diff --git a/lld/MachO/Symbols.h b/lld/MachO/Symbols.h
--- a/lld/MachO/Symbols.h
+++ b/lld/MachO/Symbols.h
@@ -92,14 +92,14 @@
 
 protected:
   Symbol(Kind k, StringRefZ name, InputFile *file)
-      : symbolKind(k), nameData(name.data), nameSize(name.size), file(file),
+      : symbolKind(k), nameData(name.data), file(file), nameSize(name.size),
         isUsedInRegularObj(!file || isa<ObjFile>(file)),
         used(!config->deadStrip) {}
 
   Kind symbolKind;
   const char *nameData;
-  mutable uint32_t nameSize;
   InputFile *file;
+  mutable uint32_t nameSize;
 
 public:
   // True if this symbol was referenced by a regular (non-bitcode) object.
@@ -109,6 +109,9 @@
   bool used : 1;
 };
 
+static_assert(sizeof(void *) != 8 || sizeof(Symbol) == 48,
+              "Try to minimize Symbol's size; we create many instances");
+
 class Defined : public Symbol {
 public:
   Defined(StringRefZ name, InputFile *file, InputSection *isec, uint64_t value,
@@ -133,14 +136,8 @@
 
   static bool classof(const Symbol *s) { return s->kind() == DefinedKind; }
 
-  InputSection *isec;
-  // Contains the offset from the containing subsection. Note that this is
-  // different from nlist::n_value, which is the absolute address of the symbol.
-  uint64_t value;
-  // size is only calculated for regular (non-bitcode) symbols.
-  uint64_t size;
-  ConcatInputSection *compactUnwind = nullptr;
-
+  // Place the bitfields first so that they can get placed in the tail padding
+  // of the parent class, on platforms which support it.
   bool overridesWeakDef : 1;
   // Whether this symbol should appear in the output binary's export trie.
   bool privateExtern : 1;
@@ -166,8 +163,24 @@
 private:
   const bool weakDef : 1;
   const bool external : 1;
+
+public:
+  InputSection *isec;
+  // Contains the offset from the containing subsection. Note that this is
+  // different from nlist::n_value, which is the absolute address of the symbol.
+  uint64_t value;
+  // size is only calculated for regular (non-bitcode) symbols.
+  uint64_t size;
+  ConcatInputSection *compactUnwind = nullptr;
 };
 
+// The Microsoft ABI doesn't support using parent class tail padding for child
+// members, hence the _MSC_VER check.
+#if !defined(_MSC_VER)
+static_assert(sizeof(void *) != 8 || sizeof(Defined) == 80,
+              "Try to minimize Defined's size; we create many instances");
+#endif
+
 // This enum does double-duty: as a symbol property, it indicates whether & how
 // a dylib symbol is referenced. As a DylibFile property, it indicates the kind
 // of referenced symbols contained within the file. If there are both weak
@@ -294,6 +307,9 @@
   alignas(LazySymbol) char e[sizeof(LazySymbol)];
 };
 
+static_assert(sizeof(SymbolUnion) == sizeof(Defined),
+              "Defined should be the largest Symbol kind");
+
 template <typename T, typename... ArgT>
 T *replaceSymbol(Symbol *s, ArgT &&...arg) {
   static_assert(sizeof(T) <= sizeof(SymbolUnion), "SymbolUnion too small");
diff --git a/lld/MachO/Symbols.cpp b/lld/MachO/Symbols.cpp
--- a/lld/MachO/Symbols.cpp
+++ b/lld/MachO/Symbols.cpp
@@ -36,12 +36,11 @@
                  bool isPrivateExtern, bool isThumb,
                  bool isReferencedDynamically, bool noDeadStrip,
                  bool canOverrideWeakDef, bool isWeakDefCanBeHidden)
-    : Symbol(DefinedKind, name, file), isec(isec), value(value), size(size),
-      overridesWeakDef(canOverrideWeakDef), privateExtern(isPrivateExtern),
-      includeInSymtab(true), thumb(isThumb),
+    : Symbol(DefinedKind, name, file), overridesWeakDef(canOverrideWeakDef),
+      privateExtern(isPrivateExtern), includeInSymtab(true), thumb(isThumb),
       referencedDynamically(isReferencedDynamically), noDeadStrip(noDeadStrip),
       weakDefCanBeHidden(isWeakDefCanBeHidden), weakDef(isWeakDef),
-      external(isExternal) {
+      external(isExternal), isec(isec), value(value), size(size) {
   if (isec) {
     isec->symbols.push_back(this);
     // Maintain sorted order.