Different codegen for pinning GetPinnableReference vs GetReference after a length check

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#110,710 2 comments 0 reactions 0 assignees View on GitHub

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
35/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
csharp
Domain
compilers, performance

Research direction

Start with the Foo and Bar repro and compare their generated assembly, focusing on the mov/test versus cmp sequence shown in the issue. Done means determining why pinning through GetPinnableReference differs from MemoryMarshal.GetReference after the length check and addressing the discrepancy if it is unintended.

Written by the indexing model from the issue text.

Description

area-CodeGen-coreclr 
public static void Foo(ReadOnlySpan<char> span)
{
    if (!span.IsEmpty)
    {
        fixed (char* ptr = span)
            Consume(ptr);
    }
}

public static void Bar(ReadOnlySpan<char> span)
{
    if (!span.IsEmpty)
    {
        fixed (char* ptr = &MemoryMarshal.GetReference(span))
            Consume(ptr);
    }
}

[MethodImpl(MethodImplOptions.NoInlining)]
private static void Consume(char* ptr) { }

https://www.diffchecker.com/cAaIxNr9/

-mov       ecx,[rcx+8]
-test      ecx,ecx
+cmp       dword ptr [rcx+8],0
Dominant language
C#
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Forks
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