Fix incorrect union member usage

See build failure
https://lab.llvm.org/buildbot/#/builders/5/builds/41428.

Memory sanitizer detects usage of `RawData` union member which is not
filled directly. Instead, the code relies on filling `Data` union
member, which is a struct consisting of signing schema parameters.

According to https://en.cppreference.com/w/cpp/language/union, this is UB:
"It is undefined behavior to read from the member of the union that
wasn't most recently written".

Instead of relying on compiler allowing us to do dirty things, do not
use union and only store `RawData`. Particular ptrauth parameters are
obtained on demand via bit operations.

Author: kovdan01

llvm/include/llvm/IR/DebugInfoMetadata.h

@@ -975,34 +975,29 @@ class DIDerivedType : public DIType {
 public:
   /// Pointer authentication (__ptrauth) metadata.
   struct PtrAuthData {
-    union {
-      struct {
-        unsigned Key : 4;
-        unsigned IsAddressDiscriminated : 1;
-        unsigned ExtraDiscriminator : 16;
-        unsigned IsaPointer : 1;
-        unsigned AuthenticatesNullValues : 1;
-      } Data;
-      unsigned RawData;
-    } Payload;
-
-    PtrAuthData(unsigned FromRawData) { Payload.RawData = FromRawData; }
+    unsigned RawData;
+
+    PtrAuthData(unsigned FromRawData) : RawData(FromRawData) {}
     PtrAuthData(unsigned Key, bool IsDiscr, unsigned Discriminator,
                 bool IsaPointer, bool AuthenticatesNullValues) {
       assert(Key < 16);
       assert(Discriminator <= 0xffff);
-      Payload.Data.Key = Key;
-      Payload.Data.IsAddressDiscriminated = IsDiscr;
-      Payload.Data.ExtraDiscriminator = Discriminator;
-      Payload.Data.IsaPointer = IsaPointer;
-      Payload.Data.AuthenticatesNullValues = AuthenticatesNullValues;
+      RawData = (Key << 0) | (IsDiscr ? (1 << 4) : 0) | (Discriminator << 5) |
+                (IsaPointer ? (1 << 21) : 0) |
+                (AuthenticatesNullValues ? (1 << 22) : 0);
     }
     bool operator==(struct PtrAuthData Other) const {
-      return Payload.RawData == Other.Payload.RawData;
+      return RawData == Other.RawData;
     }
     bool operator!=(struct PtrAuthData Other) const {
       return !(*this == Other);
     }
+
+    unsigned Key() { return (RawData >> 0) & 0b1111; }
+    bool IsAddressDiscriminated() { return (RawData >> 4) & 1; }
+    unsigned ExtraDiscriminator() { return (RawData >> 5) & 0xffff; }
+    bool IsaPointer() { return (RawData >> 21) & 1; }
+    bool AuthenticatesNullValues() { return (RawData >> 22) & 1; }
   };
 
 private:
@@ -1023,7 +1018,7 @@ class DIDerivedType : public DIType {
                AlignInBits, OffsetInBits, Flags, Ops),
         DWARFAddressSpace(DWARFAddressSpace) {
     if (PtrAuthData)
-      SubclassData32 = PtrAuthData->Payload.RawData;
+      SubclassData32 = PtrAuthData->RawData;
   }
   ~DIDerivedType() = default;
   static DIDerivedType *
@@ -1098,31 +1093,31 @@ class DIDerivedType : public DIType {
   /// \returns The PointerAuth key.
   std::optional<unsigned> getPtrAuthKey() const {
     if (auto PtrAuthData = getPtrAuthData())
-      return (unsigned)PtrAuthData->Payload.Data.Key;
+      return PtrAuthData->Key();
     return std::nullopt;
   }
   /// \returns The PointerAuth address discrimination bit.
   std::optional<bool> isPtrAuthAddressDiscriminated() const {
     if (auto PtrAuthData = getPtrAuthData())
-      return (bool)PtrAuthData->Payload.Data.IsAddressDiscriminated;
+      return PtrAuthData->IsAddressDiscriminated();
     return std::nullopt;
   }
   /// \returns The PointerAuth extra discriminator.
   std::optional<unsigned> getPtrAuthExtraDiscriminator() const {
     if (auto PtrAuthData = getPtrAuthData())
-      return (unsigned)PtrAuthData->Payload.Data.ExtraDiscriminator;
+      return PtrAuthData->ExtraDiscriminator();
     return std::nullopt;
   }
   /// \returns The PointerAuth IsaPointer bit.
   std::optional<bool> isPtrAuthIsaPointer() const {
     if (auto PtrAuthData = getPtrAuthData())
-      return (bool)PtrAuthData->Payload.Data.IsaPointer;
+      return PtrAuthData->IsaPointer();
     return std::nullopt;
   }
   /// \returns The PointerAuth authenticates null values bit.
   std::optional<bool> getPtrAuthAuthenticatesNullValues() const {
     if (auto PtrAuthData = getPtrAuthData())
-      return (bool)PtrAuthData->Payload.Data.AuthenticatesNullValues;
+      return PtrAuthData->AuthenticatesNullValues();
     return std::nullopt;
   }
 

llvm/lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -1805,7 +1805,7 @@ void ModuleBitcodeWriter::writeDIDerivedType(const DIDerivedType *N,
   Record.push_back(VE.getMetadataOrNullID(N->getAnnotations().get()));
 
   if (auto PtrAuthData = N->getPtrAuthData())
-    Record.push_back(PtrAuthData->Payload.RawData);
+    Record.push_back(PtrAuthData->RawData);
   else
     Record.push_back(0);