[clang/asan] Do not emit memcpy for trivial operator= when -fsanitize-address-field-padding >= 1

[kcc]

Summary

If we've added poisoned paddings to a type do not emit memcpy for operator=.

[rsmith]

LGTM, assuming the answer to my question is 'no'.

lib/CodeGen/CGExprCXX.cpp

321

Is mayInsertExtraPadding ever true for a union? The non-memcpy path will not do the right thing for a trivial assignment operator in that case.

[kcc]

lib/CodeGen/CGExprCXX.cpp

321

It is never true:

bool RecordDecl::mayInsertExtraPadding(bool EmitRemark) const {
...

else if (CXXRD->isUnion())
  ReasonToReject = 2;  // is a union.

lib/CodeGen/CGExprCXX.cpp

CodeGenFunction::EmitCXXOperatorMemberCallExpr(const CXXOperatorCallExpr *E,
                                                const CXXMethodDecl *MD,
                                                ReturnValueSlot ReturnValue) {
   assert(MD->isInstance() &&
          "Trying to emit a member call expr on a static method!");
   LValue LV = EmitLValue(E->getArg(0));
   llvm::Value *This = LV.getAddress();
 
   if ((MD->isCopyAssignmentOperator() || MD->isMoveAssignmentOperator()) &&
-      MD->isTrivial()) {
+      MD->isTrivial() && !MD->getParent()->mayInsertExtraPadding()) {
     llvm::Value *Src = EmitLValue(E->getArg(1)).getAddress();
     QualType Ty = E->getType();
     EmitAggregateAssign(This, Src, Ty);
     return RValue::get(This);
   }
 
   llvm::Value *Callee = EmitCXXOperatorMemberCallee(E, MD, This);
   return EmitCXXMemberOrOperatorCall(MD, Callee, ReturnValue, This,

test/CodeGen/sanitize-address-field-padding.cpp

 // Make sure the dtor of InheritsFrom_WithVirtualDtor remains in the code,
 // i.e. we ignore -mconstructor-aliases when field paddings are added
 // because the paddings in InheritsFrom_WithVirtualDtor needs to be unpoisoned
 // in the dtor.
 // WITH_CTOR_ALIASES-LABEL: define void @_Z35Create_InheritsFrom_WithVirtualDtor
 // WITH_CTOR_ALIASES-NOT: call void @_ZN15WithVirtualDtorD2Ev
 // WITH_CTOR_ALIASES: call void @_ZN28InheritsFrom_WithVirtualDtorD2Ev
 // WITH_CTOR_ALIASES: ret void
+//
+
+// Make sure we don't emit memcpy for operator= if paddings are inserted.
+struct ClassWithTrivialCopy {
+  ClassWithTrivialCopy();
+  ~ClassWithTrivialCopy();
+  void *a;
+ private:
+  void *c;
+};
+
+void MakeTrivialCopy(ClassWithTrivialCopy *s1, ClassWithTrivialCopy *s2) {
+  *s1 = *s2;
+}
+
+// CHECK-LABEL: define void @_Z15MakeTrivialCopyP20ClassWithTrivialCopyS0_
+// CHECK-NOT: memcpy
+// CHECK: ret void
+