RFR: 7903586: Revisit jextract constant classes

[Maurizio Cimadamore]

On Wed, 29 Nov 2023 10:58:56 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

> This PR removes the use of jextract constant classes, and falls back to a simpler code generation strategies which move the various constants closer to where they belong:
> 
> * for structs and callbacks, the constants are added to the class/interface being generated;
> * for the header class, we use a local holder class per function to hold the method handle and function descriptor constants.
> 
> This scheme results in much less generated code (as no constant class is emitted), but with the same performance and laziness as the old scheme.
> 
> When working with the code, I've decided to simplify the handling of constant generation, in the spirit of what was recently done with the introduction of string templates. That is, instead of using a magic `Constant` class, which has methods to generate getters/setters for a given constant, I converted much of the code into "static" string templates, so that it's easier to see which code gets generated when staring at the jextract code. As a result, the `Constant` class has been removed.

src/main/java/org/openjdk/jextract/impl/ClassSourceBuilder.java line 167:

> 165:             \{!header.isEmpty() ? STR." * \{header}\n" : ""}\
> 166:              * {@snippet lang=c :
> 167:             \{CDeclarationPrinter.declaration(decl, " * ")}

This fix is unrelated - but I noticed that all comment lines had some skewed indentation - probably a result of the fact that now the whole comment is added in a `appendLines` call with a single string template.

src/main/java/org/openjdk/jextract/impl/ClassSourceBuilder.java line 183:

> 181:     }
> 182: 
> 183:     public String mangleName(String javaName, Class<?> type) {

most of the methods added here come from the old `Constant` class

src/main/java/org/openjdk/jextract/impl/ClassSourceBuilder.java line 209:

> 207:     }
> 208: 
> 209:     private void layoutString(int textBoxIndent, StringBuilder builder, MemoryLayout l) {

Note: the logic is a bit convoluted because we have to add some indentation to the resulting string, as it can span multiple lines. And so that it is correctly aligned inside the text box in which it appears into.

src/main/java/org/openjdk/jextract/impl/HeaderFileBuilder.java line 234:

> 232:     }
> 233: 
> 234:     public String emitGlobalSegment(MemoryLayout layout, String javaName, String nativeName, Declaration declaration) {

This is a recurrent theme: sometimes the "emit" method will return the name of a relevant variable that has been generated, so that other code can refer to it.

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PR Review Comment: https://git.openjdk.org/jextract/pull/145#discussion_r1409121796
PR Review Comment: https://git.openjdk.org/jextract/pull/145#discussion_r1409117509
PR Review Comment: https://git.openjdk.org/jextract/pull/145#discussion_r1409118953
PR Review Comment: https://git.openjdk.org/jextract/pull/145#discussion_r1409124973