RFR: 8367531: Template Framework: use scopes and tokens instead of misbehaving immediate-return-queries [v29]

[Emanuel Peter]

On Wed, 12 Nov 2025 15:42:55 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> I got some feedback from users of the Template Framework, especially @galderz . And personally, I already was slightly unsatisfied by some of the issues described below, but did not expect it to be as bad as it is.
>> 
>> So I'm sorry, but I think we need to do a significant re-design. It is now still early enough, and only trivial changes are required for the "real" uses of the framework. Only the framework internal tests require significant changes.
>> 
>> Many thanks to @galderz for trying out the framework, and reporting the issues. And thanks to @chhagedorn for spending a few hours in an offline meeting discussing the issue.
>> 
>> **Major issue with Template Framework: lambda vs token order**
>> 
>> The template rendering involves some state, such as keeping track of hashtag replacements, names and fuel cost.
>> Some methods have side-effects (`addDataName`, `let`, ...) and others are simple queries (`sample`, ...).
>> Sadly, the first version of the template framework was not very consistent, and created tokens (deferred evaluation, during token evaluation) for some, and for others it queried the state and returned the result immediately (during lambda execution). One nasty consequence is that an immediately returning query can "float" above a state affecting token. For example, `addDataName` generated a token (so that we know if it is to be added for the template frame or a hook anchoring frame), but answered sampling queries immediately (because that means we can use the returned value immediately and make decisions based on it immediately, which is nice). Looking at the example below, this had the confusing result that `addDataName` only generates a token at first, then `sample` does not have that name available yet, and only later during token evaluation is the name actually added.
>> 
>> var testTemplate = Template.make(() -> body(
>>     ...
>>     addDataName("name", someType, MUTABLE),
>>     let("name", dataNames(MUTABLE).exactOf(someType).sample().name()),
>>     ...
>> ));
>> 
>> 
>> **Two possible solutions: all-in on lambda execution or all-in on tokens**
>> 
>> First, I thought I want to go all-in on lambda execution, and have everything have immediate effect and return results immediately. This would have the nice effect that the user feels like they are directly in control of the execution order. But I did not find a good way without exposing too many internals to the user, or getting rid of the nice "token lists" we currently have inside Templates (the...
>
> Emanuel Peter has updated the pull request incrementally with one additional commit since the last revision:
> 
>   inflate abreviations to full names

I could append to this section:

   159  * Ideally, we would have used <a href="https://openjdk.org/jeps/430">string templates</a> to inject these Template                                                                                           
   160  * arguments into the strings. But since string templates are not (yet) available, the Templates provide
   161  * <strong>hashtag replacements</strong> in the {@link String}s: the Template argument names are captured, and   
   162  * the argument values automatically replace any {@code "#name"} in the {@link String}s. See the different overloads
   163  * of {@link #make} for examples. Additional hashtag replacements can be defined with {@link #let}.


Proposal:

We have decided to keep hashtag replacements constrained to the scope of one Template. They
do not escape to outer or inner Template uses. If one needs to pass values to inner Templates,
this can be done with Template arguments. Keeping hashtag replacements local to Templates
has the benefit that there is no conflict in recursive templates, where outer and inner Templates
define the same hashtag replacement.

test/hotspot/jtreg/compiler/lib/template_framework/Template.java line 857:

> 855:      * for anything that follows it, until the the end of the next outer scope
> 856:      * that is non-transparent for hashtag replacements. Additionally, hashtag
> 857:      * replacements are limited to the template they were defined in.

Suggestion:

     * Note that a {@code let} definition makes the hashtag replacement available
     * for anything that follows it, until the the end of the next outer scope
     * that is non-transparent for hashtag replacements. Additionally, hashtag
     * replacements are limited to the template they were defined in.
     * If you want to pass values from an outer to an inner template, this cannot
     * be done with hashtags directly, instead one has to pass the values via
     * template arguments, and use {@code let} in each template that requires
     * the hashtag replacement.

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

PR Comment: https://git.openjdk.org/jdk/pull/27255#issuecomment-3533195851
PR Review Comment: https://git.openjdk.org/jdk/pull/27255#discussion_r2527814913