Arm64: Fix for 96441

[dhartglassMSFT]

The increment of lzcnt (*) wasn't getting transformed to a conditional increment, because an earlier phase folds the "add local, 1" into a constant based on assertion information.

[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static int RuneLength(in byte value)
{
    var lzcnt = (uint)BitOperations.LeadingZeroCount(~((uint)value << 24));
    if (lzcnt is 0) lzcnt++; //*

    return (int)lzcnt;
}

Fix to allow TryLowerCnsIntCselToCinc to look cases similar to if (myvar==0)myvar=1;

fixes #96441

[dhartglassMSFT]

@dotnet-policy-service agree company="Microsoft"

[Copilot]

Pull request overview

Expands ARM64 lowering to produce more conditional-increment (cinc/csinc) patterns, especially for assignments derived from constants compared against a local.

Changes:

• Extend TryLowerCnsIntCselToCinc to also recognize SELECTCC patterns where the “+1” value is constant relative to a CMP local/constant comparison, and rewrite to GT_SELECT_INCCC.
• Broaden LowerSelect’s trigger to attempt this lowering when either select operand is an integer constant (not only when both are).

Reviewed changes

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

File	Description
src/coreclr/jit/lowerarmarch.cpp	Adds a new lowering path to convert certain SELECTCC + CMP patterns into GT_SELECT_INCCC by substituting a local read for the constVal+1 constant.
src/coreclr/jit/lower.cpp	Calls the CINC lowering helper when either select operand is an integer constant, enabling the new pattern matching.

[jakobbotsch]

LGTM! I will kick off Fuzzlyn which is good at testing stuff like this (it may have failures in it unrelated to this PR)

[jakobbotsch]

/azp run Fuzzlyn

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[jakobbotsch]

I went through the Fuzzlyn failures (https://dev.azure.com/dnceng-public/public/_build/results?buildId=1268916&view=ms.vss-build-web.run-extensions-tab) and I don't see anything that obviously looks to be caused by this PR. I think you can bypass the infra issues and merge this.

[dhartglassMSFT]

/ba-g Known arm64 infra issues