Classfile API is missing CodeBuilder hook for various instructions-consuming computation tools

[Brian Goetz]

On 8/19/2022 10:30 AM, Adam Sotona wrote:
>
> These tools are helpers for specific use cases and they do not 
> represent transformations, but rather they collect and provide 
> information necessary for building code.
>
> For example I need to know actual stack when I plan to use 
> stack-to-locals (and back) function in my generated code.
>
> Another example (which we may embed) is to count max stack when the 
> generator is off.
>
> Optional custom logging of the exact instructions dropped to 
> CodeBuilder is another requested use case.
>
> All of the above are expensive operations, so they should be applied 
> on user request only.
>

That's what I thought.  So, since they are only applied when you really 
need them, can't these be treated as "generate the classfile, and then 
traverse it as a classfile with the API we already have?" This seems 
like it would work in all the cases when we *can* generate a classfile, 
which leaves the cases where we would fail to generate a classfile and 
therefore have nothing to analyze, which is probably a non-zero but 
not-large fraction of the cases, right?

> This API does not have much value for transformations, where user can 
> easily listen to each transformed CodeElement and compute whatever is 
> necessary. However when building code using various “magic” (like 
> blocks, constants auto-detection, and other conveniences…) it is 
> important to see what instructions it really builds into.
>

I think you misunderstood my question.  I didn't mean "is this useful in 
transformation".  I meant that, we *already* have composable transforms, 
where the output of one activity is piped into another; there's a lot of 
similarity here (and they'll use the same underlying plumbing.)  I was 
suggesting that you could think of teeing as being a (likely no-op) 
transformation on the building that is already in progress.

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