Just a simple BenchMark test for a small optimization of String.length in F#. Previously, the function always called the System.String::Length method twice. It's now down to one call only. This optimization has already been merged into F#, see: dotnet/fsharp#9469.

Placing it here to check with @adamsitnik in BDN issue #1486 if it is possible to write a single benchmark in BDN that shows the timings against two different FSharp.Core.dlls, one for the original, and one for the locally built optimized version. If we can establish a way to do that, it'll be easier to check more complex optimizations that require changes in private functions, or involve compiler-specific syntax like inline IL with stuff like (# "ldlen.multi 2 0" array : int #) .

To make this work, we would somehow need to compile with/without DisableImplicitFSharpCoreReference.