JIT: Improper handling of GC structs with undone promotion in tailcall-to-loop optimization

[jakobbotsch]

class Program
{
    static void Main()
    {
        Test(1234, default);
    }

    static int Test(int count, S16 s)
    {
        object o = "1234";
        if (count == 0 || o.GetHashCode() == 1234)
            return 42;

        return Test(count - 1, s);
    }

    struct S16
    {
        public object A, B;
    }
}

Run with DOTNET_JitNoInline=1 on win-x64. Result:

Assert failure(PID 13352 [0x00003428], Thread: 36112 [0x8d10]): Assertion failed '!parentStruct->lvUndoneStructPromotion' in 'Program:Test(int,Program+S16):int' during 'Mark local vars' (IL size 35; hash 0x046a6981; FullOpts)

    File: C:\dev\dotnet\runtime\src\coreclr\jit\lclvars.cpp Line: 4295
    Image: C:\dev\dotnet\runtime4\artifacts\tests\coreclr\windows.x64.Checked\Tests\Core_Root\corerun.exe

The problem is that the tailcall-to-loop optimization tries to zero out the copy created for the promotion of s, but that promotion was undone.

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Issue Details

---

class Program
{
    static void Main()
    {
        Test(1234, default);
    }

    static int Test(int count, S16 s)
    {
        object o = "1234";
        if (count == 0 || o.GetHashCode() == 1234)
            return 42;

        return Test(count - 1, s);
    }

    struct S16
    {
        public object A, B;
    }
}

Run with DOTNET_JitNoInline=1. Result:

Assert failure(PID 13352 [0x00003428], Thread: 36112 [0x8d10]): Assertion failed '!parentStruct->lvUndoneStructPromotion' in 'Program:Test(int,Program+S16):int' during 'Mark local vars' (IL size 35; hash 0x046a6981; FullOpts)

    File: C:\dev\dotnet\runtime\src\coreclr\jit\lclvars.cpp Line: 4295
    Image: C:\dev\dotnet\runtime4\artifacts\tests\coreclr\windows.x64.Checked\Tests\Core_Root\corerun.exe

The problem is that the tailcall-to-loop optimization tries to zero out the copy created for the promotion of s, but that promotion was undone.

Author:	jakobbotsch
Assignees:	-
Labels:	area-CodeGen-coreclr
Milestone:	-