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JIT: (bug) Loop stepped by checked(i - (-1)) is unrolled to one iteration, dropping an OverflowException #133583
Description
Repro
using System;
using System.Runtime.CompilerServices;
internal static class Program
{
[MethodImpl(MethodImplOptions.NoInlining | MethodImplOptions.AggressiveOptimization)]
private static int Test()
{
int count = 0;
int i = 2147483646;
while (i != 2147483645)
{
count++;
i = checked(i - (-1));
}
return count;
}
private static void Main()
{
try
{
Console.WriteLine($"returned {Test()}");
}
catch (OverflowException)
{
Console.WriteLine("OverflowException");
}
}
}Run with DOTNET_TieredCompilation=0.
Expected
OverflowException
i starts at 2147483646. The first iteration leaves i == int.MaxValue; the second evaluates
checked(int.MaxValue - (-1)), which overflows and throws. DOTNET_JITMinOpts=1 prints exactly
this.
Actual
returned 1
The optimizer decides the loop runs exactly once, so the second (throwing) update is deleted
entirely and Test returns normally. The unrolled iteration count is wrong for a != loop whose
step is a checked subtraction of a negative constant.
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.
AndyAyersMS commentedon Sep 10, 2026
Not a new bug, fails the same way in .NET 7 through now.
JIT: fix checked subtraction loop unrolling (#133646)
EgorBo commentedon Sep 18, 2026
Affected SDK versions:
JIT: fix checked subtraction loop unrolling (dotnet#133646)