Don't use write barriers for byref-like struct copies

[EgorBo]

Byref-like structs are stack only so we can omit write barriers for them. It looks like JIT already knew that, but only if such structs had ref fields, which they might not have.

void Test(ref MyStruct dst, MyStruct src) => dst = src;

ref struct MyStruct
{
    object a;
    long b;
}

Current codegen for Test:

; Method ConsoleApp35.Program:Test(byref,ConsoleApp35.MyStruct):this (FullOpts)
       push     rdi
       push     rsi
       mov      rdi, rdx
       mov      rsi, r8
       call     CORINFO_HELP_ASSIGN_BYREF
       movsq    
       pop      rsi
       pop      rdi
       ret      
; Total bytes of code: 18

New codegen for Test:

; Method ConsoleApp35.Program:Test(byref,ConsoleApp35.MyStruct):this (FullOpts)
       vmovdqu  xmm0, xmmword ptr [r8]         ;; NOTE: nogc region so SIMD is fine
       vmovdqu  xmmword ptr [rdx], xmm0
       ret      
; Total bytes of code: 10

[EgorBo]

@MihuBot

[EgorBo]

PTAL @jakobbotsch @kunalspathak @dotnet/jit-contrib diffs are relatively small since there are not so many byref structs without ref fields.

[kunalspathak]

LGTM

[kunalspathak]

Is there a documentation that I can read more about this?

[EgorBo]

Yep, e.g. https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/builtin-types/ref-struct Basically, Span itself is the most popular example