Use .tail prefix to optimize JIT-generated code

[SergeiPavlov]

The effect is shown here:
servicetitan/dataobjects-net#156

[hypercodeplace]

Just wondering, why is the difference with OpCodes.Tailcall? (I could't find any info about OpCodes.Tail in the docs).
And also, does this optimization work for all calls or for the recursion calls only?

[SergeiPavlov]

Ah, OpCodes.Tailcall is correct vor .NET (fixed the code)
And .Tail is for Cecil library used in DO

It works only before sequence call, ret
And this is not always recursive

I have already applied such change before and then reverted it c67bd84

My new understanding based on Debugger & StackTraces from dumps is:
Without .tail prefix the JIT optimizer SOMETIME can guess to do it, but not in 100% cases.
tail. prefix guarantees this kind of optimization

[botinko]

I cannot estimate how much this change may affect the reliability of the application. As noted in the article, this optimization is not used by default. This means that the behavior of the JIT compiler may not be sufficiently tested. Also I afraid of increased JIT time because of optimization.

[SergeiPavlov]

I cannot estimate how much this change may affect the reliability of the application. As noted in the article, this optimization is not used by default. This means that the behavior of the JIT compiler may not be sufficiently tested. Also I afraid of increased JIT time because of optimization.

.tail prefix tells JIT just to convert call, ret into jmp without additional code analyzis.
It means less work for JIT, because by default JIT has make a decision to do it or not

Looks like no potential flaw. This prefix is actively used by functional-language compilers like F#, where tail recursion is critical feature.