Avoid boxing of TState

[jods4]

[LoggerMessage] generate a struct state, in an attempt to reduce allocations.

For this to be meaningful, state should not be boxed, but it is in AsLoggableValue:

https://github.com/serilog/serilog-extensions-logging/blob/dev/src/Serilog.Extensions.Logging/Extensions/Logging/SerilogLogger.cs#L168-L174

// Current version:
static object? AsLoggableValue<TState>(TState state, Func<TState, Exception?, string>? formatter)
{
    object? stateObj = state;  // <- always boxing!!
    if (formatter != null)
        stateObj = formatter(state, null);
    return stateObj;
}

// Suggested:
static object? AsLoggableValue<TState>(TState state, Func<TState, Exception?, string>? formatter)
{
    object? stateObj = null;
    if (formatter != null)
        stateObj = formatter(state, null);
    return stateObj ?? state; // <- boxed only when needed
}

Note that in the case of [LoggerMessage] a formatter is always provided (to create the formatted string without boxing of its parameters), so stateObj will never be null and state will not be boxed.

For the curious mind, in PrepareWrite() the code if (state is IEnumerable<..> structure) does not result in boxing because JIT compiles each struct TState separately and statically optimizes these conditions and interfaces, as can be seen in this disassembly (call S.GetEnumerator):
https://sharplab.io/#v2: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

[jods4]

Another cheap win in .net < 9.0: StartsWith(char) is quite faster than StartsWith(string) on these two lines:
https://github.com/serilog/serilog-extensions-logging/blob/dev/src/Serilog.Extensions.Logging/Extensions/Logging/SerilogLogger.cs#L106-L111

In .net 9+ there's a fast path in StartsWith(string) that checks if (prefix.Length == 1) return str.StartsWith(prefix[0]).

[jods4]

BTW: it would be nice to have similar fast logging when using Serilog natively!

• MS code generator only targets M.E.L.ILogger, we'd need a generator whose template targets Serilog.ILogger.
  Source code is here: https://github.com/tkrafael/dotnet-runtime/tree/693620ed1425182d1f388f51f06fa18fb6b7e142/src/libraries/Microsoft.Extensions.Logging.Abstractions/gen
  It would also be nice to fallback to global static Log on static methods if there's no static ILogger in class.

• Code that writes a struct + formatter efficiently would need to be moved from Serilog.Extensions.Logging into Serilog core.

[nblumhardt]

Thanks for all the thoughts and suggestions @jods4!

Anyone keen to investigate/PR any of these? 😎

[jods4]

@nblumhardt The first changes are so trivial... here you go: #242

I should have opened a different issue for having performant code generator that works with Serilog.Log.
That's a whole lot more complicated, cannot be closed as quickly 😆

[jods4]

Nothing is ever trivial 🤣
Turns out StartsWith(char) was not part of .net fx of good ole time, so I reverted.

Could have gone creative with str is ['@', ..], turns out that as optimal as it gets. Felt excessive, though 😏

[nblumhardt]

Thanks again, keen to keep chipping away at these kinds of things, nice to see small gains add up over time :-)