Fix BitArray.LeftShift to clear dangling high bits #123296

Copilot · 2026-01-16T23:57:15Z

Description

BitArray.LeftShift was not clearing unused bits in the most significant storage element after shifting. This caused PopCount to return incorrect values when bits were shifted out of range.

BitArray ba = new(33);
ba[^1] = true;

ba.LeftShift(1);
Assert.False(ba.HasAnySet());   // passes
Assert.Equal(0, ba.PopCount()); // failed: returned 1

Changes

Call ClearHighExtraBits() after shift operation completes in LeftShift
Add comprehensive Theory-based tests for both LeftShift and RightShift operations validating PopCount and HasAnySet with various bit patterns and shift counts
Test coverage includes edge cases around int32 boundaries (1, 8, 31, 32, 33, 64, 65 bits), various shift amounts, and bit positions at start, middle, and end

Original prompt

This section details on the original issue you should resolve

<issue_title>BitArray ShiftLeft and PopCount produce unexpected behavior</issue_title>
<issue_description>Consider:

[Fact]
public static void PopCount_LeftShiftLeavesDanglingBits()
{
    BitArray ba = new(33);
    ba[^1] = true;

    Assert.True(ba.HasAnySet());
    Assert.Equal(1, ba.PopCount());

    ba = ba.LeftShift(1);
    Assert.False(ba.HasAnySet());
    Assert.Equal(0, ba.PopCount()); // Fails here
}

This test will fail. Despite HasAnySet returning false, PopCount will return 1.

Likewise manually counting the bits will disagree with PopCount

[Fact]
public static void PopCount_LeftShiftLeavesDanglingBits()
{
    BitArray ba = new(33);
    ba[^1] = true;

    Assert.Equal(ManualCountBits(ba), ba.PopCount());

    ba = ba.LeftShift(1);
    Assert.False(ba.HasAnySet());
    Assert.Equal(ManualCountBits(ba), ba.PopCount());


    static int ManualCountBits(BitArray ba)
    {
        int count = 0;

        foreach (bool bit in ba)
        {
            if (bit)
            {
                count++;
            }
        }

        return count;
    }
}
```</issue_description>

<agent_instructions>Fix leftshift to ensure the extra bits are cleared if necessary</agent_instructions>

## Comments on the Issue (you are @copilot in this section)

<comments>
<comment_new><author>@tannergooding</author><body>
Looks like `LeftShift` (and potentially other operations) are not clearing the "unused" upper bits of the most significant storage element in the backing array.

There is probably a balance between every operation ensuring that it only writes to the valid data (i.e. fixing LeftShift) and ensuring operations which might read "unused" bits mask them off (i.e. fixing PopCount, RightShift, etc)</body></comment_new>
</comments>

Fixes BitArray ShiftLeft and PopCount produce unexpected behavior #123281

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

stephentoub · 2026-01-17T16:30:06Z

src/libraries/System.Collections/tests/BitArray/BitArray_OperatorsTests.cs

+            ba.LeftShift(1);
+            Assert.False(ba.HasAnySet());
+            Assert.Equal(0, ba.PopCount());
+        }


@copilot, please make this test a [Theory] and validate various bit patterns and shift counts rather than only checking one combination. Also, please add similar tests with PopCount for other operations, such as RightShift, and any others that might suffer a similar problem if they inappropriately handle clearing.

Expanded to [Theory] with MemberData covering various bit patterns and shift counts. Added tests for both LeftShift and RightShift with PopCount and HasAnySet validation. Tests cover edge cases around int32 boundaries (1, 8, 31, 32, 33, 64, 65 bits). See commit 066cb7c.

dotnet-policy-service · 2026-01-17T16:31:22Z

Tagging subscribers to this area: @dotnet/area-system-collections
See info in area-owners.md if you want to be subscribed.

Copilot

Pull request overview

This pull request fixes a bug in BitArray.LeftShift where unused high-order bits in the most significant storage element were not being cleared after the shift operation, causing PopCount() to return incorrect values when bits were shifted out of range.

Changes:

Added a call to ClearHighExtraBits() in the LeftShift method to clear dangling high bits after shifting
Added regression test LeftShift_PopCountReturnsCorrectCount to verify the fix

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Private.CoreLib/src/System/Collections/BitArray.cs	Added `ClearHighExtraBits()` call after left shift operation to clear unused high bits
src/libraries/System.Collections/tests/BitArray/BitArray_OperatorsTests.cs	Added regression test that verifies PopCount returns 0 after shifting the last bit out of range

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Initial plan

51b9d17

Copilot AI assigned Copilot and stephentoub Jan 16, 2026

Copilot started work on behalf of stephentoub January 16, 2026 23:57 View session

Fix BitArray.LeftShift to clear dangling high bits

60a3afb

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Copilot AI changed the title ~~[WIP] Fix BitArray ShiftLeft and PopCount unexpected behavior~~ Fix BitArray.LeftShift to clear dangling high bits Jan 17, 2026

Copilot AI requested a review from stephentoub January 17, 2026 00:25

Copilot finished work on behalf of stephentoub January 17, 2026 00:25

build-analysis bot mentioned this pull request Jan 17, 2026

browser-wasm linux LibraryTests build failure - NETSDK1022: Duplicate 'Content' items #123237

Open

stephentoub reviewed Jan 17, 2026

View reviewed changes

stephentoub marked this pull request as ready for review January 17, 2026 16:30

Copilot AI review requested due to automatic review settings January 17, 2026 16:30

stephentoub added the area-System.Collections label Jan 17, 2026

Copilot started work on behalf of stephentoub January 17, 2026 16:30 View session

Copilot started reviewing on behalf of stephentoub January 17, 2026 16:31 View session

Copilot AI reviewed Jan 17, 2026

View reviewed changes

Copilot AI and others added 2 commits January 17, 2026 16:51

Add comprehensive shift+PopCount tests as Theory with various patterns

b649b14

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Improve test data generator readability

066cb7c

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Copilot AI requested a review from stephentoub January 17, 2026 16:54

Copilot finished work on behalf of stephentoub January 17, 2026 16:54

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Uh oh!

Fix BitArray.LeftShift to clear dangling high bits #123296

Fix BitArray.LeftShift to clear dangling high bits #123296

Copilot AI commented Jan 16, 2026 •

edited

Loading

Uh oh!

stephentoub Jan 17, 2026

Uh oh!

Copilot AI Jan 17, 2026

Uh oh!

dotnet-policy-service bot commented Jan 17, 2026

Uh oh!

Copilot AI left a comment

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

2 participants

Fix BitArray.LeftShift to clear dangling high bits #123296

Are you sure you want to change the base?

Fix BitArray.LeftShift to clear dangling high bits #123296

Conversation

Copilot AI commented Jan 16, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Description

Changes

Uh oh!

stephentoub Jan 17, 2026

Choose a reason for hiding this comment

Uh oh!

Copilot AI Jan 17, 2026

Choose a reason for hiding this comment

Uh oh!

dotnet-policy-service bot commented Jan 17, 2026

Uh oh!

Copilot AI left a comment

Choose a reason for hiding this comment

Pull request overview

Reviewed changes

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

2 participants

Copilot AI commented Jan 16, 2026 •

edited

Loading