dotnet/roslyn

Codegen of System.Nullable `is null` check is poor

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#65,091 opened on Oct 28, 2022

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4 - In ReviewArea-CompilersCode Gen QualityConcept-Design Debthelp wanted

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Description

Codegen of System.Nullable is null check is poor

Take a look at: https://sharplab.io/#v2:C4LglgNgPgAgLAAgAoHsUCcDCKAmBTAczwDsAKMYhC4AfgQDcBKAWACgBvNhbqgMwVL0qAZwTEArhAiMEXHvJgB2ANxsAvmzbwEAcTQ5s+ImQpVitBiw5zuYfqQCE9AHQAJAIbCAau4ji8MjbyCEqqrBqsWoh6uIaEJABM5JTUdExsnKzydgJCALx5YpLSslnB3KHqQA

void PoorCodegen(in int? v)
{
    if (v is null) 
        return;
}

void GoodCodegen(in int? v)
{
    if (!v.HasValue) 
        return;
}

void GoodCodegen2(in int? v)
{
    if (v == null) 
        return;
}

In the pattern matching case there suddenly is a copy.

Contributor guide

Research direction
Investigate why 'is null' creates a copy for System.Nullable while 'HasValue' and '== null' don't. Trace the lowering step in the compiler that handles 'is' patterns and nullable.
Tech stack
csharp
Domain
backendperformance
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
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.
70