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JIT: Convert multi-target switches to branchless checks#124567

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JulieLeeMSFT merged 4 commits intodotnet:mainfrom
JulieLeeMSFT:optPatternMatchSwitch
Feb 24, 2026
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JIT: Convert multi-target switches to branchless checks#124567
JulieLeeMSFT merged 4 commits intodotnet:mainfrom
JulieLeeMSFT:optPatternMatchSwitch

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@JulieLeeMSFT JulieLeeMSFT commented Feb 18, 2026 •
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Fixes #123858.

This PR improves JIT codegen for multi-target switch-style tests by converting eligible cases into branchless checks. The goal is to produce optimal codegen that matches the intent of C# pattern matching with or (e.g., x is A or B or C).

When a switch has exactly 2 unique successors (all non-default cases target one block, default targets another), convert it from Switch to an unsigned range comparison (In fgOptimizeSwitchBranches).
When both targets are simple return blocks, fgFoldCondToReturnBlock further folds this into branchless return.

Example

        private static bool IsLetterCategory(UnicodeCategory uc)
        {
            return uc == UnicodeCategory.UppercaseLetter
                 || uc == UnicodeCategory.LowercaseLetter
                 || uc == UnicodeCategory.TitlecaseLetter
                 || uc == UnicodeCategory.ModifierLetter
                 || uc == UnicodeCategory.OtherLetter;
        }

Before

cmp      ecx, 4
ja       SHORT G_M22758_IG05
mov      eax, 1
ret
G_M22758_IG05:
xor      eax, eax
ret

After

cmp      ecx, 4
setbe    al
movzx    rax, al
ret

Details

  • The comparison direction is chosen to GT_LT.
  • Edge dup counts are fixed up after conversion.
  • Added tests for zero-based and non-zero-based consecutive ranges.

ASMDiffs

  • SPMI asmdiffs show code size improvements of 83 bytes.

@JulieLeeMSFT JulieLeeMSFT added this to the 11.0.0 milestone Feb 18, 2026
Copilot AI review requested due to automatic review settings February 18, 2026 19:25
@JulieLeeMSFT JulieLeeMSFT added the area-CodeGen-coreclr CLR JIT compiler in src/coreclr/src/jit and related components such as SuperPMI label Feb 18, 2026
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JulieLeeMSFT commented Feb 18, 2026

@EgorBo, PTAL.
CC @dotnet/jit-contrib.

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Pull request overview

This PR optimizes multi-target switches where all non-default cases target a single block (distinct from the default target) by converting them into unsigned range comparisons. This enables branchless code generation when both targets are simple return blocks, addressing the disparity between pattern matching and explicit equality comparisons noted in issue #123858.

Changes:

  • Adds logic in fgOptimizeSwitchBranches to detect and optimize switches with exactly 2 unique successors
  • Transforms such switches into unsigned LE/GT comparisons, choosing the direction to favor fall-through
  • Adds comprehensive test coverage for both zero-based and non-zero-based consecutive ranges

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File Description
src/coreclr/jit/fgopt.cpp Implements the switch-to-range-check optimization with edge weight/dup count fixup and profile weight updates
src/tests/JIT/opt/OptSwitchRecognition/optSwitchRecognition.cs Adds tests for zero-based (0-4) and non-zero-based (10-14) consecutive value ranges with comprehensive boundary cases

Copilot AI review requested due to automatic review settings February 18, 2026 19:41
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Pull request overview

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

Copilot AI review requested due to automatic review settings February 18, 2026 19:48
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Pull request overview

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

Copilot AI review requested due to automatic review settings February 18, 2026 23:01
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Pull request overview

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

This was referenced Feb 19, 2026
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EgorBo
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@JulieLeeMSFT JulieLeeMSFT requested a review from EgorBo February 20, 2026 23:32
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JulieLeeMSFT commented Feb 20, 2026

@EgorBo, I addressed all comments. It's ready to review.

@build-analysis build-analysis bot mentioned this pull request Feb 21, 2026
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LGTM with a few suggestions

Copilot AI review requested due to automatic review settings February 23, 2026 19:53
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Pull request overview

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

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JulieLeeMSFT commented Feb 24, 2026

/ba-g Failures are timeouts in Android pipeline. Not related to this PR.

@JulieLeeMSFT JulieLeeMSFT merged commit c94ed87 into dotnet:main Feb 24, 2026
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@dotnet-maestro dotnet-maestro bot mentioned this pull request Feb 25, 2026
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IL and asm differences from pattern matching vs comparisons

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