JIT fails to track exact class handles with ternaries in initializers #55079
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dotnet-issue-labeler
bot
added
area-CodeGen-coreclr
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A potential fix: EgorBo@692a597 |
jakobbotsch
added
untriaged
JulieLeeMSFT
removed
the
untriaged
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Setting this to .NET 6 for now. If time is not enough, we can move to future. |
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Moving to 7.0.0, keeping type info for spilled to stack locals looks tricky. |
using System.Runtime.CompilerServices;
using System.Threading;
public static class Tests
{
static void Main()
{
BFactory factory = new BFactory();
for (int i = 0; i < 100; i++)
{
// Test1 is GDV friendly and has "return 42"
//Test1(factory);
// Test2 is not
Test2(factory);
Thread.Sleep(16);
}
}
[MethodImpl(MethodImplOptions.NoInlining)]
static int Test1(AFactory factory)
{
if (factory != null)
{
A a = factory.GetA();
if (a != null)
{
return a.GetValue();
}
}
return 0;
}
[MethodImpl(MethodImplOptions.NoInlining)]
static int Test2(AFactory factory)
{
return factory?.GetA()?.GetValue() ?? 0;
}
}
public class A
{
public virtual int GetValue() => 41;
}
public class B : A
{
public override int GetValue() => 42;
}
public class AFactory
{
public virtual A GetA() => null;
}
public class BFactory : AFactory
{
private static B b = new B();
public override A GetA() => b;
}A similar issue |
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We don't do GDV during late devirt (which typically kicks in when the object being dispatched on is the result of a call). But we should have observed the class anyways and caught this in early devirt. Looks like there is an early out in |
I'll put up a PR later today. |
Update the early bailout in impDevirtualizeCall that gives up if the object class cannot not be determined to check if we can do GDV (guarded devirtualization) based on profile data. Fixes one of the cases noted in dotnet#55079.
Update the early bailout in impDevirtualizeCall that gives up if the object class cannot not be determined to check if we can do GDV (guarded devirtualization) based on profile data. Fixes one of the cases noted in #55079.
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Moving to Future, my snippet was fixed by Andy while the Jakob's one is fixed with PGO - PGO applies GDV for that ToString() call and then later VN/CP eliminates the type check |
We don't manage to devirtualize the
ToStringcall here because we fail to track the exact class handle. The IL generated seems a bit unfortunate for us:The code ends up as
Funnily enough, value numbering and constant prop ends up loading the method table chunk as a constant.
cc @dotnet/jit-contrib
category:correctness
theme:zero-init
skill-level:expert
cost:medium
impact:medium
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