KarpelesLab / goro / 25321981980

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vm: add bytecode compiler (literals, vars, binops, if/while/for)

core/vm/vmcompiler walks compiled AST trees (phpv.Runnable)
and emits *vm.Function bytecode. Returns ErrUnsupported on
node types it doesn't understand yet, so the caller can fall
back to AST execution per-function.

Supports:
- Scalar literals (int/float/string/bool/null)
- The case-insensitive constants true/false/null
- Variable read (with undefined-variable warning)
- Binary arithmetic / bitwise / shift / compare / concat
- Unary -, ~, !
- Plain assignment $x = expr
- Compound assignment $x += expr (statement context only)
- Pre/post ++/--
- Short-circuit && and || (correct short-circuit semantics)
- if / elseif / else
- while / for / break / continue (single-level)
- return [expr]

Out of scope (returns ErrUnsupported, falls back to AST):
arrays/foreach, objects, references, try/catch, function
calls, generators, multi-level break/continue, type-hinted
return, user-defined constants, array/list destructure.

To expose the unexported AST node fields without breaking
encapsulation, core/compiler/vmaccess.go adds narrow getter
methods (LiteralVal, VariableName, OperatorOp/A/B, IfCond/Yes/No,
ForStart/Cond/Each/Code, WhileCond/Code, ReturnValue, …).
The vmcompiler matches on these getter interfaces, never on
concrete types — same pattern Goro already uses for
Writable/UndefinedChecker.

Differential tests in core/vm/vmcompiler/compile_test.go
compile each snippet through the AST and through the VM
emitter, run both, and assert identical return values.
Covers arithmetic, compare, vars, ++/--, if/else, while, for,
break, continue, short-circuit. ErrUnsupported is verified
against array literals (which fall through correctly).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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