RFR: 8344942: Template-Based Testing Framework [v2]

[Emanuel Peter]

> **Goal**
> We want to generate Java source code:
> - Make it easy to generate variants of tests. E.g. for each offset, for each operator, for each type, etc.
> - Enable the generation of domain specific fuzzers (e.g. random expressions and statements).
> 
> Note: with the Template Library draft I was already able to find a [list of bugs](https://bugs.openjdk.org/issues/?jql=labels%20%3D%20template-framework%20ORDER%20BY%20created%20DESC%2C%20summary%20DESC).
> 
> **How to get started**
> When reviewing, please start by looking at:
> https://github.com/openjdk/jdk/blob/d21a8aabaf3b191e851b6997c11bb30fcd0f942f/test/hotspot/jtreg/testlibrary_tests/template_framework/examples/TestSimple.java#L60-L76
> 
> We have a Template with two arguments. They are typed (Integer and String). We then apply the arguments `template.withArgs(42, "7")`, producing a `TemplateWithArgs`. This can then be `render`ed to a String. And then that can be compiled and executed with the CompileFramework.
> 
> And then for a "tutorial", look at:
> `test/hotspot/jtreg/testlibrary_tests/template_framework/examples/TestTutorial.java`
> 
> It shows these features:
> - The `body` of a Template is essentially a list of `Token`s that are concatenated.
> - Templates can be nested: a `TemplateWithArgs` is also a `Token`.
> - We can use `#name` replacements to directly format values into the String. If we had proper String Templates in Java, we would not need this feature.
> - We can use `$var` to make variable names unique: if we applied the same template twice, we would get variable collisions. `$var` is then replaced with e.g. `var_7` in one template use and `var_42` in the other template use.
> - The use of `Hook`s to insert code into outer (earlier) code locations. This is useful, for example, to insert fields on demand.
> - The use of recursive templates, and `fuel` to limit the recursion.
> - `Name`s: useful to register field and variable names in code scopes.
> 
> Next, look at the documentation in. This file is the heart of the Template Framework, and describes all the important features.
> https://github.com/openjdk/jdk/blob/d21a8aabaf3b191e851b6997c11bb30fcd0f942f/test/hotspot/jtreg/compiler/lib/template_framework/Template.java#L31-L76
> 
> For a better experience, you may want to generate the `javadocs`:
> `javadoc -sourcepath test/hotspot/jtreg:./test/lib compiler.lib.template_framework`
> 
> **History**
> @TobiHartmann and I have played with code generators for a while, and have had the dream of doing that in a more principled way. And to hopefully...

Emanuel Peter has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:

 - Merge branch 'master' into JDK-8344942-TemplateFramework-v3
 - fix tests
 - whitespace
 - whitespace
 - fix whitespace
 - JDK-8344942

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/24217/files
  - new: https://git.openjdk.org/jdk/pull/24217/files/a10d6fa0..ededf45b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24217&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24217&range=00-01

  Stats: 38809 lines in 1314 files changed: 5216 ins; 31448 del; 2145 mod
  Patch: https://git.openjdk.org/jdk/pull/24217.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24217/head:pull/24217

PR: https://git.openjdk.org/jdk/pull/24217