fix(ast lowerer): ARK-357, allow generating a LOAD_FAST_BY_INDEX only if there is no 'if' between the variable and its use

[SuperFola]

Description

This small snippet would use the wrong variable:

(let foo (fun (toggle) {
  (let i 0)
  (if toggle
    (let a "error, I shouldn't be loaded!"))
  (let b 5)
  (print i b) }))

(foo false) # 05
(foo true) # error, I shouldn't be loaded!5

It's because we generate LOAD_FAST_BY_INDEX without too many checks, and in the (print i b), i is loaded using LOAD_FAST_BY_INDEX 0 which is the last variable created in the current scope.

This PR marks variables before conditions that create new variables as unreachable to avoid this problem, so that a safer LOAD_FAST symbol is used instead.

Checklist

• I have read the Contributor guide
• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have updated the documentation if needed (on https://github.com/ArkScript-lang/website, content/docs/)
• I have added tests that prove my fix/feature is working
• New and existing tests pass locally with my changes
• I confirm that I am the author of this code and release it to the ArkScript project under the MPL-2.0 license. This contribution does not contain code from other sources, including code generated by a Large Language Model ("AI").

[github-actions]

Static analysis report

Lizard report

Listing only functions with cyclomatic complexity >= 15 or NLOC >= 100 or parameters >= 6.

Filename	Start line:end line	Function name	Parameters	NLOC	CCN
src/arkreactor/Compiler/Lowerer/ASTLowerer.cpp	355:434	Ark::internal::ASTLowerer::compileListInstruction	3	64	32
src/arkreactor/Compiler/Lowerer/ASTLowerer.cpp	853:999	Ark::internal::ASTLowerer::handleFunctionCall	3	114	30
src/arkreactor/Compiler/Lowerer/ASTLowerer.cpp	212:317	Ark::internal::ASTLowerer::compileExpression	5	88	29
src/arkreactor/Compiler/Lowerer/ASTLowerer.cpp	799:851	Ark::internal::ASTLowerer::handleOperator	3	42	18
src/arkreactor/Compiler/Lowerer/ASTLowerer.cpp	107:129	Ark::internal::ASTLowerer::nodeProducesOutput	1	18	17
src/arkreactor/Compiler/Lowerer/ASTLowerer.cpp	597:645	Ark::internal::ASTLowerer::compileLetMutSet	4	40	17
src/arkreactor/Compiler/Lowerer/ASTLowerer.cpp	514:595	Ark::internal::ASTLowerer::compileFunction	3	59	16

Report about files you didn't modify in this PR

Filename	Start line:end line	Function name	Parameters	NLOC	CCN
src/arkreactor/VM/VM.cpp	450:2153	Ark::VM::unsafeRun	2	1432	281
src/arkreactor/Compiler/Macros/Processor.cpp	259:634	Ark::internal::MacroProcessor::evaluate	3	344	115
src/arkreactor/Compiler/BytecodeReader.cpp	385:596	Ark::BytecodeReader::display	4	171	92
src/arkreactor/Error/Diagnostics.cpp	45:192	Ark::Diagnostics::makeContext	4	109	44
src/arkscript/main.cpp	24:383	main	2	310	43
src/arkscript/JsonCompiler.cpp	27:276	JsonCompiler::_compile	1	214	38
src/arkreactor/Compiler/NameResolution/NameResolutionPass.cpp	162:267	Ark::internal::NameResolutionPass::visitKeyword	3	85	35
src/arkreactor/Builtins/Slice.cpp	21:93	Ark::internal::Builtins::slice	1	63	34
src/arkreactor/Compiler/AST/Parser.cpp	841:932	Ark::internal::Parser::string	1	88	32
src/arkreactor/Compiler/AST/Node.cpp	179:285	Ark::internal::Node::repr	0	90	30
src/arkreactor/Compiler/Macros/Processor.cpp	107:191	Ark::internal::MacroProcessor::processNode	3	63	28
src/arkreactor/Compiler/AST/Node.cpp	287:368	Ark::internal::Node::debugPrint	1	70	26
src/arkreactor/Compiler/BytecodeReader.cpp	612:713	Ark::BytecodeReader::printInstruction	7	98	25
src/arkreactor/TypeChecker.cpp	149:236	Ark::types::generateError	6	75	25
src/arkreactor/TypeChecker.cpp	32:147	Ark::types::displayContract	6	102	24
src/arkreactor/Compiler/AST/Parser.cpp	289:417	Ark::internal::Parser::import_	1	98	23
src/arkreactor/Compiler/NameResolution/NameResolutionPass.cpp	56:160	Ark::internal::NameResolutionPass::visit	2	83	23
include/utf8.hpp	138:184	utf8::isValid	1	44	21
src/arkreactor/VM/Value/Value.cpp	77:145	Ark::Value::toString	2	54	21
src/arkreactor/VM/VM.cpp	2307:2401	Ark::VM::backtrace	3	81	20
src/arkreactor/Compiler/AST/Optimizer.cpp	35:85	Ark::internal::Optimizer::countAndPruneDeadCode	1	42	20
src/arkreactor/Compiler/NameResolution/StaticScope.cpp	70:111	Ark::internal::NamespaceScope::get	3	32	19
src/arkscript/Formatter.cpp	501:560	Formatter::formatCall	2	51	19
src/arkreactor/Builtins/String.cpp	46:112	fmt::formatter::parse	1	59	18
src/arkreactor/Compiler/AST/Optimizer.cpp	87:144	Ark::internal::Optimizer::pruneUnusedGlobalVariables	1	50	18
src/arkscript/Formatter.cpp	186:248	Formatter::format	3	59	18
src/arkreactor/Compiler/Macros/Executors/Function.cpp	16:89	Ark::internal::FunctionExecutor::applyMacro	2	56	17
include/Ark/Compiler/AST/Predicates.hpp	132:156	Ark::internal::IsSymbol::operator ( )	1	24	16
src/arkscript/Formatter.cpp	298:339	Formatter::formatFunction	2	35	16
src/arkreactor/VM/VM.cpp	2165:2222	Ark::VM::throwArityError	4	47	15
src/arkreactor/Compiler/Macros/Executors/Function.cpp	99:156	Ark::internal::FunctionExecutor::unify	5	50	15
src/arkreactor/Compiler/IntermediateRepresentation/IROptimizer.cpp	22:260	Ark::internal::IROptimizer::IROptimizer	1	219	14
src/arkreactor/Error/Diagnostics.cpp	194:211	Ark::Diagnostics::helper	7	17	2

---

CppCheck report

Filename	Line	Type	Description
src/arkreactor/Compiler/Lowerer/ASTLowerer.cpp	620	style	Consider using std::count_if algorithm instead of a raw loop.
src/arkreactor/Compiler/Lowerer/LocalsLocator.cpp	26	style	The scope of the variable 'data' can be reduced.

Report files about files you didn't modify in this PR

Filename	Line	Type	Description
include/Ark/VM/VM.inl	258	style	Variable 'maybe_value_ptr' can be declared as pointer to const
src/arkreactor/Builtins/Bytecode.cpp	23	style	Parameter 'vm' can be declared as pointer to const
src/arkreactor/Builtins/IO.cpp	165	style	Consider using std::transform algorithm instead of a raw loop.
src/arkreactor/Compiler/BytecodeReader.cpp	20	performance	Variable 'm_arg_kinds' is assigned in constructor body. Consider performing initialization in initialization list.
src/arkreactor/Compiler/IntermediateRepresentation/IROptimizer.cpp	249	style	Consider using std::transform algorithm instead of a raw loop.
src/arkreactor/Compiler/IntermediateRepresentation/IROptimizer.cpp	256	style	Consider using std::transform algorithm instead of a raw loop.
src/arkreactor/Compiler/NameResolution/ScopeResolver.cpp	136	style	Consider using std::find_if algorithm instead of a raw loop.
src/arkreactor/VM/VM.cpp	0	information	Too many #ifdef configurations - cppcheck only checks 12 of 13 configurations. Use --force to check all configurations.
src/arkreactor/VM/VM.cpp	281	error	Iterators of different containers 'm_execution_contexts.emplace_back(std::make_unique())' and 'm_execution_contexts.front()' are used together.
include/Ark/VM/Value/Future.hpp	50	style	Unused private function: 'Future::deleteSelfViaVM'
src/arkreactor/VM/Value/Future.cpp	23	performance	Variable 'm_value' is assigned in constructor body. Consider performing initialization in initialization list.

[github-actions]

Super Instructions report

Some Super Instructions are under the usage threshold (16).

Super Instruction	Uses in compiled code
INCREMENT_BY_INDEX	15
INCREMENT	15
STORE_FROM	14
LT_CONST_JUMP_IF_FALSE	8
NEQ_SYM_JUMP_IF_FALSE	4
SET_VAL_TAIL	4
STORE_HEAD	2
MUL_BY	2
GT_SYM_JUMP_IF_FALSE	2
CHECK_TYPE_OF	2
NEQ_CONST_JUMP_IF_TRUE	2
SET_VAL_TAIL_BY_INDEX	1
SET_VAL_HEAD_BY_INDEX	1
MUL_SET_VAL	1
SET_VAL_HEAD	1
STORE_TAIL	1

Super Instructions over the threshold

Super Instruction	Uses in compiled code
CALL_SYMBOL	4437
LOAD_CONST_LOAD_CONST	2554
LOAD_CONST_STORE	2406
LOAD_CONST_SET_VAL	673
CALL_BUILTIN	655
INCREMENT_STORE	521
AT_SYM_SYM	514
GET_FIELD_FROM_SYMBOL_INDEX	487
CALL_BUILTIN_WITHOUT_RETURN_ADDRESS	453
STORE_LIST	363
APPEND_IN_PLACE_SYM	327
CALL_SYMBOL_BY_INDEX	299
LT_LEN_SYM_JUMP_IF_FALSE	297
GET_FIELD_FROM_SYMBOL	289
LT_SYM_JUMP_IF_FALSE	148
EQ_SYM_INDEX_JUMP_IF_TRUE	140
STORE_FROM_INDEX	107
SET_VAL_FROM_INDEX	104
SET_VAL_FROM	94
FUSED_MATH	86
AT_SYM_INDEX_SYM_INDEX	82
STORE_LEN	80
LT_CONST_JUMP_IF_TRUE	75
CALL_CURRENT_PAGE	73
DECREMENT_BY_INDEX	61
EQ_CONST_JUMP_IF_TRUE	50
DECREMENT_STORE	45
GT_CONST_JUMP_IF_TRUE	45
AT_SYM_INDEX_CONST	43
GT_CONST_JUMP_IF_FALSE	39
MUL_BY_INDEX	38
STORE_HEAD_BY_INDEX	35
STORE_TAIL_BY_INDEX	34
CHECK_TYPE_OF_BY_INDEX	27
APPEND_IN_PLACE_SYM_INDEX	22
DECREMENT	18

[codspeed-hq]

Merging this PR will improve performance by 1.74%

⚡ 1 improved benchmark
✅ 17 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	create_closure	4.8 ms	4.7 ms	+1.74%

---

_{Comparing fix/load_fast (18837bd) with dev (64a767c)}

[coveralls]

Coverage Report for CI Build 25389358327

Warning

No base build found for commit 64a767c on dev.
Coverage changes can't be calculated without a base build.
If a base build is processing, this comment will update automatically when it completes.

Coverage: 94.077%

Details

• Patch coverage: 25 of 25 lines across 3 files are fully covered (100%).

Uncovered Changes

No uncovered changes found.

Coverage Regressions

Requires a base build to compare against. How to fix this →

---

Coverage Stats

Relevant Lines:	10519
Covered Lines:	9896
Line Coverage:	94.08%
Coverage Strength:	658669.2 hits per line

---

💛 - Coveralls

[github-actions]

Fuzzing report

/usr/local/bin/afl-whatsup status check tool for afl-fuzz by Michal Zalewski

Summary stats

    Fuzzers alive : 0
   Dead or remote : 1 (included in stats)
   Total run time : 5 minutes, 0 seconds
      Total execs : 37 thousands
 Cumulative speed : 125 execs/sec
    Pending items : 155 faves, 1315 total
 Coverage reached : 8.52%
    Crashes saved : 0
      Hangs saved : 0

Cycles without finds : 0
Time without finds : 0

[+] Captured 47341 tuples (map size 305942, highest value 255, total values 496321994) in '/dev/null'.
[+] A coverage of 47341 edges were achieved out of 305984 existing (15.47%) with 1331 input files.