Reduce allocations due to resizing dictionaries

[Erarndt]

Fixes #

Context

The usage pattern of Dictionary<TKey, TValue> in Lookup can cause a large number of allocations due to resizing. The dictionaries are often created in paths that add items one at a time. We can use available context to make better guesses about the desired size to reduce the number of allocations.

Before:

After:

Where the allocations happen shift around a bit, but the total allocations drop roughly 100MB

Changes Made

Testing

Notes

[Copilot]

Pull Request Overview

This PR aims to reduce allocations by pre-sizing dictionaries based on known usage patterns, cutting down on repeated resizes.

• Introduces ItemTypesCount in ItemDictionary for better initial capacity guesses.
• Adds InitializeCapacity in Lookup to optionally pre-allocate the primary dictionary.
• Calls InitializeCapacity in TaskExecutionHost and BatchingEngine before batch item additions.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/Build/Collections/ItemDictionary.cs	Added ItemTypesCount property to expose the count of item-type buckets for capacity hints.
src/Build/BackEnd/TaskExecutionHost/TaskExecutionHost.cs	Invoke InitializeCapacity(outputs.Length) on the lookup before adding task outputs.
src/Build/BackEnd/Components/RequestBuilder/Lookup.cs	Added InitializeCapacity(int capacity) method and optional capacity-based constructor overload.
src/Build/BackEnd/Components/RequestBuilder/BatchingEngine.cs	Use InitializeCapacity(items.Count) when inserting into existing buckets to pre-size them.

Comments suppressed due to low confidence (3)

src/Build/Collections/ItemDictionary.cs:87

• Add XML documentation for the ItemTypesCount property to describe what it represents and why it's needed.

        public int ItemTypesCount

src/Build/BackEnd/Components/RequestBuilder/Lookup.cs:712

• [nitpick] The method name InitializeCapacity is misleading since it only initializes when the table is null. Consider renaming to InitializeIfNull or changing its behavior to truly ensure a minimum capacity (e.g., via Dictionary.EnsureCapacity).

        internal void InitializeCapacity(int capacity) => PrimaryAddTable ??= new ItemDictionary<ProjectItemInstance>(capacity);

src/Build/BackEnd/Components/RequestBuilder/Lookup.cs:712

• Add unit tests for InitializeCapacity to verify it sets the initial capacity correctly and doesn't override existing table contents.

        internal void InitializeCapacity(int capacity) => PrimaryAddTable ??= new ItemDictionary<ProjectItemInstance>(capacity);

[surayya-MS]

Looks great! Thanks for the PR!

[AR-May]

Looks good, thank you. Just couple of questions.