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JIT: (bug) Ordinary field load is hoisted out of a loop across a Volatile.Read #133579
Description
Repro
using System;
using System.Runtime.CompilerServices;
using System.Threading;
internal sealed class Box
{
public int Value;
public int Ready;
}
internal static class Program
{
[MethodImpl(MethodImplOptions.NoInlining | MethodImplOptions.AggressiveOptimization)]
private static int Reader(Box box)
{
int value = 0;
int seen = 0;
while (seen < 2)
{
value = box.Value;
if (Volatile.Read(ref box.Ready) != 0)
{
seen++;
}
}
return value;
}
private static void Main()
{
var box = new Box();
var reader = new Thread(() => Console.WriteLine($"reader saw {Reader(box)}"));
reader.Start();
Thread.Sleep(50);
box.Value = 42;
Volatile.Write(ref box.Ready, 1);
reader.Join();
}
}Run with DOTNET_TieredCompilation=0.
Expected
reader saw 42
The loop exits only after two iterations saw Ready != 0, so the returned box.Value load runs
after an acquire in an earlier iteration. DOTNET_JitDoLoopHoisting=0 and DOTNET_JITMinOpts=1
both print this, 5 of 5 runs each.
Actual
reader saw 0
Reproduced on 5 of 5 runs. The plain box.Value load is hoisted out of the loop even though it is
followed by a Volatile.Read acquire in the same loop body, so it is executed once before the
writer publishes and never re-read.
Platform
Windows x64, .NET 11 (dotnet/runtime main @ a0b86b1, Checked build). Requires
DOTNET_TieredCompilation=0; no special CPU features.
Activity
dotnet-policy-service commentedon Sep 10, 2026
Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
See info in area-owners.md if you want to be subscribed.
MihuBot commentedon Sep 10, 2026
I'm a bot. Here is a possible related and/or duplicate issue (I may be wrong):
JIT: Prevent loop hoisting across volatile reads (#133825)
JIT: Prevent loop hoisting across volatile reads (dotnet#133825)
EgorBo commentedon Sep 18, 2026
Affected SDK versions:
hamarb123 commentedon Sep 18, 2026
@AndyAyersMS would this be considered for backport to at least 10 and 11? I think I probably have some code that could run into this. And to me, this seems like a bug that is unlikely to be correctly root-caused by an AI, since it relies on it:
volatile, rather than it thinking "maybe i just got the memory model wrong and volatile is more like volatile in c++, etc."[release/11.0] JIT: Prevent loop hoisting across volatile reads (#134351
snakefoot commentedon Oct 8, 2026
@JulieLeeMSFT + @AndyAyersMS Possible to get backport to NET10 since also affected?