Speed up opt_normalize_projection_type
#50818
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There is a hot path through `opt_normalize_projection_type`: - `try_start` does a cache lookup (#1). - The result is a `NormalizedTy`. - There are no unresolved type vars, so we call `complete`. - `complete` does *another* cache lookup (rust-lang#2), then calls `SnapshotMap::insert`. - `insert` does *another* cache lookup (rust-lang#3), inserting the same value that's already in the cache. This patch optimizes this hot path by introducing `complete_normalized`, for use when the value is known in advance to be a `NormalizedTy`. It always avoids lookup rust-lang#2. Furthermore, if the `NormalizedTy`'s obligations are empty (the common case), we know that lookup rust-lang#3 would be a no-op, so we avoid it, while inserting a Noop into the `SnapshotMap`'s undo log.
This patch changes `opt_normalize_project_type` so it appends obligations to a given obligations vector, instead of returning a new obligations vector. This change avoids lots of allocations. In the most extreme case, for a clean "Check" build of serde it reduces the total number of allocations by 20%.
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| @@ -202,17 +202,19 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> { | |||
| obligation.cause.span, | |||
| infer::LateBoundRegionConversionTime::HigherRankedType, | |||
| data); | |||
| let normalized = super::normalize_projection_type( | |||
| let mut obligations = vec![]; | |||
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If this specifically is hot, it'd be interesting to see if we could make this an Option argument since obligations appear to be unused in this case.
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This particular callsite is not hot. The hot callsite is the one within <TypeFolder for AssociatedTypeNormalizer>::fold_ty in librustc/traits/project.rs, which passes an existing vector.
nikomatsakis
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May 17, 2018
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@bors r+ |
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📌 Commit 47bc774 has been approved by |
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@bors rollup |
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…omatsakis Speed up `opt_normalize_projection_type` `opt_normalize_projection_type` is hot in the serde and futures benchmarks in rustc-perf. These two patches speed up the execution of most runs for them by 2--4%.
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Rollup of 10 pull requests Successful merges: - #50387 (Remove leftover tab in libtest outputs) - #50553 (Add Option::xor method) - #50610 (Improve format string errors) - #50649 (Tweak `nearest_common_ancestor()`.) - #50790 (Fix grammar documentation wrt Unicode identifiers) - #50791 (Fix null exclusions in grammar docs) - #50806 (Add `bless` x.py subcommand for easy ui test replacement) - #50818 (Speed up `opt_normalize_projection_type`) - #50837 (Revert #49767) - #50839 (Make sure people know the book is free oline) Failed merges:
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opt_normalize_projection_typeis hot in the serde and futures benchmarks in rustc-perf. These two patches speed up the execution of most runs for them by 2--4%.