dotnet/roslyn

Suboptimal code for matching nullable value to constant pattern

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#42,912 opened on Mar 30, 2020

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Area-CompilersCode Gen QualityFeature - Pattern MatchingFeature Requesthelp wanted

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Description

See https://sharplab.io/#v2:EYLgZgpghgLgrgJwgZwLQAdYwggdsgZgB8ABARgDYACEgJhrIHYBYAKAG82ruaCHrgAe0EAbKgElkAQQDG8KCIAUAS1wwA/FQAeASioBeAHzaqy5FTIBuKlx4k+5AcLGSAolvQQ5EACYq1mroGxloG+haWbAC+bGxAA=

This issue is extracted from https://github.com/dotnet/roslyn/issues/31494

For source

static class Program
{
    public static bool IsActual(int? x) => x is 1; 
    public static bool IsExpected(int? x) => x == 1;
}

The generated code is

internal static class Program
{
    public static bool IsActual(int? x)
    {
        if (x.HasValue)
        {
            return x.GetValueOrDefault() == 1;
        }
        return false;
    }

    public static bool IsExpected(int? x)
    {
        int? num = x;
        int num2 = 1;
        return (num.GetValueOrDefault() == num2) & num.HasValue;
    }
}

The point is that we optimized == to avoid control-flow. We should do the same for the pattern-matching operations.

Contributor guide

Research direction
Examine the pattern matching lowering in the compiler and apply the same optimization used for equality operator (==) to pattern matching with nullable value types.
Tech stack
csharp
Domain
backend
Issue type
Performance
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
C#
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60