Restore BitArray constructor performance

[joshuajyue]

Summary

• Keep source lengths in normal locals rather than address-exposed out values, removing the Boolean helper's out parameter entirely and delaying byte and integer out assignments until after allocation and copying.
• Use an int induction variable in the Boolean SIMD and scalar remainder paths 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 and are being validated with a focused Windows x64 EgorBot run.
• BitArraySetLengthGrow(Size: 4): this benchmark includes construction from byte[]. Automated bisection instead pointed to unrelated JIT commit 0bbeda6cbad4 and flagged possible code-alignment noise, so a focused EgorBot run is validating whether this fix affects it.

A local same-testhost ARM64 paired run of the merged Boolean implementation versus this fix (50 alternating AB/BA pairs) found no statistically significant difference: 37.24 ns versus 37.96 ns, p=0.426. Focused Windows x64 EgorBot runs will validate the original regression environment.

Validation

• Checked System.Private.CoreLib build
• 216 BitArray_CtorTests passed with hardware intrinsics enabled
• 216 BitArray_CtorTests passed with DOTNET_EnableHWIntrinsic=0
• Read-only review found no correctness, bounds-safety, endianness, or exception-behavior issues

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)
            {

[joshuajyue]

@EgorBot -windows_x64 -attempts 3 --filter "Perf_BitArray.BitArrayIntArrayCtor" --filter "Perf_BitArray.BitArraySetLengthGrow"

Note

This benchmark request was generated with GitHub Copilot.

[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.