Avoid boxing in System.ObjectModel.KeydCollection during startup

[jkotas]

ArgumentNullException.ThrowIfNull can incur boxing when applied to argument of generic type. Tier-1 JIT optimizations are able to optimize this boxing in steady state, but Tier-0 JIT optimization are not. It can result into excessive allocations during startup. Switch KeyedCollection to use ThrowHelper that is pattern used by number of other collections.

[jkotas]

This was noted in #103361 (comment) .

[jkotas]

Repro:

using System.Collections.ObjectModel;

var c = new MyCollection();

var start = GC.GetAllocatedBytesForCurrentThread();
for (int i = 0; i < 1000; i++)
{
    c.TryGetValue(1, out _);
}
var end = GC.GetAllocatedBytesForCurrentThread();
Console.WriteLine(end - start);

class MyCollection : KeyedCollection<uint, uint>
{
    protected override uint GetKeyForItem(uint item) => item;
}

24000 before the change, 0 with the change.

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[stephentoub]

Issues on this (not just for KeyedCollection, but multiple places ThrowIfNull is used with a generic) have been opened multiple times in the past, and we've previously defended the non-generic ThrowIfNull for such use, citing the boxing removal by the JIT. If we're going to start replacing these, I think we should instead reconsider adding such a generic overload, or augment the JIT to do the boxing removal even in tier 0.

[jkotas]

Issues on this (not just for KeyedCollection, but multiple places ThrowIfNull is used with a generic) have been opened multiple times in the past

I was not aware that we had issues opened on this in the past. For reference: #82227 and #100406

On the other hand, we took tweaks to avoid Tier-0 specific boxing in the past.

we should instead reconsider adding such a generic overload

Is there a way to make this generic overload bind to generic types only? If we just add a generic overload, it would be preferred over the object overload, and we would end up with a ton of generic instantiations. The startup cost of these generic instantiations during startup of a typical app would be likely a lot more than the cost of the occasional boxing caused by this during startup of a typical app.

augment the JIT to do the boxing removal even in tier 0.

@dotnet/jit-contrib What would it take to augment Tier-0 to avoid boxing in this case?

[stephentoub]

Is there a way to make this generic overload bind to generic types only?

Not to my knowledge. @333fred, any way to use overload priorities to achieve this?

[AndyAyersMS]

@dotnet/jit-contrib What would it take to augment Tier-0 to avoid boxing in this case?

We'd have to mark the method as intrinsic and add new logic to impBoxPatternMatch.

[333fred]

Is there a way to make this generic overload bind to generic types only?

Not to my knowledge. @333fred, any way to use overload priorities to achieve this?

To be clear, you'd want public static void ThrowIfNull<T>(T? argument, string? paramName = default), but for that to only be prioritized when T is itself a type parameter? No, that isn't possible to do with priorities as designed. Priority is applied unconditionally.

[jkotas]

We'd have to mark the method as intrinsic and add new logic to impBoxPatternMatch.

That sounds better to me than applying workarounds like this in generic collections and other types. Opened #104512