Restore BitArray constructor performance

[joshuajyue]

Summary

• Restore the original byte[], bool[], and int[] constructor implementations to eliminate helper-call and inlining regressions.
• Keep separate implementations for the new ReadOnlySpan<byte>, ReadOnlySpan<bool>, and ReadOnlySpan<int> constructors.
• Use an int induction variable in the Boolean span constructor so span indexing and bounds analysis stay in the JIT's canonical signed form.

Fixes #131815

Performance

Performance tracking identified regressions in three affected benchmark groups:

• BitArrayBoolArrayCtor: automated bisection tied the confirmed Size: 512 regression to Add ReadOnlySpan constructors to BitArray #131500 (16.73 ns to 22.65 ns, approximately +35%).
• BitArrayIntArrayCtor: additional regressions were reported at multiple input sizes.
• BitArraySetLengthGrow(Size: 4): this benchmark includes construction from byte[]; automated bisection also flagged possible code-alignment noise, so it is included in the validation set.

This change prioritizes preserving the existing array-constructor performance over sharing implementation with the span overloads. Focused Windows x64 EgorBot runs cover all three benchmark groups on the original regression architecture.

Validation

• Checked System.Private.CoreLib build
• Verified the restored array constructor bodies match their pre-Add ReadOnlySpan constructors to BitArray #131500 implementations
• Two independent read-only reviews found no correctness, bounds-safety, endianness, or exception-behavior issues
• Full PR CI and focused EgorBot benchmarks will validate the updated commit

Note

This pull request description was generated with GitHub Copilot and reviewed before publication.

[joshuajyue]

@EgorBot -windows_x64 -attempts 3 --filter "Perf_BitArray.BitArrayBoolArrayCtor"

Note

This benchmark request was generated with GitHub Copilot.

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[Copilot]

Pull request overview

This PR tweaks BitArray constructors to reduce JIT impediments in hot paths, with the goal of restoring constructor microbenchmark performance after recent refactoring.

Changes:

• Avoids using an out-parameter as the source value for allocation sizing in the byte/int span helpers by computing the bit length in a local first.
• Removes the out bitLength parameter from the Boolean packing helper and assigns _bitLength directly from the source length in the constructors.
• Switches the Boolean packing loop induction variable to int to keep span indexing and bounds analysis in canonical signed form for the JIT.

Suppressed comments (1)

src/libraries/System.Private.CoreLib/src/System/Collections/BitArray.cs:342

• length here is a bit count (values.Length * BitsPerInt32), but the name reads like an element/byte count. Consider renaming it to bitLengthLocal (and updating the two uses) to make the unit explicit.

            int length = values.Length * BitsPerInt32;
            byte[] array = AllocateByteArray(length);

            if (BitConverter.IsLittleEndian)
            {

[EgorBo]

I've filed an alternative fix to make it fully memory safe #131838 (part of the on-going effort). Let's see if it improves.

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated 1 comment.

[joshuajyue]

@EgorBot -windows_x64 -linux_x64 -arm --filter "Perf_BitArray.BitArrayBoolArrayCtor"

[joshuajyue]

@EgorBot -windows_x64 -attempts 3 --filter "Perf_BitArray.BitArrayBoolArrayCtor"

[joshuajyue]

@EgorBot -windows_x64 -linux_x64 -arm --filter "Perf_BitArray.BitArrayIntArrayCtor"

[joshuajyue]

@EgorBot -windows_x64 -commits PR_131833,31004ff8dc2,fffed311025,44687f2d763 --envvars DOTNET_ReadyToRun:0 DOTNET_TieredCompilation:0 DOTNET_JitDisasm:BitArray DOTNET_JitDisasmDiffable:1 DOTNET_JitPrintInlinedMethods:BitArray --filter "Perf_BitArray.BitArraySetLengthGrow"

Note

This benchmark request was generated with GitHub Copilot and reviewed before publication.

[joshuajyue]

@EgorBot -windows_x64 -commits PR_131833,31004ff8dc2,fffed311025,44687f2d763 --envvars DOTNET_ReadyToRun:0 DOTNET_TieredCompilation:0 DOTNET_JitDisasm:BitArray DOTNET_JitDisasmDiffable:1 DOTNET_JitPrintInlinedMethods:BitArray --filter "Perf_BitArray.BitArrayIntArrayCtor"

Note

This benchmark request was generated with GitHub Copilot and reviewed before publication.

[joshuajyue]

@EgorBot -windows_x64 -commits ca4ed7d,31004ff8dc2 -attempts 5 --envvars DOTNET_ReadyToRun:0 DOTNET_TieredCompilation:0 DOTNET_JitDisasm:System.Collections.BitArray:.ctor* DOTNET_JitDisasmWithAddress:1 DOTNET_JitDisasmWithAlignmentBoundaries:1 DOTNET_JitDisasmWithCodeBytes:1 --filter "Perf_BitArray.BitArrayBoolArrayCtor"

Note

This benchmark request was generated with GitHub Copilot and reviewed before publication.

[joshuajyue]

@EgorBot -windows_x64 -commits 31004ff,31004ff8dc241f47d07361678d82e1e55b66f729 -attempts 5 --filter "Perf_BitArray.BitArrayBoolArrayCtor"

Note

This benchmark request was generated with GitHub Copilot and reviewed before publication.

[joshuajyue]

@EgorBot -linux_x64 -commits ca4ed7d,31004ff8dc2 -attempts 3 -perf_events cycles,instructions,branches,branch-misses --filter "Perf_BitArray.BitArrayBoolArrayCtor"

Note

This benchmark request was generated with GitHub Copilot and reviewed before publication.

[joshuajyue]

@EgorBot -windows_x64 -commits PR_131833,31004ff8dc2,31004ff8dc241f47d07361678d82e1e55b66f729 -attempts 5 --filter "Perf_BitArray.BitArrayBoolArrayCtor"

Note

This benchmark request was generated with GitHub Copilot and reviewed before publication.

[joshuajyue]

@EgorBot -windows_x64 -commits PR_131833,ca4ed7d4a26,31004ff8dc2 -attempts 5 --envvars DOTNET_ReadyToRun:0 DOTNET_TieredCompilation:0 DOTNET_JitDisasm:System.Collections.BitArray:.ctor* DOTNET_JitDisasmWithAddress:1 DOTNET_JitDisasmWithAlignmentBoundaries:1 DOTNET_JitDisasmWithCodeBytes:1 --filter "Perf_BitArray.BitArrayBoolArrayCtor"

Note

This benchmark request was generated with GitHub Copilot and reviewed before publication.

[joshuajyue]

@EgorBot -linux_x64 -commits PR_131833,ca4ed7d4a26,31004ff8dc2 -attempts 3 -perf_events cycles,instructions,branches,branch-misses --filter "Perf_BitArray.BitArrayBoolArrayCtor"

Note

This benchmark request was generated with GitHub Copilot and reviewed before publication.

[joshuajyue]

@EgorBot -windows_x64 -commits PR_131833,ca4ed7d4a26,9072500a881 -attempts 3 --filter "Perf_BitArray.BitArrayIntArrayCtor"

Note

This benchmark request was generated with GitHub Copilot and reviewed before publication.

[joshuajyue]

@EgorBot -windows_x64 -commits PR_131833,ca4ed7d4a26,9072500a881 -attempts 3 --filter "Perf_BitArray.BitArrayBoolArrayCtor"

Note

This benchmark request was generated with GitHub Copilot and reviewed before publication.

[Copilot]

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[EgorBo]

@joshuajyue I would recommend running locally. You can just use the benchmark skill & your favorite AI tool to set it up and validate till it's ready.

[joshuajyue]

@EgorBot -windows_x64 -commits PR_131833,ca4ed7d4a26,9072500a881,a491f2f5ede -attempts 3 --filter "Perf_BitArray.BitArrayBoolArrayCtor"

Note

This benchmark request was generated with GitHub Copilot and reviewed before publication.