dotnet/roslyn

Suboptimal codegen for `is` pattern compared to .NET 6/7

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#80052 opened on Aug 26, 2025

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 (3 comments) (1 reaction) (1 assignee)C# (20,414 stars) (4,257 forks)batch import
Area-CompilersCode Gen QualityOngoing-Quality-Candidatehelp wanted

Description

A simple method with a pattern match is generates less efficient code starting from .NET 8 compared to .NET 6/7.

Repro: godbolt

public static bool HasLeadingSlash(ReadOnlySpan<char> path)
{
    if (path.Length > 0 && path[0] is '/' or '\\')
        return true;

    return false;
}

.NET 8,9,10 (IL size: 49 bytes):

public static bool HasLeadingSlash(ReadOnlySpan<char> path)
{
    bool flag = path.Length > 0;
    if (flag)
    {
        char c = path[0];
        bool flag2 = ((c == '/' || c == '\\') ? true : false);
        flag = flag2;
    }

    if (flag)
    {
        return true;
    }

    return false;
}

.NET 6,7 (IL size: 34 bytes):

public static bool HasLeadingSlash(ReadOnlySpan<char> path)
{
    if (path.Length > 0)
    {
        char c = path[0];
        if (c == '/' || c == '\\')
        {
            return true;
        }
    }
    return false;
}

Roslyn now introduces unnecessary temporary bool variables and extra conditional branches. The JIT is unable to untangle this pattern, resulting in larger, slower code.

Contributor guide

Tech stack
csharp
Domain
performancetooling
Issue type
performance
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
5
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
over 1 week
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
blocked
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
C# programmingRoslyn compiler internalsIL code generation
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
10
Research direction
Examine the pattern matching lowering in Roslyn's codegen, specifically the `BoundIsExpression` handling. Compare IL output between .NET 7 and .NET 8+ for the provided method. The regression likely stems from changes in how temporary boolean variables are introduced. Check the assignee's comments for any progress or notes on the root cause. Focus on files like `EmitExpression.cs` or `LoweredConditional.cs` in the CSharp codegen folder.