RFR: 8324433: Introduce a way to determine if an expression is evaluated as a constant by the Jit compiler [v6]

[Maurizio Cimadamore]

On Thu, 25 Jan 2024 12:52:21 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>>> > Naive question: the right way to use this would be almost invariably be like this:
>>> > ```
>>> > if (isCompileConstant(foo) && fooHasCertainStaticProperties(foo)) {
>>> >     // fast-path
>>> > }
>>> > // slow path
>>> > ```
>>> > 
>>> > 
>>> >     
>>> >       
>>> >     
>>> > 
>>> >       
>>> >     
>>> > 
>>> >     
>>> >   
>>> > Right?
>>> 
>>> Yes, I think so.
>> 
>> But then whatever is in the fast path and `fooHasCertainStaticProperties` are never profiled because never executed by the interpreter or c1. So `fooHasCertainStaticProperties` will likely not be inlined and c2 will do a poor (or rather not as good as you'd like) job of compiling whatever is in the fast path.
>
>> > > Naive question: the right way to use this would be almost invariably be like this:
>> > > ```
>> > > if (isCompileConstant(foo) && fooHasCertainStaticProperties(foo)) {
>> > >     // fast-path
>> > > }
>> > > // slow path
>> > > ```
>> > > 
>> > > 
>> > >     
>> > >       
>> > >     
>> > > 
>> > >       
>> > >     
>> > > 
>> > >     
>> > >   
>> > > Right?
>> > 
>> > 
>> > Yes, I think so.
>> 
>> But then whatever is in the fast path and `fooHasCertainStaticProperties` are never profiled because never executed by the interpreter or c1. So `fooHasCertainStaticProperties` will likely not be inlined and c2 will do a poor (or rather not as good as you'd like) job of compiling whatever is in the fast path.
> 
> I suppose perhaps it is implied that `fooHasCertainStaticProperties` should have `@ForceInline` ? But yes, there seems to be several assumptions in how this logic is supposed to be used, and at the moment, it seems to me more of a footgun than something actually useful (but I admit my ignorance on the subject).

> @mcimadamore Yes this is hard to use apart from the simple cases. Considering we have already used this technique in the `MethodHandle` implementation, I think there are valid use cases.

I don't 100% buy the `MethodHandleImpl` analogy. In that case the check is not simply used to save a branch, but to spare spinning of a completely new lambda form. That is a very heavy operation. What I'm trying to say is that I'm not too sure how robust of a mechanism this is in the context of micro(nano?)-optimizations (such as the one you are considering).

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PR Comment: https://git.openjdk.org/jdk/pull/17527#issuecomment-1910359911