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perf: Speed up search for short associated functions, especially very common identifiers such as new #17927

Merged
merged 3 commits into from
Aug 23, 2024
Merged

perf: Speed up search for short associated functions, especially very common identifiers such as new #17927

merged 3 commits into from
Aug 23, 2024

Commits on Aug 22, 2024

  1. When descending into macros in search, first check if there is a need… 

    … to - i.e. if we are inside a macro call

This avoids the need to analyze the file when we are not inside a macro call.

This is especially important for the optimization in the next commit(s), as there the common case will be to descent into macros but then not analyze.
    @ChayimFriedman2
    ChayimFriedman2 committed Aug 22, 2024
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  2. Speed up search for short associated functions, especially very commo… 

    …n identifiers such as `new`

The search is used by IDE features such as rename and find all references.

The search is slow because we need to verify each candidate, and that requires analyzing it; the key to speeding it up is to avoid the analysis where possible.

I did that with a bunch of tricks that exploits knowledge about the language and its possibilities. The first key insight is that associated methods may only be referenced in the form `ContainerName::func_name` (parentheses are not necessary!) (Rust doesn't include a way to `use Container::func_name`, and even if it will in the future most usages are likely to stay in that form.

Searching for `::` will help only a bit, but searching for `Container` can help considerably, since it is very rare that there will be two identical instances of both a container and a method of it.

However, things are not as simple as they sound. In Rust a container can be aliased in multiple ways, and even aliased from different files/modules. If we will try to resolve the alias, we will lose any gain from the textual search (although very common method names such as `new` will still benefit, most will suffer because there are more instances of a container name than its associated item).

This is where the key trick enters the picture. The key insight is that there is still a textual property: a container namer cannot be aliased, unless its name is mentioned in the alias declaration, or a name of alias of it is mentioned in the alias declaration.

This becomes a fixpoint algorithm: we expand our list of aliases as we collect more and more (possible) aliases, until we eventually reach a fixpoint. A fixpoint is not guaranteed (and we do have guards for the rare cases where it does not happen), but it is almost so: most types have very few aliases, if at all.

We do use some semantic information while analyzing aliases. It's a balance: too much semantic analysis, and the search will become slow. But too few of it, and we will bring many incorrect aliases to our list, and risk it expands and expands and never reach a fixpoint. At the end, based on benchmarks, it seems worth to do a lot to avoid adding an alias (but not too much), while it is worth to do a lot to avoid the need to semantically analyze func_name matches (but again, not too much).

After we collected our list of aliases, we filter matches based on this list. Only if a match can be real, we do semantic analysis for it.

The results are promising: searching for all references on `new()` in `base-db` in the rust-analyzer repository, which previously took around 60 seconds, now takes as least as two seconds and a half (roughly), while searching for `Vec::new()`, almost an upper bound to how much a symbol can be used, that used to take 7-9 minutes(!) now completes in 100-120 seconds, and with less than half of non-verified results (aka. false positives).

This is the less strictly correct (but faster) of this patch; it can miss some (rare) cases (there is a test for that - `goto_ref_on_short_associated_function_complicated_type_magic_can_confuse_our_logic()`). There is another branch that have no false negatives but is slower to search (`Vec::new()` never reaches a fixpoint in aliases collection there). I believe it is possible to create a strategy that will have the best of both worlds, but it will involve significant complexity and I didn't bother, especially considering that in the vast majority of the searches the other branch will be more than enough. But all in all, I decided to bring this branch (of course if the maintainers will agree), since our search is already not 100% accurate (it misses macros), and I believe there is value in the additional perf.
    @ChayimFriedman2
    ChayimFriedman2 committed Aug 22, 2024
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  3. Add cov_marks to test rust-lang#17927

    @ChayimFriedman2
    ChayimFriedman2 committed Aug 22, 2024
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