[Sema] Fix false positive warnings for misaligned member access (#150025)

These warnings are reported on a per expression basis, however some
potential misaligned accesses are discarded before that happens. The
problem is when a new expression starts while processing another
expression. The new expression will end first and emit all potential
misaligned accesses collected up to that point. That includes candidates
that were found in the parent expression, even though they might have
gotten discarded later.

Fixed by checking if the candidate is located withing the currently
processed expression.

Fixes #144729

Author: vvuksanovic

clang/docs/ReleaseNotes.rst

@@ -168,6 +168,10 @@ Improvements to Clang's diagnostics
   "signedness of format specifier 'u' is incompatible with 'c' [-Wformat-signedness]"
   and the API-visible diagnostic id will be appropriate.
 
+- Fixed false positives in ``-Waddress-of-packed-member`` diagnostics when
+  potential misaligned members get processed before they can get discarded.
+  (#GH144729)
+
 Improvements to Clang's time-trace
 ----------------------------------
 

clang/include/clang/Sema/Sema.h

@@ -2659,9 +2659,9 @@ class Sema final : public SemaBase {
   /// identifies the magic value.
   typedef std::pair<const IdentifierInfo *, uint64_t> TypeTagMagicValue;
 
-  /// Diagnoses the current set of gathered accesses. This typically
-  /// happens at full expression level. The set is cleared after emitting the
-  /// diagnostics.
+  /// Diagnoses the current set of gathered accesses. This happens at the end of
+  /// each expression evaluation context. Diagnostics are emitted only for
+  /// accesses gathered in the current evaluation context.
   void DiagnoseMisalignedMembers();
 
   /// This function checks if the expression is in the sef of potentially
@@ -3117,9 +3117,6 @@ class Sema final : public SemaBase {
 
     bool operator==(const MisalignedMember &m) { return this->E == m.E; }
   };
-  /// Small set of gathered accesses to potentially misaligned members
-  /// due to the packed attribute.
-  SmallVector<MisalignedMember, 4> MisalignedMembers;
 
   /// Adds an expression to the set of gathered misaligned members.
   void AddPotentialMisalignedMembers(Expr *E, RecordDecl *RD, ValueDecl *MD,
@@ -6765,6 +6762,10 @@ class Sema final : public SemaBase {
     /// InLifetimeExtendingContext is true.
     SmallVector<MaterializeTemporaryExpr *, 8> ForRangeLifetimeExtendTemps;
 
+    /// Small set of gathered accesses to potentially misaligned members
+    /// due to the packed attribute.
+    SmallVector<MisalignedMember, 4> MisalignedMembers;
+
     /// \brief Describes whether we are in an expression constext which we have
     /// to handle differently.
     enum ExpressionKind {

clang/lib/Sema/SemaChecking.cpp

@@ -14125,7 +14125,6 @@ void Sema::CheckCompletedExpr(Expr *E, SourceLocation CheckLoc,
     CheckUnsequencedOperations(E);
   if (!IsConstexpr && !E->isValueDependent())
     CheckForIntOverflow(E);
-  DiagnoseMisalignedMembers();
 }
 
 void Sema::CheckBitFieldInitialization(SourceLocation InitLoc,
@@ -15570,11 +15569,12 @@ void Sema::CheckArgumentWithTypeTag(const ArgumentWithTypeTagAttr *Attr,
 
 void Sema::AddPotentialMisalignedMembers(Expr *E, RecordDecl *RD, ValueDecl *MD,
                                          CharUnits Alignment) {
-  MisalignedMembers.emplace_back(E, RD, MD, Alignment);
+  currentEvaluationContext().MisalignedMembers.emplace_back(E, RD, MD,
+                                                            Alignment);
 }
 
 void Sema::DiagnoseMisalignedMembers() {
-  for (MisalignedMember &m : MisalignedMembers) {
+  for (MisalignedMember &m : currentEvaluationContext().MisalignedMembers) {
     const NamedDecl *ND = m.RD;
     if (ND->getName().empty()) {
       if (const TypedefNameDecl *TD = m.RD->getTypedefNameForAnonDecl())
@@ -15583,7 +15583,7 @@ void Sema::DiagnoseMisalignedMembers() {
     Diag(m.E->getBeginLoc(), diag::warn_taking_address_of_packed_member)
         << m.MD << ND << m.E->getSourceRange();
   }
-  MisalignedMembers.clear();
+  currentEvaluationContext().MisalignedMembers.clear();
 }
 
 void Sema::DiscardMisalignedMemberAddress(const Type *T, Expr *E) {
@@ -15594,13 +15594,15 @@ void Sema::DiscardMisalignedMemberAddress(const Type *T, Expr *E) {
       cast<UnaryOperator>(E)->getOpcode() == UO_AddrOf) {
     auto *Op = cast<UnaryOperator>(E)->getSubExpr()->IgnoreParens();
     if (isa<MemberExpr>(Op)) {
-      auto *MA = llvm::find(MisalignedMembers, MisalignedMember(Op));
-      if (MA != MisalignedMembers.end() &&
+      auto &MisalignedMembersForExpr =
+          currentEvaluationContext().MisalignedMembers;
+      auto *MA = llvm::find(MisalignedMembersForExpr, MisalignedMember(Op));
+      if (MA != MisalignedMembersForExpr.end() &&
           (T->isDependentType() || T->isIntegerType() ||
            (T->isPointerType() && (T->getPointeeType()->isIncompleteType() ||
                                    Context.getTypeAlignInChars(
                                        T->getPointeeType()) <= MA->Alignment))))
-        MisalignedMembers.erase(MA);
+        MisalignedMembersForExpr.erase(MA);
     }
   }
 }

clang/lib/Sema/SemaExpr.cpp

@@ -18121,6 +18121,8 @@ void Sema::PopExpressionEvaluationContext() {
     MaybeODRUseExprs.insert_range(Rec.SavedMaybeODRUseExprs);
   }
 
+  DiagnoseMisalignedMembers();
+
   // Pop the current expression evaluation context off the stack.
   ExprEvalContexts.pop_back();
 }

clang/test/Sema/address-packed.c

@@ -338,3 +338,11 @@ struct Invalid0 {
 void *g14(struct Invalid0 *ivl) {
   return &(ivl->x);
 }
+
+void to_void_with_expr(void *ptr, int expr);
+
+void g15(void) {
+  struct Arguable arguable;
+  to_void_with_expr(&arguable.x, 3); // no-warning
+  to_void_with_expr(&arguable.x, ({3;})); // no-warning
+}