JIT: (bug) Forward substitution drops a promoted struct's defining store

using System;
using System.Runtime.CompilerServices;

class Program
{
    struct S { public long A; public long B; public long C; }

    [MethodImpl(MethodImplOptions.NoInlining)]
    static long Combine(S s, long a, long b) => s.A + s.B + s.C + a + b;

    [MethodImpl(MethodImplOptions.NoInlining | MethodImplOptions.AggressiveOptimization)]
    static long Test(S input)
    {
        S s = input;
        input.A = 42;
        long p = s.A;
        return Combine(s, p, p);
    }

    static void Main() => Console.WriteLine(Test(new S { A = 1, B = 2, C = 3 }));
}

Expected output: 8
Actual output: 6

windows-x64, main @ a0b86b1.

Goes away with DOTNET_JitNoForwardSub=1.

@JulieLeeMSFT

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Taking a look...

Seems like this came from my #129312 so is new in .NET 11.

JIT: Fix promoted liveness in forward substitution (#133703)

Affected SDK versions:

11.0.100-rc.2.26467.112

@AndyAyersMS

@AndyAyersMS would you consider backporting the fix from #133703 to release/11.0?

It currently only exists in main (12.0). The repro still fails on the shipped 11.0 RC2 SDK (11.0.100-rc.2.26467.112), while 8.0.425, 9.0.318 and 10.0.401 all behave correctly, so this is a regression introduced during the 11.0 cycle rather than a long-standing bug — 11.0 would ship with it.

_{Note: this comment was produced by an AI agent; the repro was re-run against all four released SDKs to confirm the results above.}

[release/11.0] JIT: Fix promoted liveness in forward substitution (#1…

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JIT: (bug) Forward substitution drops a promoted struct's defining store #133641

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JIT: (bug) Forward substitution drops a promoted struct's defining store #133641

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dotnet-policy-service commented on Sep 10, 2026

AndyAyersMS commented on Sep 10, 2026

AndyAyersMS commented on Sep 10, 2026

EgorBo commented on Sep 18, 2026

EgorBo commented on Sep 18, 2026

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