Replace ExtractMostSignificantBits+BitOp patterns with Vector helper methods by Copilot · Pull Request #126841 · dotnet/runtime

Copilot · 2026-04-13T16:51:28Z

Replace usages of ExtractMostSignificantBits() followed by PopCount/TrailingZeroCount/LeadingZeroCount with the recently optimized [Intrinsic] vector helpers: CountMatches, IndexOfFirstMatch, IndexOfLastMatch, and IndexOfWhereAllBitsSet.

Description

Pattern replacements

BitOperations.PopCount(v.ExtractMostSignificantBits()) → VectorN.CountMatches(v) (within CoreLib, using internal helper to avoid x64 regression)
BitOperations.TrailingZeroCount(v.ExtractMostSignificantBits()) → VectorN.IndexOfFirstMatch(v) (within CoreLib) or VectorN.IndexOfWhereAllBitsSet(v) (in Tensors, using public API)
Count - 1 - BitOperations.LeadingZeroCount(v.ExtractMostSignificantBits()) → VectorN.IndexOfLastMatch(v) (within CoreLib)

-count += BitOperations.PopCount(Vector512.Equals(Vector512.LoadUnsafe(ref current), targetVector).ExtractMostSignificantBits());
+count += Vector512.CountMatches(Vector512.Equals(Vector512.LoadUnsafe(ref current), targetVector));

-uint matches = Vector128.Equals(Vector128<byte>.Zero, search).ExtractMostSignificantBits();
-if (matches == 0)
+Vector128<byte> cmp = Vector128.Equals(Vector128<byte>.Zero, search);
+if (cmp == Vector128<byte>.Zero)
 {
     offset += (nuint)Vector128<byte>.Count;
 }
 else
 {
-    return (int)(offset + (uint)BitOperations.TrailingZeroCount(matches));
+    return (int)(offset + (uint)Vector128.IndexOfFirstMatch(cmp));
 }

Files changed

SpanHelpers.T.cs — 3 replacements in CountValueType loop bodies using the internal CountMatches helper directly; replaced ComputeFirstIndex (3 overloads, EMSB+TZC → IndexOfFirstMatch) and ComputeLastIndex (3 overloads, EMSB+LZC → IndexOfLastMatch)
SpanHelpers.Byte.cs — 9 replacements in the null-terminator search (EMSB+TZC → IndexOfFirstMatch), keeping the vector comparison result and comparing against VectorN<byte>.Zero instead of extracting to a bitmask
TensorPrimitives.Max.cs — Inlined VectorN.IndexOfWhereAllBitsSet directly at all 9 call sites (previously consumed the shared IndexOfFirstMatch helpers)

Not changed

Vector64.cs / Vector128.cs — Left as-is; these contain the internal helper methods (CountMatches, IndexOfFirstMatch, etc.) that the other callsites consume
TensorPrimitives.IndexOfMax.cs — Left as-is; no changes made to this file
TensorPrimitives.HammingDistance.cs — Left as-is; CountWhereAllBitsSet would introduce an x64 regression, and the internal CountMatches helper is not accessible from the Tensors assembly
SpanHelpers.Byte.cs loop patterns — Left as-is; IndexOf/LastIndexOf patterns that iterate through multiple matches with ResetLowestSetBit/FlipBit don't map to single-match helpers
SpanHelpers.Byte.cs SequenceEqual — Left as-is; computes TZC(~matches) for first difference, not first match

Testing

All relevant tests pass:

System.Runtime.Intrinsics.Tests: 12,944 tests passed
System.Memory.Tests: 52,229 tests passed
System.Numerics.Tensors.Tests: 5,427 tests passed
System.Runtime.Tests: 69,682 tests passed

Replace patterns of ExtractMostSignificantBits() followed by PopCount/TrailingZeroCount/LeadingZeroCount with the optimized Vector helpers: CountWhereAllBitsSet, IndexOfWhereAllBitsSet, and LastIndexOfWhereAllBitsSet. Remove AdvSimd special paths from Vector64/Vector128 internal helpers (CountMatches, IndexOfFirstMatch, IndexOfLastMatch) and the now-unused AdvSimdExtractBitMask/AdvSimdFixupBitCount methods. Agent-Logs-Url: https://github.com/dotnet/runtime/sessions/fcd7f0bb-7e64-41fa-8773-089705b9a737 Co-authored-by: EgorBo <523221+EgorBo@users.noreply.github.com>

dotnet-policy-service · 2026-04-13T17:17:28Z

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

…ctly Agent-Logs-Url: https://github.com/dotnet/runtime/sessions/fd3915a5-bbf3-4530-8406-f3983be66ffb Co-authored-by: tannergooding <10487869+tannergooding@users.noreply.github.com>

Agent-Logs-Url: https://github.com/dotnet/runtime/sessions/527f6141-5f1f-41b3-bb40-336677a9c787 Co-authored-by: tannergooding <10487869+tannergooding@users.noreply.github.com>

…ression Agent-Logs-Url: https://github.com/dotnet/runtime/sessions/83aad277-6910-4c67-8aac-88c2e15047b3 Co-authored-by: tannergooding <10487869+tannergooding@users.noreply.github.com>

Agent-Logs-Url: https://github.com/dotnet/runtime/sessions/83aad277-6910-4c67-8aac-88c2e15047b3 Co-authored-by: tannergooding <10487869+tannergooding@users.noreply.github.com>

…tions

EgorBo · 2026-06-24T13:23:21Z

@copilot resolve the merge conflicts in this pull request

…ct-msb-with-vector-functions # Conflicts: # src/libraries/System.Numerics.Tensors/src/System/Numerics/Tensors/netcore/TensorPrimitives.IndexOfMax.cs Co-authored-by: EgorBo <523221+EgorBo@users.noreply.github.com>

Copilot · 2026-06-24T13:28:54Z

@copilot resolve the merge conflicts in this pull request

Resolved in 942e23e. The only conflict was in TensorPrimitives.IndexOfMax.cs where main still had the old IndexOfFirstMatch helper methods that this PR removed — kept our version (helpers removed, IndexOfWhereAllBitsSet inlined directly).

…tions

Copilot

Pull request overview

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

+        private static unsafe int IndexOfMinMaxCore<T, TIndexOfMinMax>(ReadOnlySpan<T> x)
+    where T : INumber<T>
+    where TIndexOfMinMax : struct, IIndexOfOperator<T>
+        {
+            if (x.IsEmpty)


        public static int IndexOfMax<T>(ReadOnlySpan<T> x)
            where T : INumber<T> =>
            IndexOfMinMaxCore<T, IndexOfMaxOperator<T>>(x);

        /// <summary>Returns the index of MathF.Max(x, y)</summary>
-        internal readonly struct IndexOfMaxOperator<T> : IIndexOfMinMaxOperator<T> where T : INumber<T>
+        internal readonly struct IndexOfMaxOperator<T> : IIndexOfOperator<T> where T : INumber<T>
        {


EgorBo · 2026-06-24T23:53:12Z

@copilot please completely revert all changes in TensorPrimitives.IndexOfMax.cs

Co-authored-by: EgorBo <523221+EgorBo@users.noreply.github.com>

Copilot · 2026-06-25T00:02:39Z

@copilot please completely revert all changes in TensorPrimitives.IndexOfMax.cs

Done — TensorPrimitives.IndexOfMax.cs has been fully reverted to its original state (identical to main) in the latest commit.

EgorBo · 2026-06-25T23:25:25Z

@tannergooding feel free to pick it up, but looks like #129688 will optimize all uses.

Copilot AI assigned Copilot and EgorBo Apr 13, 2026

Copilot AI requested review from Copilot and removed request for Copilot April 13, 2026 16:51

Copilot created this pull request from a session on behalf of EgorBo April 13, 2026 16:54 View session

Copilot finished work on behalf of EgorBo April 13, 2026 16:54

Copilot AI requested a review from EgorBo April 13, 2026 16:54

tannergooding reviewed Apr 13, 2026

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Comment thread ...s/System.Numerics.Tensors/src/System/Numerics/Tensors/netcore/TensorPrimitives.IndexOfMax.cs

Copilot started work on behalf of tannergooding April 13, 2026 17:10 View session

tannergooding reviewed Apr 13, 2026

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Comment thread src/libraries/System.Private.CoreLib/src/System/Runtime/Intrinsics/Vector128.cs Outdated

github-actions Bot added the area-System.Numerics label Apr 13, 2026

tannergooding reviewed Apr 13, 2026

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Comment thread ...tem.Numerics.Tensors/src/System/Numerics/Tensors/netcore/TensorPrimitives.HammingDistance.cs Outdated

Remove IndexOfFirstMatch wrappers; inline IndexOfWhereAllBitsSet dire…

31f94e3

…ctly Agent-Logs-Url: https://github.com/dotnet/runtime/sessions/fd3915a5-bbf3-4530-8406-f3983be66ffb Co-authored-by: tannergooding <10487869+tannergooding@users.noreply.github.com>

Copilot AI requested review from Copilot and removed request for Copilot April 13, 2026 17:34

Copilot finished work on behalf of tannergooding April 13, 2026 17:36

Copilot AI requested a review from tannergooding April 13, 2026 17:36

Copilot started work on behalf of tannergooding April 13, 2026 17:37 View session

Revert Vector128.cs, Vector64.cs, and HammingDistance.cs changes

499c77e

Agent-Logs-Url: https://github.com/dotnet/runtime/sessions/527f6141-5f1f-41b3-bb40-336677a9c787 Co-authored-by: tannergooding <10487869+tannergooding@users.noreply.github.com>

Copilot AI requested review from Copilot and removed request for Copilot April 13, 2026 18:02

Copilot finished work on behalf of tannergooding April 13, 2026 18:05

Copilot started work on behalf of tannergooding April 13, 2026 18:07 View session

Use CountMatches internal helper in SpanHelpers.T.cs to avoid x64 reg…

8dc533d

…ression Agent-Logs-Url: https://github.com/dotnet/runtime/sessions/83aad277-6910-4c67-8aac-88c2e15047b3 Co-authored-by: tannergooding <10487869+tannergooding@users.noreply.github.com>

Copilot AI requested review from Copilot and removed request for Copilot April 13, 2026 18:37

Clean up extra blank lines from removed IndexOfFirstMatch methods

d3a08e4

Agent-Logs-Url: https://github.com/dotnet/runtime/sessions/83aad277-6910-4c67-8aac-88c2e15047b3 Co-authored-by: tannergooding <10487869+tannergooding@users.noreply.github.com>

Copilot AI review requested due to automatic review settings April 13, 2026 18:38

This was referenced Apr 23, 2026

Many WasmTestOnChrome-MONO-ST tests failing unable to load dotnet.*.js #127257

Open

ProcessSafeHandle_WaitForExitOrKillOnCancellationAsync_KillsOnCancellation failuring in CI #127287

Closed

Copilot AI mentioned this pull request Apr 23, 2026

Fix race condition: set _canceled before SignalCore in ProcessWaitState #127312

Merged

Merge branch 'main' into copilot/replace-extract-msb-with-vector-func…

ea005d4

…tions

EgorBo mentioned this pull request Jun 24, 2026

arm64: Optimize ARM64 compare mask ExtractMostSignificantBits consumers #129688

Open

Copilot started work on behalf of EgorBo June 24, 2026 13:23 View session

Copilot AI requested review from Copilot and removed request for Copilot June 24, 2026 13:28

Copilot finished work on behalf of EgorBo June 24, 2026 13:29

Merge branch 'main' into copilot/replace-extract-msb-with-vector-func…

d6c1ee3

…tions

Copilot AI review requested due to automatic review settings June 24, 2026 23:04

Copilot started reviewing on behalf of EgorBo June 24, 2026 23:04 View session

Copilot AI reviewed Jun 24, 2026

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Copilot started work on behalf of EgorBo June 24, 2026 23:53 View session

Revert TensorPrimitives.IndexOfMax.cs to original state

5ff4a0c

Co-authored-by: EgorBo <523221+EgorBo@users.noreply.github.com>

Copilot finished work on behalf of EgorBo June 25, 2026 00:03

EgorBo closed this Jun 25, 2026

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Replace ExtractMostSignificantBits+BitOp patterns with Vector helper methods#126841

Replace ExtractMostSignificantBits+BitOp patterns with Vector helper methods#126841
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Copilot AI commented Apr 13, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Description

Pattern replacements

Files changed

Not changed

Testing

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Copilot AI commented Apr 13, 2026 •

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