JIT: Bad codegen - using wrong var for field access

[TIHan]

// Generated by Fuzzlyn v1.5 on 2022-11-14 02:52:08
// Run on Arm64 Windows
// Seed: 11038715273855459808
// Reduced from 96.6 KiB to 0.9 KiB in 00:08:38
// Debug: Outputs 0
// Release: Outputs 1
public struct S0
{
    public sbyte F0;
    public long F2;
}

public struct S1
{
    public S0 F0;
    public byte F1;
    public bool F3;
    public S1(byte f1): this()
    {
        F1 = f1;
    }
}

public class Program
{
    public static IRuntime s_rt;
    public static S1 s_23;
    public static void Main()
    {
        s_rt = new Runtime();
        var vr1 = new S1(1);
        M16(s_23, ref vr1);
    }

    public static void M16(S1 argThis, ref S1 arg0)
    {
        if (s_23.F3)
        {
            arg0.F0 = argThis.F0;
        }
        else
        {
            arg0 = argThis;
        }

        s_rt.WriteLine(arg0.F1);
    }
}

public interface IRuntime
{
    void WriteLine<T>(T value);
}

public class Runtime : IRuntime
{
    public void WriteLine<T>(T value) => System.Console.WriteLine(value);
}

I was able to reproduce this on X64 as well.
Built under Release:

• DOTNET_TieredCompilation=0 prints 1
• DOTNET_TieredCompilation=1 prints 0 <- expected output

I've yet to do a deep dive into the issue, but I think this line, s_rt.WriteLine(arg0.F1); , the arg0 is always using its original value.

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
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Issue Details

---

// Generated by Fuzzlyn v1.5 on 2022-11-14 02:52:08
// Run on Arm64 Windows
// Seed: 11038715273855459808
// Reduced from 96.6 KiB to 0.9 KiB in 00:08:38
// Debug: Outputs 0
// Release: Outputs 1
public struct S0
{
    public sbyte F0;
    public long F2;
}

public struct S1
{
    public S0 F0;
    public byte F1;
    public bool F3;
    public S1(byte f1): this()
    {
        F1 = f1;
    }
}

public class Program
{
    public static IRuntime s_rt;
    public static S1 s_23;
    public static void Main()
    {
        s_rt = new Runtime();
        var vr1 = new S1(1);
        M16(s_23, ref vr1);
    }

    public static void M16(S1 argThis, ref S1 arg0)
    {
        if (s_23.F3)
        {
            arg0.F0 = argThis.F0;
        }
        else
        {
            arg0 = argThis;
        }

        s_rt.WriteLine(arg0.F1);
    }
}

public interface IRuntime
{
    void WriteLine<T>(T value);
}

public class Runtime : IRuntime
{
    public void WriteLine<T>(T value) => System.Console.WriteLine(value);
}

I was able to reproduce this on X64 as well.
Built under Release:

• DOTNET_TieredCompilation=0 prints 1
• DOTNET_TieredCompilation=1 prints 0 <- expected output

I've yet to do a deep dive into the issue, but I think this line, s_rt.WriteLine(arg0.F1); , the arg0 is always using its original value.

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

[jakobbotsch]

Bisected to #77103

[jakobbotsch]

*************** Starting PHASE Post-morph tail merge
All preds of BB04 end with the same tree, moving
STMT00006 ( 0x00C[E-] ... 0x013 )
               [000034] -A-XG+-----                         ▌  ASG       struct (copy)
               [000033] ---XG+-N---                         ├──▌  BLK       struct<S0, 16>
               [000027] -----+-----                         │  └──▌  LCL_VAR   byref  V01 arg1         
               [000038] n---G+-----                         └──▌  OBJ       struct<S0, 16>
               [000030] -----+-----                            └──▌  LCL_VAR   byref  V00 arg0         

unlinking STMT00006 ( 0x00C[E-] ... 0x013 )
               [000034] -A-XG+-----                         ▌  ASG       struct (copy)
               [000033] ---XG+-N---                         ├──▌  BLK       struct<S0, 16>
               [000027] -----+-----                         │  └──▌  LCL_VAR   byref  V01 arg1         
               [000038] n---G+-----                         └──▌  OBJ       struct<S0, 16>
               [000030] -----+-----                            └──▌  LCL_VAR   byref  V00 arg0         
 from BB02

BB02 becomes empty

unlinking STMT00001 ( 0x01A[E-] ... 0x01C )
               [000011] -A-XG+-----                         ▌  ASG       struct (copy)
               [000010] ---XG+-N---                         ├──▌  BLK       struct<S1, 24>
               [000008] -----+-----                         │  └──▌  LCL_VAR   byref  V01 arg1         
               [000039] n---G+-----                         └──▌  OBJ       struct<S1, 24>
               [000009] -----+-----                            └──▌  LCL_VAR   byref  V00 arg0         
 from BB03

BB03 becomes empty
Did 1 tail merges in BB04

Looks like we are not checking equality between GT_BLK/GT_OBJ nodes properly.

[jakobbotsch]

I'll submit a fix since Andy is out.