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Carry ExtractMostSignificantBits through to LIR and add constant folding support#117673

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tannergooding merged 4 commits intodotnet:mainfrom
tannergooding:arm64-extractmsb
Jul 16, 2025
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Carry ExtractMostSignificantBits through to LIR and add constant folding support#117673
tannergooding merged 4 commits intodotnet:mainfrom
tannergooding:arm64-extractmsb

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@tannergooding tannergooding commented Jul 15, 2025 •
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This doesn't update the IR to take advantage of any special patterns yet.

It does, however, simplify the codegen for V128<byte>.ExtractMostSignificantBits.

The logic was previously:

    op1 = op1 & Vector128.Create<ulong>(0x8080808080808080).AsByte();
    op1 = AdvSimd.ShiftLogical(op1, Vector128.Create<ulong>(0x00FFFEFDFCFBFAF9).AsSByte());

    return (Vector128.Sum(op1.GetUpper()) << 8) | Vector128.Sum(op1.GetLower());

The updated logic is:

    op1 = op1 & Vector128.Create<ulong>(0x8080808080808080).AsByte();
    op1 = AdvSimd.ShiftLogical(op1, Vector128.Create<ulong>(0x00FFFEFDFCFBFAF9).AsSByte());

    var tmp = AdvSimd.ZeroExtendWideningUpper(op1);
    tmp = AdvSimd.ShiftLeftLogical(tmp, 8);
    tmp = AdvSimd.AddWideningLower(tmp, op1.GetLower());

    return Vector128.Sum(tmp);

The original logic would generate:

            movi    v16.16b, #0x80
            and     v16.16b, v0.16b, v16.16b
            ldr     q17, [@RWD00]
            ushl    v16.16b, v16.16b, v17.16b
            mov     v17.16b, v16.16b
            addv    b17, v17.8b
            umov    w0, v17.b[0]
            ext     v16.16b, v16.16b, v16.16b, #8
            addv    b16, v16.8b
            umov    w1, v16.b[0]
            orr     w0, w0, w1,  LSL #8

While the newer logic is a bit smaller and avoids a second more expensive addv instruction:

            movi    v16.16b, #0x80
            and     v16.16b, v0.16b, v16.16b
            ldr     q17, [@RWD00]
            ushl    v16.16b, v16.16b, v17.16b
            uxtl2   v17.8h, v16.16b
            shl     v17.8h, v17.8h, #8
            uaddw   v16.8h, v17.8h, v16.8b
            addv    h16, v16.8h
            umov    w0, v16.h[0]

@github-actions github-actions bot added the area-CodeGen-coreclr CLR JIT compiler in src/coreclr/src/jit and related components such as SuperPMI label Jul 15, 2025
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tannergooding commented Jul 15, 2025

Diffs look positive and it's a nice throughput improvement as well.

The few regressions are from places we have multiple ExtractMostSignificantBits() calls and the relevant constants are no longer able to be CSE'd, which is just another variant of #70182.

We should get even bigger wins if we add some optimizations for particular x.ExtractMostSignificantBits() patterns (i.e. cases like x.ExtractMSB() == 0 and such).

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tannergooding commented Jul 15, 2025

/azp run Fuzzlyn

@tannergooding tannergooding marked this pull request as ready for review July 15, 2025 18:40
@tannergooding tannergooding requested review from EgorBo and Copilot July 15, 2025 18:40
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Pull Request Overview

This PR carries the ExtractMostSignificantBits intrinsic through to the LIR (Low-level Intermediate Representation) and adds constant folding support. The main goal is to enable better codegen optimization for SIMD operations that extract the most significant bits from vector elements.

  • Removes early expansion of ExtractMostSignificantBits intrinsics during import phase
  • Adds constant folding capabilities for ExtractMostSignificantBits in both value numbering and expression folding
  • Implements LIR-level rewriting for ExtractMostSignificantBits with platform-specific optimizations

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file
File Description
src/coreclr/jit/valuenum.cpp Adds constant folding support for ExtractMostSignificantBits in value numbering
src/coreclr/jit/simd.h Implements EvaluateExtractMSB template functions for constant evaluation
src/coreclr/jit/rationalize.h Declares RewriteHWIntrinsicExtractMsb method for LIR rewriting
src/coreclr/jit/rationalize.cpp Implements platform-specific LIR rewriting for ExtractMostSignificantBits
src/coreclr/jit/hwintrinsicxarch.cpp Removes early expansion logic for x86/x64 short/ushort cases
src/coreclr/jit/hwintrinsiclistxarch.h Updates intrinsic flags to enable special import and disable early codegen
src/coreclr/jit/hwintrinsiclistarm64.h Updates intrinsic flags to enable special import and disable early codegen
src/coreclr/jit/hwintrinsicarm64.cpp Removes early expansion logic for ARM64 ExtractMostSignificantBits
src/coreclr/jit/gentree.cpp Adds constant folding support for ExtractMostSignificantBits in expression folding

EgorBo
EgorBo reviewed Jul 15, 2025
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Nice!

This was referenced Jul 16, 2025
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tannergooding commented Jul 16, 2025

/ba-g unrelated android timeout and image acquisition failure that passed on last run.

@tannergooding tannergooding merged commit c0d4efe into dotnet:main Jul 16, 2025
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@tannergooding tannergooding deleted the arm64-extractmsb branch July 16, 2025 14:38
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