Daniel Sun created GROOVY-12161:
-----------------------------------
Summary: Omit identity catch-all for try/catch without finally and
slim exception bytecode
Key: GROOVY-12161
URL: https://issues.apache.org/jira/browse/GROOVY-12161
Project: Groovy
Issue Type: Improvement
Reporter: Daniel Sun
h2. Summary
{{StatementWriter.writeTryCatchFinally}} currently emits a catch-all identity
rethrow ({{astore}} / {{aload}} / {{athrow}}) and an empty shared-finally path
for *every* try/catch, including plain try/catch with no finally. That diverges
from javac, bloats the exception table, and adds avoidable control-flow noise
that is less JIT-friendly.
h2. Problem
For a plain try/catch (empty finally), the generator still:
* registers a catch-all handler that only stores and rethrows the throwable
* routes normal fall-through through an empty "finally" block and a second
{{GOTO}} past the catch-all
* always creates a catch-side {{BlockRecorder}} even when there are no catch
clauses
On abrupt exits ({{return}} / {{break}} / {{continue}}),
{{CompileStack.applyBlockRecorder}} also emits a trailing {{NOP}} after inlined
finally/synchronized guards, which is unnecessary once the restarted range can
bind to the next real instruction.
Non-empty finally must keep a real catch-all so uncaught exceptions still run
finally then rethrow. Nested try/catch inside try/finally must keep
{{BlockRecorder}} range splitting so an enclosing finally inlined on {{return}}
is *not* covered by an inner typed handler (regression risk for GROOVY-8229).
Stack-map casts for assignments inside try (GROOVY-9805) continue to depend on
an active {{BlockRecorder}}.
h2. Proposed approach
* Keep a {{BlockRecorder}} on try/catch regions always (empty finally ⇒ no-op
excluded statement) so range splits, nested finally semantics, and GROOVY-9805
casts stay correct.
* When finally is empty: omit the catch-all identity rethrow and the empty
shared-finally block; fall-through jumps straight to a join label after the
handlers.
* When finally is non-empty: keep typed handlers first, then catch-all + shared
fall-through finally (existing semantics).
* Only push a catch-side {{BlockRecorder}} when there is at least one {{catch}}.
* In {{applyBlockRecorder}}, keep a leading {{NOP}} so abrupt-exit-only try
ranges stay non-empty (illegal empty exception ranges), but drop the trailing
{{NOP}}.
h2. Expected bytecode shape (illustrative)
Plain try/catch after change — typed handlers only, no catch-all:
{code:java}
int m(int x) {
try {
return x
} catch (RuntimeException e) {
return -1
}
}
{code}
Exception table should list only {{RuntimeException}} (possibly multiple ranges
after return-path range splits), not a {{null}}/any handler, and must not
contain an adjacent {{ASTORE}}/{{ALOAD}}/{{ATHROW}} identity rethrow.
Try/finally must still have a catch-all any-handler so finally runs for
uncaught exceptions.
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