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https://issues.apache.org/jira/browse/GROOVY-12065?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Eric Milles reassigned GROOVY-12065:
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Assignee: Daniel Sun
> Implement peephole optimization for bytecode generation
> -------------------------------------------------------
>
> Key: GROOVY-12065
> URL: https://issues.apache.org/jira/browse/GROOVY-12065
> Project: Groovy
> Issue Type: Improvement
> Reporter: Daniel Sun
> Assignee: Daniel Sun
> Priority: Major
>
> h2. Overview
> This improvement introduces a dedicated, single-pass bytecode compaction
> layer via {{{}PeepholeOptimizingMethodVisitor{}}}. This adapter wraps the
> underlying ASM {{MethodVisitor}} during class generation
> ({{{}AsmClassGenerator{}}}), intercepting instruction streams within local
> basic blocks to eliminate redundant operations, rewrite conditional branches,
> and narrow constant instructions to their most optimal forms.
> Crucially, by decoupling peephole optimization from structural code
> generation, this approach significantly simplifies the constant emission
> logic inside {{{}OperandStack{}}}. Instead of maintaining verbose, duplicate
> routing logic for specialized primitive opcodes (such as {{{}ICONST_x{}}},
> {{{}BIPUSH{}}}, or {{{}FCONST_x{}}}), {{OperandStack}} now delegates constant
> pushing uniformly via {{{}visitLdcInsn{}}}. The stateful peephole layer then
> transparently condenses these instructions into their tightest bytecode
> representations under the hood.
> h2. Key Optimizations to Implement
> h3. 1. Redundant Instruction & Dead Code Elimination
> * *Discarded Assignment Values:* Eliminates wasteful {{DUP}} -> {{[X]STORE}}
> -> {{POP}} patterns typically produced during assignments where the
> expression result on the operand stack is unused. The optimizer flattens
> these directly into a single {{{}[X]STORE{}}}.
> * *Redundant Loads:* Detects situations where a local variable or constant
> is loaded ({{{}[X]LOAD{}}} / {{{}LDC{}}}) but immediately discarded via
> {{{}POP{}}}/{{{}POP2{}}}, or followed immediately by a void {{{}RETURN{}}}.
> The optimizer safely drops both operations while preserving interleaved local
> increments ({{{}IINC{}}}).
> h3. 2. Conditional Jump & Zero-Comparison Rewriting
> * Optimizes integer comparisons against zero by transforming expensive
> binary comparison sequences (e.g., {{ILOAD}} -> {{ICONST_0}} ->
> {{{}IF_ICMPxx{}}}) into compact unary zero-comparison instructions
> ({{{}IFxx{}}}). For instance, an {{IF_ICMPEQ}} branch against a buffered
> {{0}} constant is cleanly rewritten directly to {{{}IFEQ{}}}.
> h3. 3. Instruction Narrowing & Constant Compaction
> * Intercepts standard literal definitions and transparently narrows them
> down to the smallest possible specialized opcodes ({{{}ICONST_M1{}}} through
> {{{}ICONST_5{}}}, {{{}BIPUSH{}}}, {{{}SIPUSH{}}}, {{{}LCONST_0/1{}}},
> {{{}FCONST_0/1/2{}}}, and {{{}DCONST_0/1{}}}).
> * Explicitly safeguards signed floating-point zeros ({{{}-0.0f{}}} and
> {{{}-0.0d{}}}) from accidental flattening to guarantee strict IEEE 754
> runtime compliance.
> h3. 4. Big Number Literal Lowering
> * Centralizes string-constructed object instantiations for {{BigDecimal}}
> and {{BigInteger}} literals by rewriting buffered constants into inline
> {{NEW}} -> {{DUP}} -> {{LDC [string]}} -> {{INVOKESPECIAL <init>}} pipelines
> seamlessly before stack emission.
> h2. Implementation Strategy & Safety Guardrails
> The {{PeepholeOptimizingMethodVisitor}} operates on a lightweight,
> stack-local sliding window. To fully preserve runtime semantics, debugging
> capabilities, and catch-block structures, the lookahead window is
> automatically flushed to the delegate visitor when encountering non-local
> boundaries or frame-altering instructions, including:
> * Control flow jumps and basic block {{Label}} targets.
> * Stack map frames ({{{}visitFrame{}}}).
> * Line numbers and local variable debug maps.
> * Method invocations and {{InvokeDynamic}} instructions.
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