cache miss for an individual file applies based on the header being a dependency in the AST e.g. it’s included either directly or via another header (possibly a PCH), or does the fact that one file has changed invalidate the entire cache i.e. for all files?

You are right. This is one reason for cache entry invalidation. Another reason is change in the compilation command. You can think of it this way: if the compiler thinks that a source file should be recompiled then it is a cache miss.

@Sam_Anthonisz

Also, we can see the analysis has loaded the cache from the server; however what logic is used to determine what cache this is e.g. it is the cache of the last analysis of the PR’s base branch e.g. main|develop etc. If so is it always the latest or does it relate to a specific git commit i.e. the one the head branch of the PR was branched from?

The main branch updates the cache and it is the one that is used. PR doesn’t update the cache. there is one entry per source file and it comes from the latest analysis of the main branch.

@Sam_Anthonisz

For example if we re-run the above PR analysis it continues to get 0 cache hits, implying that, for PRs at least, the analysis cache is not uploaded to the server?

No, because the cache isn’t updated from PR.

@Sam_Anthonisz

If so, would that seem like a logical optimisation as it is quite common for a PR to have additional commits required as a result of reviews etc. and that in turn will trigger another analysis which could benefit from the cache of the prior analysis?

This doesn’t take care of the case where multiple PR are running in parallel. Currently, the default cache is simple and covers simple cases (easy win).

For more complicated cases, you can control CFamily cache by using explicitly the filesystem cache.
This way you can decide when to update the cache in a way that is tailored to your workflow. Also, You can have multiple filesystem caches (a cache per PR).

I hope it is clearer now!

Thanks,