> Inline functions are still fine though for writing zero-cost helpers, and I don't think C# has them (there's an <AggressiveInlining> attribute but IIRC it's just a hint, the compiler isn't required to actually obey).

In F#, you cannot disable inlining of small functions. It does IL source code inlining, like copy-paste, not machine code inlining. That increases generated dll/exe size a lot for generics. And sometimes you do not want to do that because it's worse for performance. There is an issue for that: https://github.com/fsharp/fslang-suggestions/issues/838

In C#, the AggressiveInlining attribute works well, in predictable manner. Non-inlineable cases are well known (`throw\switch\fixed\try..catch\calling delegates` and some more https://github.com/dotnet/runtime/blob/master/src/coreclr/ji...).

> because it supports directly inlining IL instructions intermixed with regular code

This works for very simple things only. And using InlineIL.Fody, Sigil, raw IL.Emit or just raw IL code as text is not that more difficult if you already know IL.

> you _can_ write F# that's about as fast as non-unsafe C#

I tried this several times. It always ends with fighting the compiler more than just rewriting in C#, even without the `unsafe` keyword. In most cases due to generated IL that I cannot control from F#.

F# does a good job with tail-recursion, though - you can make the for/while loop safer this way. https://www.gresearch.co.uk/article/in-favour-of-recursive-f...

But it adds .tail prefix before every callvirt instruction and this breaks some JIT optimizations. E.g. generic type specialization 'if(typeof(T) == typeof(...))' breaks, I had to dig into compiler source code to find out that it doesn't add .tail before ctors, used a struct wrapper and called a method in the struct ctor to avoid .tail prefix, but it adds its own overhead.

Also, most simple tail calls are rewritten by a compiler to a loop, but if you look at the generated IL, there are multiple temp variables, locals, needless assignments. Maybe JIT is able to eliminate those, or maybe not... In C# I could get IL that reflects what I write, without surprises.

Slight correction: .tail prefix prevents inlining by JIT, it's mentioned in the second link in my sibling comment as "explicit tail prefix in callee". So even if that type specialization JIT optimization worked, the small performance-critical method containing it was not inlined. But the point of the specialization was to make the call for primitives faster than a method call, and fallback to a default implementation via callvirt. And there is no way to disable adding the .tail prefix to the calls that do not need it.

This also means that a lot of F# functions that are not inlined by F# source-code inlining itself effectively have NoInlining attribute. E.g. some helper method `if x then y.call() else z.call()`, if not inlined by F#, will never be inlined by JIT due to .tail prefixes before the methods calls.