Inline GetAlternateLookup

[bitfaster]

Inlining GetAlternateLookup eliminates the heap allocation for the AlternateLookup struct on .NET 10:

Method	Runtime	Mean	Ratio	Gen0	Allocated
GetAlternateInline	.NET 10.0	42.73 ns	0.77	-	-
GetAlternateNoInline	.NET 10.0	52.94 ns	0.96	0.0092	40 B
GetAlternateInline	.NET 9.0	55.35 ns	1.00	0.0092	40 B
GetAlternateNoInline	.NET 9.0	56.13 ns	1.01	0.0092	40 B

From https://learn.microsoft.com/en-us/dotnet/core/whats-new/dotnet-10/runtime

Inlining improvements

Various inlining improvements have been made in .NET 10.

The JIT can now inline methods that become eligible for devirtualization due to previous inlining. This improvement allows the JIT to uncover more optimization opportunities, such as further inlining and devirtualization.

...

Return types

During inlining, the JIT now updates the type of temporary variables that hold return values. If all return sites in a callee yield the same type, this precise type information is used to devirtualize subsequent calls. This enhancement complements the improvements in late devirtualization and array enumeration de-abstraction.

[coveralls]

coverage: 99.123%. remained the same — users/alexpeck/inline10 into main

[bitfaster]

inlining TryGet does not eliminate allocs.

An alternative is to make TryGet an extension method, and in the extension method use the ICache comparer to do the check, then call GetAltLookup where the inlining trick works.

[bitfaster]

Tested both LRU and LFU end to end:

Method	Runtime	Mean	Gen0	Allocated
LruGetAlternateInline	.NET 10.0	42.69 ns	-	-
LfuGetAlternateInline	.NET 10.0	76.70 ns	-	-
LruGetAlternateInline	.NET 9.0	53.58 ns	0.0092	40 B
LfuGetAlternateInline	.NET 9.0	127.16 ns	0.0353	152 B

[bitfaster]

Also tried this (including forcing the return type to always be the same struct), but does not devirtualize on .NET 10:

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public bool TryGetAlternateLookup<TAlternateKey>([MaybeNullWhen(false)] out IAlternateLookup<TAlternateKey, K, V> lookup)
            where TAlternateKey : notnull, allows ref struct
        {
            AlternateLookup<TAlternateKey> alt = AlternateLookup<TAlternateKey>.Empty;
            if (this.dictionary.IsCompatibleKey<TAlternateKey, K, I>())
            {
                alt = new AlternateLookup<TAlternateKey>(this);
                lookup = alt;
                return true;
            }

            lookup = alt;
            return false;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public (bool, IAlternateLookup<TAlternateKey, K, V>)TryGetAlternateLookup2<TAlternateKey>()
    where TAlternateKey : notnull, allows ref struct
        {
            if (this.dictionary.IsCompatibleKey<TAlternateKey, K, I>())
            {
                return (true, new AlternateLookup<TAlternateKey>(this));
            }

            return (false, AlternateLookup<TAlternateKey>.Empty);
        }