First pass implementing RULE-0-2-1, unused variables.

Implementation relies on shared code with autosar that was previously declared
in the ql files. This behavior has been moved into UnusedObjects.qll.

The previous code seemed like a very disorganized set of exceptional cases,
which was going to be worse with the new code. I made a minimal attempt to
better organize it by organizing it into modules containing individual filter
passes.

Filtering out `.getAnAccess()` was always the last step. From a performance
perspective, this is a bad order. We start with all variables, and for each
one we join it against some set of rare exceptional cases, like functions with
assembly, that only minimally reduce the set of variables we want to query.
Changed to use a better order, where we begin with the set of variables that
have no accesses, which should be a very small set relatively speaking, and then
perform the odds-and-ends filters on top of that set. Small performance
improvements have been measured across the queries.

Author: MichaelRFairhurst

change_notes/2026-02-12-unused-variable-reorganization.md

@@ -0,0 +1,3 @@
+ - `M0-1-3`, `RULE-2-8` - `UnusedLocalVariable.ql`, `UnusedMemberVariable.ql`, `UnusedGlobalOrNamespaceVariable.ql`, `UnusedObjectDefinition.ql`, `UnusedObjectDefinitionStrict.ql`:
+   - The organization of unused variable analysis has been reorganized to be usable in MISRA C++ rule 0.2.1, with no expected noticeable change in results.
+   - New filtering passes begin by filtering out variables that have an existing access (`.getAnAccess()`), which should improve performance in later filtering passes and overall query performance.

cpp/autosar/src/rules/M0-1-3/UnusedGlobalOrNamespaceVariable.ql

@@ -18,11 +18,6 @@ import cpp
 import codingstandards.cpp.autosar
 import codingstandards.cpp.deadcode.UnusedVariables
 
-from PotentiallyUnusedGlobalOrNamespaceVariable v
-where
-  not isExcluded(v, DeadCodePackage::unusedGlobalOrNamespaceVariableQuery()) and
-  // No variable access
-  not exists(v.getAnAccess()) and
-  // Exclude members whose value is compile time and is potentially used to inintialize a template
-  not maybeACompileTimeTemplateArgument(v)
+from ThirdPassUnused::UnusedGlobalOrNamespaceVariable v
+where not isExcluded(v, DeadCodePackage::unusedGlobalOrNamespaceVariableQuery())
 select v, "Variable '" + v.getQualifiedName() + "' is unused."

cpp/autosar/src/rules/M0-1-3/UnusedLocalVariable.ql

@@ -18,58 +18,10 @@ import cpp
 import codingstandards.cpp.autosar
 import codingstandards.cpp.deadcode.UnusedVariables
 
-// Collect constant values that we should use to exclude otherwise unused constexpr variables.
-//
-// For constexpr variables used as template arguments or in static_asserts, we don't see accesses
-// (just the appropriate literals). We therefore take a conservative approach and do not report
-// constexpr variables whose values are used in such contexts.
-//
-// For performance reasons, these special values should be collected in a single pass.
-predicate excludedConstantValue(string value) {
-  value = any(ClassTemplateInstantiation cti).getTemplateArgument(_).(Expr).getValue()
-  or
-  value = any(StaticAssert sa).getCondition().getAChild*().getValue()
-}
-
-/**
- * Defines the local variables that should be excluded from the unused variable analysis based
- * on their constant value.
- *
- * See `excludedConstantValue` for more details.
- */
-predicate excludeVariableByValue(Variable variable) {
-  variable.isConstexpr() and
-  excludedConstantValue(getConstExprValue(variable))
-}
-
-// TODO: This predicate may be possible to merge with M0-1-4's getUseCount(). These two rules
-// diverged to handle `excludeVariableByValue`, but may be possible to merge.
-int getUseCountConservatively(Variable v) {
-  result =
-    count(VariableAccess access | access = v.getAnAccess()) +
-      count(UserProvidedConstructorFieldInit cfi | cfi.getTarget() = v) +
-      // In case an array type uses a constant in the same scope as the constexpr variable,
-      // consider it as used.
-      countUsesInLocalArraySize(v)
-}
-
-predicate isConservativelyUnused(Variable v) {
-  getUseCountConservatively(v) = 0 and
-  not excludeVariableByValue(v)
-}
-
-from PotentiallyUnusedLocalVariable v
-where
-  not isExcluded(v, DeadCodePackage::unusedLocalVariableQuery()) and
-  // Local variable is never accessed
-  not exists(v.getAnAccess()) and
-  // Sometimes multiple objects representing the same entities are created in
-  // the AST. Check if those are not accessed as well. Refer issue #658
-  not exists(LocalScopeVariable another |
-    another.getDefinitionLocation() = v.getDefinitionLocation() and
-    another.hasName(v.getName()) and
-    exists(another.getAnAccess()) and
-    another != v
-  ) and
-  isConservativelyUnused(v)
+// 10.1, 41.0
+// 10.9, 45.9
+// 11.0, 52.3
+// 13.1, 59.3
+from ThirdPassUnused::UnusedLocalVariable v
+where not isExcluded(v, DeadCodePackage::unusedLocalVariableQuery())
 select v, "Local variable '" + v.getName() + "' in '" + v.getFunction().getName() + "' is not used."

cpp/autosar/src/rules/M0-1-3/UnusedMemberVariable.ql

@@ -19,13 +19,6 @@ import codingstandards.cpp.autosar
 import codingstandards.cpp.FunctionEquivalence
 import codingstandards.cpp.deadcode.UnusedVariables
 
-from PotentiallyUnusedMemberVariable v
-where
-  not isExcluded(v, DeadCodePackage::unusedMemberVariableQuery()) and
-  // No variable access
-  not exists(v.getAnAccess()) and
-  // No explicit initialization in a constructor
-  not exists(UserProvidedConstructorFieldInit cfi | cfi.getTarget() = v) and
-  // Exclude members whose value is compile time and is potentially used to inintialize a template
-  not maybeACompileTimeTemplateArgument(v)
+from ThirdPassUnused::UnusedMemberVariable v
+where not isExcluded(v, DeadCodePackage::unusedMemberVariableQuery())
 select v, "Member variable '" + v.getName() + "' is unused."

cpp/common/src/codingstandards/cpp/deadcode/UnusedObjects.qll

@@ -17,18 +17,15 @@ import codingstandards.cpp.alertreporting.DeduplicateMacroResults
 class UnusedObjectDefinition extends VariableDeclarationEntry {
   UnusedObjectDefinition() {
     (
-      getVariable() instanceof BasePotentiallyUnusedLocalVariable
+      getVariable() instanceof FirstPassUnused::UnusedLocalVariable
       or
-      getVariable() instanceof BasePotentiallyUnusedGlobalOrNamespaceVariable
+      getVariable() instanceof FirstPassUnused::UnusedGlobalOrNamespaceVariable
     ) and
-    not exists(VariableAccess access | access.getTarget() = getVariable()) and
     getVariable().getDefinition() = this
   }
 
   /* Dead objects with these attributes are reported in the "strict" queries. */
-  predicate hasAttrUnused() {
-    getVariable().getAnAttribute().hasName(["unused", "used", "maybe_unused", "cleanup"])
-  }
+  predicate hasAttrUnused() { hasAttrUnused(getVariable()) }
 }
 
 /* Configuration to use the `DedupMacroResults` module to reduce alert noise */