RFR: 8357185: Redundant local variables with unconditionally matching primitive patterns

[Chen Liang]

On Thu, 3 Jul 2025 09:13:30 GMT, Jan Lahoda <jlahoda at openjdk.org> wrote:

>> A commit in the implementation of JEP 455, https://github.com/openjdk/jdk/pull/15638/commits/ceee1e4c08457a0793fdfb556db99e057a947af1, added redundant synthetic local variable for trivial `instanceof int`-ish type conversion operations in the javac AST. Such conversions have been present since the introduction of record patterns, and previously they consistently lower to the part before `instanceof`. With this change, the introduced redundant variable is visible in the class file, as seen in the JBS issue.
>> 
>> Testing: langtools/tools/javac
>
> src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Lower.java line 2846:
> 
>> 2844: 
>> 2845:             if (types.isUnconditionallyExact(tree.expr.type, tree.pattern.type)) {
>> 2846:                 var = make.Exec(instanceOfExpr);  // drop redundant variable
> 
> While this is sufficient to solve the issue, it might be good to go through the if cascade more thoroughly, and only introduce the variable if needed. I.e. in case the value is used more than once. If the value is used only once (like in the `else if (tree.expr.type.isPrimitive()) {`, it ought to be possible to use it directly. Unless there are ordering or other concerns, of course.

Re @lahodaj: I don't see how the primitive branch uses the value only once - it computes the value, and use it twice: 1st for ExactConversionSupport, 2nd for the expression result. I don't think we can elide the variable.

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PR Review Comment: https://git.openjdk.org/jdk/pull/26107#discussion_r2182985619