bc131422e9
Remove redundant 'resolve_obligations_as_possible' call
Hi! I was looking for a "good first issue" and saw this one: https://github.com/rust-lang/rust-analyzer/issues/7542. I like searching for performance improvements, so I wanted to try to find something useful there.
There are two tests in integrated_benchmarks.rs, I looked at 'integrated_highlighting_benchmark' (not the one discussed in the issue above).
Profile from that test looks like this:
```
$ RUN_SLOW_BENCHES=1 cargo test --release --package rust-analyzer --lib -- integrated_benchmarks::integrated_highlighting_benchmark --exact --nocapture
Finished release [optimized] target(s) in 0.06s
Running unittests src/lib.rs (target/release/deps/rust_analyzer-a80ca6bb8f877458)
running 1 test
workspace loading: 358.45ms
initial: 9.60s
change: 13.96µs
cpu profiling is disabled, uncomment `default = [ "cpu_profiler" ]` in Cargo.toml to enable.
273ms - highlight
143ms - infer:wait @ per_query_memory_usage
143ms - infer_query
0 - crate_def_map:wait (3165 calls)
4ms - deref_by_trait (967 calls)
96ms - resolve_obligations_as_possible (22106 calls)
0 - trait_solve::wait (2068 calls)
21ms - Semantics::analyze_impl (18 calls)
0 - SourceBinder::to_module_def (20 calls)
36ms - classify_name (19 calls)
19ms - classify_name_ref (308 calls)
0 - crate_def_map:wait (461 calls)
4ms - descend_into_macros (628 calls)
0 - generic_params_query (4 calls)
0 - impl_data_with_diagnostics_query (1 calls)
45ms - infer:wait (37 calls)
0 - resolve_obligations_as_possible (2 calls)
0 - source_file_to_def (1 calls)
0 - trait_solve::wait (42 calls)
after change: 275.23ms
test integrated_benchmarks::integrated_highlighting_benchmark ... ok
```
22106 calls to `resolve_obligations_as_possible` seem like the main issue there.
One thing I noticed (and fixed in this PR) is that `InferenceContext::resolve_ty_shallow` first calls `resolve_obligations_as_possible`, and then calls `InferenceTable::resolve_ty_shallow`. But `InferenceTable::resolve_ty_shallow` [inside](2e9f1204ca/crates/hir-ty/src/infer/unify.rs (L372)) again calls `resolve_obligations_as_possible`.
`resolve_obligations_as_possible` inside has a while loop, which works until it can't find any helpful information. So calling this function for the second time does nothing, so one of the calls could be safely removed.
`InferenceContext::resolve_ty_shallow` is actually quite a hot place, and after fixing it, the total number of `resolve_obligations_as_possible` in this test is reduced to 15516 (from 22106). "After change" time also improves from ~270ms to ~240ms, which is not a very huge win, but still something measurable.
Same profile after PR:
```
$ RUN_SLOW_BENCHES=1 cargo test --release --package rust-analyzer --lib -- integrated_benchmarks::integrated_highlighting_benchmark --exact --nocapture
Finished release [optimized] target(s) in 0.06s
Running unittests src/lib.rs (target/release/deps/rust_analyzer-a80ca6bb8f877458)
running 1 test
workspace loading: 339.86ms
initial: 9.28s
change: 10.69µs
cpu profiling is disabled, uncomment `default = [ "cpu_profiler" ]` in Cargo.toml to enable.
236ms - highlight
110ms - infer:wait @ per_query_memory_usage
110ms - infer_query
0 - crate_def_map:wait (3165 calls)
4ms - deref_by_trait (967 calls)
64ms - resolve_obligations_as_possible (15516 calls)
0 - trait_solve::wait (2068 calls)
21ms - Semantics::analyze_impl (18 calls)
0 - SourceBinder::to_module_def (20 calls)
34ms - classify_name (19 calls)
18ms - classify_name_ref (308 calls)
0 - crate_def_map:wait (461 calls)
3ms - descend_into_macros (628 calls)
0 - generic_params_query (4 calls)
0 - impl_data_with_diagnostics_query (1 calls)
45ms - infer:wait (37 calls)
0 - resolve_obligations_as_possible (2 calls)
0 - source_file_to_def (1 calls)
0 - trait_solve::wait (42 calls)
after change: 238.15ms
test integrated_benchmarks::integrated_highlighting_benchmark ... ok
```
The performance of this test could be further improved but at the cost of making code more complicated, so I wanted to check if such a change is desirable before sending another PR.
`resolve_obligations_as_possible` is actually called a lot of times even when no new information was provided. As I understand, `resolve_obligations_as_possible` could do something useful only if some variables/values were unified since the last check. We can store a boolean variable inside `InferenceTable`, which indicates if `try_unify` was called after last `resolve_obligations_as_possible`. If it wasn't called, we can safely not call `resolve_obligations_as_possible` again.
I tested this change locally, and it reduces the number of `resolve_obligations_as_possible` to several thousand (it is not shown in the profile anymore, so don't know the exact number), and the total time is reduced to ~180ms. Here is a generated profile:
```
$ RUN_SLOW_BENCHES=1 cargo test --release --package rust-analyzer --lib -- integrated_benchmarks::integrated_highlighting_benchmark --exact --nocapture
Finished release [optimized] target(s) in 0.06s
Running unittests src/lib.rs (target/release/deps/rust_analyzer-a80ca6bb8f877458)
running 1 test
workspace loading: 349.92ms
initial: 8.56s
change: 11.32µs
cpu profiling is disabled, uncomment `default = [ "cpu_profiler" ]` in Cargo.toml to enable.
175ms - highlight
21ms - Semantics::analyze_impl (18 calls)
0 - SourceBinder::to_module_def (20 calls)
33ms - classify_name (19 calls)
17ms - classify_name_ref (308 calls)
0 - crate_def_map:wait (461 calls)
3ms - descend_into_macros (628 calls)
0 - generic_params_query (4 calls)
0 - impl_data_with_diagnostics_query (1 calls)
97ms - infer:wait (38 calls)
0 - resolve_obligations_as_possible (2 calls)
0 - source_file_to_def (1 calls)
0 - trait_solve::wait (42 calls)
after change: 177.04ms
test integrated_benchmarks::integrated_highlighting_benchmark ... ok
```
Let me know if adding a new bool field seems like a reasonable tradeoff, so I can send a PR.
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