Daniel Sun created GROOVY-12162:
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Summary: Harden and extend bytecode peephole optimization
Key: GROOVY-12162
URL: https://issues.apache.org/jira/browse/GROOVY-12162
Project: Groovy
Issue Type: Improvement
Reporter: Daniel Sun
h2. Overview
Follow-up to GROOVY-12065. The initial peephole layer
({{{}PeepholeOptimizingMethodVisitor{}}}) only wrapped selected
{{visitMethod}} call sites. After {{OperandStack}} started
emitting constants via {{visitLdcInsn}} and relying on the
peephole visitor for compaction, any method that was *not* wrapped
regressed to non-compact forms. Class-generation error diagnostics
that depended on {{TraceMethodVisitor}} were also broken once
the peephole wrapper sat in front of the trace visitor.
This improvement applies peephole compaction uniformly, restores
diagnostics, and adds several safe local-window rewrites that
further shrink generated bytecode without changing runtime
semantics.
h2. Problems Addressed
* *Inconsistent application:* MOP bridges, call-site helpers,
{{{}class${}}}, and other synthetic methods often bypassed the
peephole wrapper while user methods did not.
* *Broken classgen diagnostics:*
{{mv instanceof TraceMethodVisitor}} no longer matched after
wrapping, so "Last known generated bytecode" was never printed
when {{visitMaxs}} failed under classgen logging.
* *Incomplete dead-code patterns:*
{{{}ALOAD{}}}/reference {{{}LDC{}}}; {{{}CHECKCAST{}}}; {{POP}}
only dropped the cast (the load remained); bare
{{{}DUP{}}}/{{{}DUP2{}}}; matching {{{}POP{}}}/{{{}POP2{}}} was left
untouched; {{{}ACONST_NULL{}}}; {{IF_ACMP*}} was not rewritten
to {{{}IFNULL{}}}/{{{}IFNONNULL{}}}.
* *Side-effect caution:* dropping a standalone {{CHECKCAST}}
immediately before void {{RETURN}} could discard a type-check
side effect for a value already committed to the stack.
h2. Changes
h3. Uniform application
* Introduce {{{}PeepholeOptimizingClassVisitor{}}}, installed once
from {{{}WriterController{}}}, so every method body (including
synthetic helpers) is routed through
{{{}PeepholeOptimizingMethodVisitor{}}}.
* Remove ad-hoc {{new PeepholeOptimizingMethodVisitor(...)}}
wraps from {{AsmClassGenerator}} and {{{}CallSiteWriter{}}}.
h3. Diagnostics
* Add {{PeepholeOptimizingMethodVisitor.printTraceBytecode}} to
walk the visitor chain and print a nested
{{TraceMethodVisitor}} when present.
h3. Additional rewrites (still single-pass / stack-local)
* Attach {{CHECKCAST}} to a pending {{ALOAD}} or reference
constant so load + cast + {{POP}} collapses completely when
the value is unused; keep the cast when the value is used.
* Rewrite {{{}ACONST_NULL{}}}; {{{}IF_ACMPEQ{}}}/{{{}IF_ACMPNE{}}} to
{{{}IFNULL{}}}/{{{}IFNONNULL{}}}.
* Eliminate bare {{{}DUP{}}}/{{{}DUP2{}}} followed by a matching pop
(with or without an intervening store → store-only form).
* On void {{{}RETURN{}}}, drop only pending variable/constant loads
(preserving {{{}IINC{}}}); flush a standalone {{CHECKCAST}}
instead of discarding it.
* Buffer replacement flushes prior pending state instead of
silently clearing it; integer constant emission reuses
{{BytecodeHelper.pushConstant}} on the delegate.
h2. Safety
The visitor remains a single candidate window: pending state is
flushed at labels, frames, line numbers, invokes, and other
non-local boundaries. Signed zero constants continue to use raw-bit
comparisons (GROOVY-9797).
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