Add ReadOnlySpan constructors to BitArray by joshuajyue · Pull Request #131500 · dotnet/runtime

joshuajyue · 2026-07-28T22:28:24Z

Summary

Add ReadOnlySpan<bool>, ReadOnlySpan<byte>, and ReadOnlySpan<int> constructors to BitArray.
Route the existing array constructors through the new span overloads while preserving null and overflow behavior.
Preserve Boolean SIMD packing, scalar remainder handling, and integer endianness behavior.
Add coverage for parity, slices, stack-allocated spans, source independence, SIMD boundaries, and noncanonical Boolean values.

Fixes #80263

Validation

Checked CoreCLR and libraries baseline build
Checked CoreLib rebuild and testhost refresh
216 BitArray_CtorTests passed with default hardware intrinsics
216 BitArray_CtorTests passed with DOTNET_EnableAVX2=0
216 BitArray_CtorTests passed with DOTNET_EnableHWIntrinsic=0
33,944 System.Collections tests passed

Performance

Full BenchmarkDotNet results from Windows ARM64 Release:

Direct span construction was 33–51% faster than the existing span.ToArray() workaround across the measured sizes.
Allocations were approximately 50% lower for byte and integer inputs and up to 89% lower for Boolean inputs.
Existing array-constructor allocations were unchanged, with no observed regression for medium or large inputs.
At 32 elements, delegating the existing Boolean and byte array constructors added approximately 1 ns. If that overhead is considered significant, those constructors could retain dedicated implementations at the cost of duplicated logic.

Note

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 4236475d-0243-488d-93d8-cd78e94d532b

azure-pipelines · 2026-07-28T22:29:05Z

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Pull request overview

Adds new ReadOnlySpan<T>-based constructors to System.Collections.BitArray to enable allocation-free construction from span-backed inputs, while preserving existing array-constructor behaviors by routing them through the new span overloads. This extends the public System.Collections contract and provides test coverage for span-specific scenarios (slices, stackalloc, copy semantics, and SIMD boundary safety).

Changes:

Added BitArray(ReadOnlySpan<bool>), BitArray(ReadOnlySpan<byte>), and BitArray(ReadOnlySpan<int>) constructors and routed existing array constructors through them.
Updated the System.Collections reference assembly to expose the new public constructors.
Expanded unit tests to validate span parity with array constructors, slicing/stackalloc usage, source independence, non-canonical bool handling, and vector boundary safety.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Private.CoreLib/src/System/Collections/BitArray.cs	Implements the new span constructors and reuses them from existing array constructors without changing established behaviors (null/overflow handling, packing semantics).
src/libraries/System.Collections/tests/BitArray/BitArray_CtorTests.cs	Adds coverage for span constructors (slices, stackalloc, copy semantics, non-canonical bools, and BoundedMemory OOB checks).
src/libraries/System.Collections/ref/System.Collections.cs	Adds the new public constructor signatures to the ref surface for `System.Collections.BitArray`.

azure-pipelines · 2026-07-29T20:50:37Z

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Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 4236475d-0243-488d-93d8-cd78e94d532b

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Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

Copilot

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

jeffhandley

Suggestions for comments on the tests to make them easier to maintain over time (probably applies to existing tests too), but non-blocking.

jeffhandley · 2026-07-30T20:50:58Z

+            bool[] boolValues = [false, true, false, true, true, false];
+            AssertBitArray(new BitArray(boolValues.AsSpan(1, 4)), [true, false, true, true]);


Suggestion: Add a comment that declares if/why the ordering of these values is intentional so that future maintainers know what can and cannot be changed without leaking a regression.

This could also be accomplished with a more descriptive test name.

jeffhandley · 2026-07-30T20:54:33Z

+        }
+
+        [Fact]
+        public static void Ctor_StackAllocatedSpans()


Suggestion: More descriptive test name or a comment here too

jeffhandley · 2026-07-30T20:54:55Z

+        [InlineData(64)]
+        [InlineData(67)]
+        [InlineData(100)]
+        public static void Ctor_BoolSpan_NonCanonicalTrueValues(int length)


jeffhandley · 2026-07-30T20:55:14Z

            Assert.Equal(bitArray.Length, clone.Length);
        }
+
+        private static void AssertBitArray(BitArray bitArray, ReadOnlySpan<bool> expected)


Good idea refactoring this out

jeffhandley · 2026-07-30T20:57:25Z

        /// "<paramref name="values"/>[0] &amp; 1" represents bit 0, "<paramref name="values"/>[0] &amp; 2" represents bit 1,
        /// "<paramref name="values"/>[0] &amp; 4" represents bit 2, and so on.
+        ///
+        /// This constructor is an <c>O(n)</c> operation, where <c>n</c> is the number of elements in <paramref name="values"/>.


Add ReadOnlySpan constructors to BitArray

2edbdf9

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Copilot AI review requested due to automatic review settings July 28, 2026 22:28

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This was referenced Jul 29, 2026

[wasm] OpenQA.Selenium.WebDriverTimeoutException: timeout: Timed out receiving message from renderer #117486

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[ci-scan] Build break: ILCompiler.ReadyToRun.Tests fails CS7036 on ReadyToRunCompilerContext ctor #131504

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Merge branch 'main' into bitarray-readonlyspan-constructors

1b861d3

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Comment thread src/libraries/System.Private.CoreLib/src/System/Collections/BitArray.cs

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Comment thread src/libraries/System.Private.CoreLib/src/System/Collections/BitArray.cs Outdated

joshuajyue added 2 commits July 30, 2026 00:59

Document BitArray constructor behavior

5f21107

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Preserve BitArray array constructor inlining

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The Operation will be canceled. The next steps may not contain expected logs. dotnet/dnceng#3008

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Merge branch 'main' into bitarray-readonlyspan-constructors

78f513c

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		bool[] boolValues = [false, true, false, true, true, false];
		AssertBitArray(new BitArray(boolValues.AsSpan(1, 4)), [true, false, true, true]);

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joshuajyue commented Jul 28, 2026 •

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