Assertion failed '!nodeInfo.IsLclVarWrite() || !unusedLclVarReads.Contains(nodeInfo.LclNum())'

[kunalspathak]

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.
//
// This file is auto-generated.
// Seed: -1
//

using System;
using System.Collections.Generic;
using System.Runtime.CompilerServices;
public class TestClass20045 {
    public struct S2 {
        public ulong uint64_1;
    }
    public struct S3 {
    }
    public struct S4 {
        public ulong uint64_2;
        public S2 s2_4;
    }
    public struct S5 {
    }
    static bool s_boolean_0 = true;
    static S2 s_s2_16 = new S2();
    static S3 s_s3_17 = new S3();
    public S3 Method1(out S5 p_s5_0, ref S2 p_s2_1) {
        unchecked {
            S4 s4_4 = new S4();
            do {
                try {
                    try {
                        try {
                            s4_4.uint64_2 = s4_4.s2_4.uint64_1;
                            {
                            }
                        } catch (TypeAccessException) {
                        } finally {
                        }
                    } catch (RankException) {
                    } catch (NotImplementedException) {
                    } finally {
                    }
                } catch (InvalidCastException) {
                } finally {
                }
            }
            while (15 > 4 || (s_boolean_0 = 15 <= 4));
            return s_s3_17;
        }
    }
    public void Method0() {
        unchecked {
            S5 s5_3 = new S5();
            s_s3_17 = Method1(out s5_3, ref s_s2_16);
            return;
        }
    }
    public static void Main(string[] args) {
        TestClass20045 objTestClass20045 = new TestClass20045();
        objTestClass20045.Method0();
    }
}
/*
Got output diff:
--------- Baseline ---------  

Environment:



--------- Test ---------  

Environment:



Assert failure(PID 12228 [0x00002fc4], Thread: 7360 [0x1cc0]): Assertion failed '!nodeInfo.IsLclVarWrite() || !unusedLclVarReads.Contains(nodeInfo.LclNum())' in 'TestClass20045:Method1(byref,byref):S3:this' during 'Lowering nodeinfo' (IL size 45)

    File: D:\git\runtime\src\coreclr\jit\lir.cpp Line: 1369
    Image: D:\git\runtime\artifacts\tests\coreclr\windows.x86.Checked\tests\Core_Root\CoreRun.exe




*/

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Issue Details

---

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.
//
// This file is auto-generated.
// Seed: -1
//

using System;
using System.Collections.Generic;
using System.Runtime.CompilerServices;
public class TestClass20045 {
    public struct S2 {
        public ulong uint64_1;
    }
    public struct S3 {
    }
    public struct S4 {
        public ulong uint64_2;
        public S2 s2_4;
    }
    public struct S5 {
    }
    static bool s_boolean_0 = true;
    static S2 s_s2_16 = new S2();
    static S3 s_s3_17 = new S3();
    public S3 Method1(out S5 p_s5_0, ref S2 p_s2_1) {
        unchecked {
            S4 s4_4 = new S4();
            do {
                try {
                    try {
                        try {
                            s4_4.uint64_2 = s4_4.s2_4.uint64_1;
                            {
                            }
                        } catch (TypeAccessException) {
                        } finally {
                        }
                    } catch (RankException) {
                    } catch (NotImplementedException) {
                    } finally {
                    }
                } catch (InvalidCastException) {
                } finally {
                }
            }
            while (15 > 4 || (s_boolean_0 = 15 <= 4));
            return s_s3_17;
        }
    }
    public void Method0() {
        unchecked {
            S5 s5_3 = new S5();
            s_s3_17 = Method1(out s5_3, ref s_s2_16);
            return;
        }
    }
    public static void Main(string[] args) {
        TestClass20045 objTestClass20045 = new TestClass20045();
        objTestClass20045.Method0();
    }
}
/*
Got output diff:
--------- Baseline ---------  

Environment:



--------- Test ---------  

Environment:



Assert failure(PID 12228 [0x00002fc4], Thread: 7360 [0x1cc0]): Assertion failed '!nodeInfo.IsLclVarWrite() || !unusedLclVarReads.Contains(nodeInfo.LclNum())' in 'TestClass20045:Method1(byref,byref):S3:this' during 'Lowering nodeinfo' (IL size 45)

    File: D:\git\runtime\src\coreclr\jit\lir.cpp Line: 1369
    Image: D:\git\runtime\artifacts\tests\coreclr\windows.x86.Checked\tests\Core_Root\CoreRun.exe




*/

Author:	kunalspathak
Assignees:	-
Labels:	arch-x86, os-windows, area-CodeGen-coreclr
Milestone:	-

[kunalspathak]

Repros on windows/x86

@dotnet/jit-contrib

[BruceForstall]

Simpler repro case:

using System;
using System.Runtime.CompilerServices;

public class TestClass20045 {
    public struct S2 {
        public ulong uint64_2;
    }
    public struct S4 {
        public ulong uint64_1;
        public S2 s2;
    }
    [MethodImpl(MethodImplOptions.NoInlining)]
    public static ulong Method1(bool b) {
        S4 s = new S4();
        do {
            s.uint64_1 = s.s2.uint64_2;
        } while (b);
        return s.s2.uint64_2;
    }
    public static void Main(string[] args) {
        ulong u = Method1(false);
        Console.WriteLine(u);
    }
}

[BruceForstall]

This also fails in 5.0. Moving out to 7.0.

[AndyAyersMS]

Do we do something reasonable in release jits?

[BruceForstall]

Do we do something reasonable in release jits?

Yes, at least for this case.

LIR is trying to assert that if you have a local var store, there aren't any outstanding reads of that var that are "contained" and thus won't generate code until the "user" of the read. E.g.,

  t1 = LCL_VAR V01
  t2 = LCL_VAR V01
  STORE_LCL_VAR V01 <- t7 // here we would trash the "t2 = ..." which doesn't actually generate code if it is contained in the STORE_LCL_VAR below
  STORE_LCL_VAR V09 <- t2

Specifically, the comment says:

// Specifically, ensure that an unaliasable lclVar is not redefined between the
// point at which a use appears in linear order and the point at which it is used by its user.
// This ensures that it is always safe to treat a lclVar use as happening at the user (rather than at
// the lclVar node).

In this case, it's not an issue because we don't have a LCL_VAR; we have non-overlapping LCL_FLD. But CheckLclVarSemanticsHelper doesn't track LCL_FLD ranges.

The long decomposition looks like:

N001 (  3,  4) [000010] ------------        t10 =    LCL_FLD   long   V01 loc0         u:3[+8] Fseq[s2, uint64_2] $200
                                                  /--*  t10    long
N003 ( 10, 12) [000012] UA--G-------              *  STORE_LCL_FLD long   V01 loc0         ud:3->0[+0] Fseq[uint64_1]

=>

N001 (  3,  4) [000010] ------------        t10 =    LCL_FLD   int    V01 loc0         u:3[+8] Fseq[s2, uint64_2] $200
               [000043] ------------        t43 =    LCL_FLD   int    V01 loc0         [+12]
                                                  /--*  t10    int
                                                  +--*  t43    int
               [000044] ------------        t44 = *  LONG      long
                                                  /--*  t44    long
N003 ( 10, 12) [000012] UA--G-------              *  STORE_LCL_FLD long   V01 loc0         ud:3->0[+0] Fseq[uint64_1]

=> 

N001 (  3,  4) [000010] ------------        t10 =    LCL_FLD   int    V01 loc0         u:3[+8] Fseq[s2, uint64_2] $200
               [000043] ------------        t43 =    LCL_FLD   int    V01 loc0         [+12]
                                                  /--*  t10    int
N003 ( 10, 12) [000012] UA--G-------              *  STORE_LCL_FLD int    V01 loc0         ud:3->0[+0] Fseq[uint64_1]
                                                  /--*  t43    int
               [000045] U-----------              *  STORE_LCL_FLD int    V01 loc0         [+4]

In this case, the first STORE_LCL_FLD writing V01 doesn't trash the non-overlapping t43, reading V01 at a different offset/size, used by the second STORE_LCL_FLD.

It seems like the LIR checking should consider all non-overlapping LCL_FLD reads/writes as independent, like each was a LCL_VAR. But that would be a huge change.

We could simply (temporarily?) ignore this assert for LCL_FLD.

We could possibly change decomposition to try to avoid creating this form; maybe by creating copies through temps, which would presumably create worse CQ.

[jakobbotsch]

Another repro, in debug mode/with tiering enabled:

// Generated by Fuzzlyn v1.5 on 2021-10-19 13:43:00
// Run on .NET 6.0.0-rc.2.21480.5 on X64 Windows
// Seed: 248268849421871353
// Reduced from 18.2 KiB to 0.2 KiB in 00:03:35
// Exits with error:
//
// Assert failure(PID 8056 [0x00001f78], Thread: 30444 [0x76ec]): Assertion failed '!nodeInfo.IsLclVarWrite() || !unusedLclVarReads.Contains(nodeInfo.LclNum())' in 'Program:Main(Fuzzlyn.ExecutionServer.IRuntime)' during 'Lowering nodeinfo' (IL size 30)
//
//     File: C:\dev\dotnet\runtime2\src\coreclr\jit\lir.cpp Line: 1367
//     Image: C:\dev\dotnet\runtime2\artifacts\tests\coreclr\windows.x86.Checked\Tests\Core_Root\corerun.exe
//
//
public struct S0
{
    public long F2;
}

public struct S1
{
    public S0 F0;
}

public class Program
{
    public static void Main()
    {
        var vr3 = new S1();
        var vr6 = vr3.F0;
        vr6.F2 = vr6.F2;
    }
}