Merge pull request #18502 from jketema/consteval

C++: Support `if consteval` and `if ! consteval`

cpp/ql/lib/change-notes/2024-01-16-consteval-if.md

@@ -0,0 +1,4 @@
+---
+category: feature
+---
+* A new class `ConstevalIfStmt` was introduced, which represents the C++23 `if consteval` and `if ! consteval` statements.

cpp/ql/lib/semmle/code/cpp/PrintAST.qll

@@ -912,6 +912,10 @@ private predicate namedStmtChildPredicates(Locatable s, Element e, string pred)
     or
     s.(ConstexprIfStmt).getElse() = e and pred = "getElse()"
     or
+    s.(ConstevalIfStmt).getThen() = e and pred = "getThen()"
+    or
+    s.(ConstevalIfStmt).getElse() = e and pred = "getElse()"
+    or
     s.(Handler).getParameter() = e and pred = "getParameter()"
     or
     s.(IfStmt).getInitialization() = e and pred = "getInitialization()"

cpp/ql/lib/semmle/code/cpp/controlflow/internal/CFG.qll

@@ -876,6 +876,25 @@ private predicate subEdge(Pos p1, Node n1, Node n2, Pos p2) {
     p2.nodeAfter(n2, s)
   )
   or
+  // NotConstevalIfStmt -> { then, else } ->
+  exists(ConstevalIfStmt s |
+    p1.nodeAt(n1, s) and
+    p2.nodeBefore(n2, s.getThen())
+    or
+    p1.nodeAt(n1, s) and
+    p2.nodeBefore(n2, s.getElse())
+    or
+    p1.nodeAt(n1, s) and
+    not exists(s.getElse()) and
+    p2.nodeAfter(n2, s)
+    or
+    p1.nodeAfter(n1, s.getThen()) and
+    p2.nodeAfter(n2, s)
+    or
+    p1.nodeAfter(n1, s.getElse()) and
+    p2.nodeAfter(n2, s)
+  )
+  or
   // WhileStmt -> condition ; body -> condition ; after dtors -> after
   exists(WhileStmt s |
     p1.nodeAt(n1, s) and

cpp/ql/lib/semmle/code/cpp/ir/implementation/raw/internal/TranslatedCondition.qll

@@ -47,7 +47,7 @@ abstract class TranslatedFlexibleCondition extends TranslatedCondition, Conditio
 {
   TranslatedFlexibleCondition() { this = TTranslatedFlexibleCondition(expr) }
 
-  final override predicate handlesDestructorsExplicitly() { none() } // TODO: this needs to be revisted when we get unnamed destructors
+  final override predicate handlesDestructorsExplicitly() { none() } // TODO: this needs to be revisited when we get unnamed destructors
 
   final override TranslatedElement getChild(int id) { id = 0 and result = this.getOperand() }
 

cpp/ql/lib/semmle/code/cpp/ir/implementation/raw/internal/TranslatedElement.qll

@@ -166,7 +166,7 @@ private predicate ignoreExpr(Expr expr) {
 }
 
 /**
- * Holds if the side effects of `expr` should be ignoredf for the purposes of IR generation.
+ * Holds if the side effects of `expr` should be ignored for the purposes of IR generation.
  *
  * In cases involving `constexpr`, a call can wind up as a constant expression. `ignoreExpr()` will
  * not hold for such a call, since we do need to translate the call (as a constant), but we need to

cpp/ql/lib/semmle/code/cpp/ir/implementation/raw/internal/TranslatedStmt.qll

@@ -1098,6 +1098,61 @@ class TranslatedConstExprIfStmt extends TranslatedIfLikeStmt {
   override predicate hasElse() { exists(stmt.getElse()) }
 }
 
+class TranslatedConstevalIfStmt extends TranslatedStmt {
+  override ConstevalIfStmt stmt;
+
+  override Instruction getFirstInstruction(EdgeKind kind) {
+    if not this.hasEvaluatedBranch()
+    then
+      kind instanceof GotoEdge and
+      result = this.getInstruction(OnlyInstructionTag())
+    else result = this.getEvaluatedBranch().getFirstInstruction(kind)
+  }
+
+  override TranslatedElement getChildInternal(int id) {
+    id = 0 and
+    result = this.getThen()
+    or
+    id = 1 and
+    result = this.getElse()
+  }
+
+  override predicate hasInstruction(Opcode opcode, InstructionTag tag, CppType resultType) {
+    not this.hasEvaluatedBranch() and
+    opcode instanceof Opcode::NoOp and
+    tag = OnlyInstructionTag() and
+    resultType = getVoidType()
+  }
+
+  override Instruction getALastInstructionInternal() {
+    if not this.hasEvaluatedBranch()
+    then result = this.getInstruction(OnlyInstructionTag())
+    else result = this.getEvaluatedBranch().getALastInstruction()
+  }
+
+  override TranslatedElement getLastChild() { result = this.getEvaluatedBranch() }
+
+  override Instruction getInstructionSuccessorInternal(InstructionTag tag, EdgeKind kind) {
+    tag = OnlyInstructionTag() and
+    result = this.getParent().getChildSuccessor(this, kind)
+  }
+
+  override Instruction getChildSuccessorInternal(TranslatedElement child, EdgeKind kind) {
+    (child = this.getThen() or child = this.getElse()) and
+    result = this.getParent().getChildSuccessor(this, kind)
+  }
+
+  TranslatedStmt getEvaluatedBranch() {
+    result = getTranslatedStmt(stmt.getRuntimeEvaluatedBranch())
+  }
+
+  predicate hasEvaluatedBranch() { stmt.hasRuntimeEvaluatedBranch() }
+
+  TranslatedStmt getThen() { result = getTranslatedStmt(stmt.getThen()) }
+
+  TranslatedStmt getElse() { result = getTranslatedStmt(stmt.getElse()) }
+}
+
 abstract class TranslatedLoop extends TranslatedStmt, ConditionContext {
   override Loop stmt;
 

cpp/ql/lib/semmle/code/cpp/stmts/Stmt.qll

@@ -437,6 +437,154 @@ class ConstexprIfStmt extends ConditionalStmt, @stmt_constexpr_if {
   }
 }
 
+/**
+ * A C/C++ '(not) consteval if'. For example, the `if consteval` statement
+ * in the following code:
+ * ```cpp
+ * if consteval {
+ *   ...
+ * }
+ * ```
+ */
+class ConstevalIfStmt extends Stmt, @stmt_consteval_or_not_consteval_if {
+  override string getAPrimaryQlClass() { result = "ConstevalIfStmt" }
+
+  override string toString() {
+    if this.isNot() then result = "if ! consteval ..." else result = "if consteval ..."
+  }
+
+  /**
+   * Holds if this is a 'not consteval if' statement.
+   *
+   * For example, this holds for
+   * ```cpp
+   * if ! consteval { return true; }
+   * ```
+   * but not for
+   * ```cpp
+   * if consteval { return true; }
+   * ```
+   */
+  predicate isNot() { this instanceof @stmt_not_consteval_if }
+
+  /**
+   * Gets the 'then' statement of this '(not) consteval if' statement.
+   *
+   * For example, for
+   * ```cpp
+   * if consteval { return true; }
+   * ```
+   * the result is the `BlockStmt` `{ return true; }`.
+   */
+  Stmt getThen() { consteval_if_then(underlyingElement(this), unresolveElement(result)) }
+
+  /**
+   * Gets the 'else' statement of this '(not) constexpr if' statement, if any.
+   *
+   * For example, for
+   * ```cpp
+   * if consteval { return true; } else { return false; }
+   * ```
+   * the result is the `BlockStmt` `{ return false; }`, and for
+   * ```cpp
+   * if consteval { return true; }
+   * ```
+   * there is no result.
+   */
+  Stmt getElse() { consteval_if_else(underlyingElement(this), unresolveElement(result)) }
+
+  /**
+   * Holds if this '(not) constexpr if' statement has an 'else' statement.
+   *
+   * For example, this holds for
+   * ```cpp
+   * if consteval { return true; } else { return false; }
+   * ```
+   * but not for
+   * ```cpp
+   * if consteval { return true; }
+   * ```
+   */
+  predicate hasElse() { exists(this.getElse()) }
+
+  override predicate mayBeImpure() {
+    this.getThen().mayBeImpure() or
+    this.getElse().mayBeImpure()
+  }
+
+  override predicate mayBeGloballyImpure() {
+    this.getThen().mayBeGloballyImpure() or
+    this.getElse().mayBeGloballyImpure()
+  }
+
+  override MacroInvocation getGeneratingMacro() {
+    this.getThen().getGeneratingMacro() = result and
+    (this.hasElse() implies this.getElse().getGeneratingMacro() = result)
+  }
+
+  /**
+   * Gets the statement of this '(not) consteval if' statement evaluated during compile time, if any.
+   *
+   * For example, for
+   * ```cpp
+   * if ! consteval { return true; } else { return false; }
+   * ```
+   * the result is the `BlockStmt` `{ return false; }`, and for
+   * ```cpp
+   * if ! consteval { return true; }
+   * ```
+   * there is no result.
+   */
+  Stmt getCompileTimeEvaluatedBranch() {
+    if this.isNot() then result = this.getElse() else result = this.getThen()
+  }
+
+  /**
+   * Holds if this '(not) constexpr if' statement has a compile time evaluated statement.
+   *
+   * For example, this holds for
+   * ```cpp
+   * if ! consteval { return true; } else { return false; }
+   * ```
+   * but not for
+   * ```cpp
+   * if ! consteval { return true; }
+   * ```
+   */
+  predicate hasCompileTimeEvaluatedBranch() { exists(this.getCompileTimeEvaluatedBranch()) }
+
+  /**
+   * Gets the statement of this '(not) consteval if' statement evaluated during runtime, if any.
+   *
+   * For example, for
+   * ```cpp
+   * if consteval { return true; } else { return false; }
+   * ```
+   * the result is the `BlockStmt` `{ return false; }`, and for
+   * ```cpp
+   * if consteval { return true; }
+   * ```
+   * there is no result.
+   */
+  Stmt getRuntimeEvaluatedBranch() {
+    if this.isNot() then result = this.getThen() else result = this.getElse()
+  }
+
+  /**
+   * Holds if this '(not) constexpr if' statement has a runtime evaluated statement.
+   *
+   * For example, this holds for
+   * ```cpp
+   * if consteval { return true; } else { return false; }
+   * ```
+   * but not for
+   * ```cpp
+   * if consteval { return true; }
+   * ```
+   */
+  predicate hasRuntimeEvaluatedBranch() { exists(this.getRuntimeEvaluatedBranch()) }
+}
+
 private class TLoop = @stmt_while or @stmt_end_test_while or @stmt_range_based_for or @stmt_for;
 
 /**

cpp/ql/lib/semmlecode.cpp.dbscheme

@@ -2152,6 +2152,8 @@ case @stmt.kind of
 // ...  34 @stmt_finally_end deprecated
 |  35 = @stmt_constexpr_if
 |  37 = @stmt_co_return
+|  38 = @stmt_consteval_if
+|  39 = @stmt_not_consteval_if
 ;
 
 type_vla(
@@ -2194,6 +2196,18 @@ constexpr_if_else(
     int else_id: @stmt ref
 );
 
+@stmt_consteval_or_not_consteval_if = @stmt_consteval_if | @stmt_not_consteval_if;
+
+consteval_if_then(
+    unique int constexpr_if_stmt: @stmt_consteval_or_not_consteval_if ref,
+    int then_id: @stmt ref
+);
+
+consteval_if_else(
+    unique int constexpr_if_stmt: @stmt_consteval_or_not_consteval_if ref,
+    int else_id: @stmt ref
+);
+
 while_body(
     unique int while_stmt: @stmt_while ref,
     int body_id: @stmt ref

cpp/ql/lib/upgrades/a01d8f91b8d49259e509b574962dec90719f69a6/upgrade.properties

@@ -0,0 +1,2 @@
+description: Support (not) consteval if
+compatibility: partial