[FEA] Add Java bindings for multi-output AST JIT

[thirtiseven]

Description

This PR exposes the libcudf AST JIT capabilities introduced by #23615 and #23621 through the Java API.

The execution backend is selected when an expression is compiled:

• AstExpression.compile() preserves the existing behavior and produces an expression compatible with default AST consumers.
• AstExpression.compileJit() produces a JIT-specific expression. Each CompiledExpression owns only the native AST tree and literal representation required by its compilation mode.
• CompiledExpression.computeColumn(Table) executes using the compiled expression's mode, avoiding a separate computeColumnJit API and avoiding duplicate native AST trees.
• CompiledExpression.computeTableJit(Table, CompiledExpression...) evaluates multiple JIT-compiled expressions in one compute_table_jit call. Output order follows the supplied expression order, and libcudf Row IR can eliminate structurally equivalent subexpressions across outputs.

For JIT-compiled expressions, literals are converted to one-row columns once during compilation and retained for repeated evaluation. Temporary scalar owners are released after construction completes. Default-compiled expressions retain the existing scalar-backed representation and do not incur the JIT literal-column allocation cost.

The binding also:

• rejects compilation-mode mismatches, including passing default-compiled expressions to computeTableJit;
• prevents JIT-compiled expressions from being passed to default-only consumers such as join and scan predicates;
• validates null and closed inputs;
• preserves the lifetime of input tables and compiled expressions across JNI calls.

This API is consumed by NVIDIA/cudf-spark#15312 to execute compatible Project expressions as multi-output AST JIT waves.

Validation

• mvn -B -Dtest=CompiledExpressionTest surefire:test: 166 tests, 0 failures, 0 errors, 0 skipped.
• Relevant cuDF pre-commit checks pass.
• The downstream cudf-spark consumer builds successfully, and its targeted AST JIT and Project retry suites pass.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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📥 Commits

Reviewing files that changed from the base of the PR and between 40c1844 and e37a829.

📒 Files selected for processing (7)

• java/src/main/java/ai/rapids/cudf/ast/AstExpression.java
• java/src/main/java/ai/rapids/cudf/ast/CompiledExpression.java
• java/src/main/java/ai/rapids/cudf/ast/JitOperation.java
• java/src/main/java/ai/rapids/cudf/ast/Literal.java
• java/src/main/native/src/CompiledExpression.cpp
• java/src/main/native/src/jni_compiled_expr.hpp
• java/src/test/java/ai/rapids/cudf/ast/CompiledExpressionTest.java

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📝 Walkthrough

Summary by CodeRabbit

• New Features

  • Added explicit JIT compilation for AST expressions.
  • Added support for evaluating multiple JIT expressions together as a table.
  • Added validation to prevent incompatible execution paths and inputs.

• Bug Fixes

  • Improved resource safety during JIT evaluation.
  • Preserved input data when validation or computation fails.

• Documentation

  • Updated guidance for using JIT compilation with JIT-specific operations and decimal literals.

• Tests

  • Expanded coverage for JIT expressions, multiple outputs, literals, nullability, casting, overflow, and validation.

Walkthrough

The AST API now supports explicit default and JIT compilation modes. JIT expressions use mode-specific native execution and support multi-output table computation. Tests cover validation, literals, nullability, overflow, casts, and decimal behavior.

Changes

JIT AST execution

Layer / File(s)	Summary
Java compilation and evaluation APIs java/src/main/java/ai/rapids/cudf/ast/{AstExpression,CompiledExpression,JitOperation,Literal}.java	Adds compileJit(), stores compilation mode, adds computeTableJit, restricts incompatible native-handle access, and updates JIT documentation.
Native compilation modes java/src/main/native/src/{jni_compiled_expr.hpp,CompiledExpression.cpp}	Stores compilation mode, creates JIT literal columns, validates JIT operations, releases staging scalars, and exposes separate compilation entry points.
Multi-output JIT execution java/src/main/native/src/CompiledExpression.cpp	Replaces single-expression JIT computation with validated multi-output table evaluation.
JIT API validation and regression coverage java/src/test/java/ai/rapids/cudf/ast/CompiledExpressionTest.java	Covers mode validation, handle restrictions, repeated inputs, literals, multi-output results, failures, overflow, casts, and decimal behavior.

Estimated code review effort: 4 (Complex) | ~45 minutes

Merge Risk: ⚪ Minimal · up to e37a8

The PR adds Java bindings for multi-output AST JIT with reported tests and downstream validation passing; no actionable merge-blocking risk remains.

Suggested reviewers: nartal1

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 21.31% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 61 functions across 7 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description clearly explains the Java bindings for multi-output AST JIT, compilation modes, validation, testing, and downstream use.
Title check	✅ Passed	The title clearly summarizes the primary change: adding Java bindings for multi-output AST JIT.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

• Fix all pre-merge checks with AI

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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