[JIT] Inefficient in-place readonly record struct updates with "with" keyword

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#111,076 2 comments 1 reaction 1 assignee View on GitHub

@jakobbotsch is already working on this.

Since Jan 6, 2025.

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Description

area-CodeGen-coreclr tenet-performance
Description

Consider the following code snippet:

public readonly record struct X(int A, int B, int C, int D, int E);

public record struct Y(int A, int B, int C, int D, int E);

public class C
{
    public void M(ref X x)
    {
        x = x with { C = 12 };
    }
    
    public void N(ref Y y)
    {
        y.C = 12;
    }
}

The function M uses "with" to change the reference x in place, this should be equivalent to just setting C = 12 directly. However, on sharplab we get:

C.M(X ByRef)
    L0000: sub esp, 0x14
    L0003: cmp [edx], dl
    L0005: vmovdqu xmm0, [edx]
    L0009: vmovdqu [esp], xmm0
    L000e: mov eax, [edx+0x10]
    L0011: mov [esp+0x10], eax
    L0015: vmovdqu xmm0, [esp]
    L001a: vmovdqu [edx], xmm0
    L001e: mov eax, [esp+0x10]
    L0022: mov [edx+0x10], eax
    L0025: mov dword ptr [edx+8], 0xc
    L002c: add esp, 0x14
    L002f: ret

C.N(Y ByRef)
    L0000: mov dword ptr [edx+8], 0xc
    L0007: ret
Configuration

Sharplab / C# / x64 / JIT ASM / Release

Dominant language
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