JIT: True branch not executed

@EgorBo

// Generated by Fuzzlyn v3.3 on 2026-02-15 16:12:11
// Run on X86 Windows
// Seed: 345838815179539746-vectort,vector128,vector256,x86aes,x86avx,x86avx2,x86avx512bw,x86avx512bwvl,x86avx512cd,x86avx512cdvl,x86avx512dq,x86avx512dqvl,x86avx512f,x86avx512fvl,x86bmi1,x86bmi2,x86fma,x86lzcnt,x86pclmulqdq,x86popcnt,x86sse,x86sse2,x86sse3,x86sse41,x86sse42,x86ssse3,x86x86base
// Reduced from 51.2 KiB to 0.8 KiB in 00:02:10
// Debug: Outputs 1
// Release: Outputs 0
using System.Runtime.Intrinsics;

public class C0
{
    public sbyte F2;
}

public class Program
{
    public static bool s_2 = true;
    public static C0 s_8 = new C0();
    public static sbyte s_9 = 1;
    public static Vector256<ushort> s_17;
    public static void Main()
    {
        M0();
        System.Console.WriteLine(s_8.F2);
    }

    public static void M0()
    {
        bool var5 = default(bool);
        if (!var5)
        {
            var5 = s_2;
            if (!var5)
            {
                return;
            }
        }

        if (!var5)
        {
            var5 = false;
        }

        s_17 = Vector256.Create<ushort>(1);
        if (var5)
        {
            s_8.F2 = s_9;
        }
    }
}

Bisected to/exposed by #123856, cc @EgorBo

@JulieLeeMSFT

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
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Probably same issue:

// Generated by Fuzzlyn v3.3 on 2026-02-28 17:41:56
// Run on Arm Linux
// Seed: 4947552585052090314
// Reduced from 335.1 KiB to 0.5 KiB in 00:04:35
// Debug: Prints 0 line(s)
// Release: Prints 1 line(s)
public class C0
{
    public ushort F7;
}

public class Program
{
    public static void Main()
    {
        C0 vr5 = new C0();
        M9(vr5);
    }

    public static void M9(C0 argThis)
    {
        bool var0 = !(1 != argThis.F7);
        bool var15 = var0;
        if (var0)
        {
            var0 = true;
        }

        if (!var15)
        {
            if (var0)
            {
                sbyte vr7 = default(sbyte);
                System.Console.WriteLine(vr7);
            }
        }
    }
}

Copilot says:

  The real issue is that `optGetEdgeAssertions` is returning assertions for an edge that no longer exists—after RBO redirected
  BB01's true target from BB03 to BB04, the function still returns the old assertions instead of recognizing the edge is gone. I
  need to add a check in `optVisitReachingAssertions` to verify that each PHI predecessor block is actually still a predecessor
  before using its assertions.

Yeah I also had a copilot run on this, not sure I agree with its analysis (or at least with whatever I had locally). It does feel like optVisitReachingAssertions is the culprit and my PR that you highlighted just uncover the bug for. But copilot seems to focus on unreachable Phi args as the issue

Ignore PhiArgs without matching actual preds (#125093)

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JIT: True branch not executed #124507

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JIT: True branch not executed #124507

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dotnet-policy-service commented on Feb 17, 2026

jakobbotsch commented on Mar 2, 2026

EgorBo commented on Mar 2, 2026

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