[Sema] Fix false positive warnings for misaligned member access

Potential misaligned accesses are collected and reported on a per full
expression basis, however some of them are discarded before reporting.
When a nested full expression is completed it will emit diagnostics for
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.

This is fixed by maintaining separate potential candidate lists per
expression evaluation context and processing only the list for the
current context.

Author: vvuksanovic

clang/include/clang/Sema/Sema.h

@@ -3110,9 +3110,6 @@ 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

@@ -14084,7 +14084,6 @@ void Sema::CheckCompletedExpr(Expr *E, SourceLocation CheckLoc,
     CheckUnsequencedOperations(E);
   if (!IsConstexpr && !E->isValueDependent())
     CheckForIntOverflow(E);
-  DiagnoseMisalignedMembers();
 }
 
 void Sema::CheckBitFieldInitialization(SourceLocation InitLoc,
@@ -15529,11 +15528,13 @@ 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) {
+  auto &MisalignedMembersForExpr = currentEvaluationContext().MisalignedMembers;
+  for (MisalignedMember &m : MisalignedMembersForExpr) {
     const NamedDecl *ND = m.RD;
     if (ND->getName().empty()) {
       if (const TypedefNameDecl *TD = m.RD->getTypedefNameForAnonDecl())
@@ -15542,7 +15543,7 @@ void Sema::DiagnoseMisalignedMembers() {
     Diag(m.E->getBeginLoc(), diag::warn_taking_address_of_packed_member)
         << m.MD << ND << m.E->getSourceRange();
   }
-  MisalignedMembers.clear();
+  MisalignedMembersForExpr.clear();
 }
 
 void Sema::DiscardMisalignedMemberAddress(const Type *T, Expr *E) {
@@ -15553,13 +15554,15 @@ 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

@@ -18105,6 +18105,8 @@ 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
+}