dotnet/roslyn

No ReadOnlySpan<byte> static data optimization for new byte[N]

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#33,088 opened on Feb 4, 2019

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Area-CompilersFeature Requesthelp wanted

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Description

Version Used: dotnet 2.2.200-preview-009748

Steps to Reproduce:

Declare a static ReadOnlySpan<byte> property of the form:

public class C
{
    public static ReadOnlySpan<byte> StaticData => new byte[] { 0 };
    public static ReadOnlySpan<byte> DoesntWork => new byte[1];
}

Expected Behavior:

Per the optimisation in #24621, I expected that both StaticData and DoesntWork would be compiled to a reference to static data, e.g., new ReadOnlySpan<byte>(&global::<PrivateImplementationDetails>.5BA93C9DB0CFF93F52B521D7420E43F6EDA2784F, 1);

Actual Behavior:

Only StaticData has the implementation optimisation; DoesntWork doesn't, even though it has the exact same data. The problem is more noticeable with an expression like new byte[1000], where you might not want to type out 1000 literal zero bytes in source code.

SharpLab: https://sharplab.io/#v2:D4AQDABCCMDcCwAoJIDMUBMEDCSDeSERU6MAbBAEoCmAhgCYDyAdgDYCeAygA63MA8AI3YAXagD4InEbREBLAMYARWbQgBeSc2oB3CMLEBtALoQ8ESAF8EiYiSjQKNBiw48+Q0RIhKA9tQBnZhEAdV8AJwBrDS1dfS9DaGMbSwggA===

Contributor guide

Research direction
Investigate the compiler's existing optimization for constant byte arrays in ReadOnlySpan (as implemented in #24621) and extend it to handle the case where the array is initialized with default values using `new byte[length]`. Focus on the part of the compiler that detects and rewrites such patterns during lowering or code generation. Look at the logic that compares the array initializer to a zero filled array and emit the static data reference accordingly.
Tech stack
csharp
Domain
compilers
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.
3-5 days
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#Git
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
65