[clang][bytecode] Fix incorrect offset in elem()

We need to use the base offset in both cases.
Also, add additional assertions to make sure we don't miss this case
again.

Fixes #155132

Author: tbaederr

clang/lib/AST/ByteCode/EvaluationResult.cpp

@@ -178,8 +178,8 @@ bool EvaluationResult::checkFullyInitialized(InterpState &S,
 static void collectBlocks(const Pointer &Ptr,
                           llvm::SetVector<const Block *> &Blocks) {
   auto isUsefulPtr = [](const Pointer &P) -> bool {
-    return P.isLive() && !P.isZero() && !P.isDummy() && P.isDereferencable() &&
-           !P.isUnknownSizeArray() && !P.isOnePastEnd();
+    return P.isLive() && P.isBlockPointer() && !P.isZero() && !P.isDummy() &&
+           P.isDereferencable() && !P.isUnknownSizeArray() && !P.isOnePastEnd();
   };
 
   if (!isUsefulPtr(Ptr))

clang/lib/AST/ByteCode/Pointer.h

@@ -696,15 +696,20 @@ class Pointer {
     assert(asBlockPointer().Pointee);
     assert(isDereferencable());
     assert(getFieldDesc()->isPrimitiveArray());
+    assert(I < getFieldDesc()->getNumElems());
 
     unsigned ElemByteOffset = I * getFieldDesc()->getElemSize();
-    if (isArrayRoot())
-      return *reinterpret_cast<T *>(asBlockPointer().Pointee->rawData() +
-                                    asBlockPointer().Base + sizeof(InitMapPtr) +
-                                    ElemByteOffset);
+    if (isArrayRoot()) {
+      unsigned ReadOffset = BS.Base + sizeof(InitMapPtr) + ElemByteOffset;
+      assert(ReadOffset + sizeof(T) <=
+             BS.Pointee->getDescriptor()->getAllocSize());
+      return *reinterpret_cast<T *>(BS.Pointee->rawData() + ReadOffset);
+    }
 
-    return *reinterpret_cast<T *>(asBlockPointer().Pointee->rawData() + Offset +
-                                  ElemByteOffset);
+    unsigned ReadOffset = BS.Base + ElemByteOffset;
+    assert(ReadOffset + sizeof(T) <=
+           BS.Pointee->getDescriptor()->getAllocSize());
+    return *reinterpret_cast<T *>(BS.Pointee->rawData() + ReadOffset);
   }
 
   /// Whether this block can be read from at all. This is only true for

clang/test/AST/ByteCode/invalid.cpp

@@ -58,3 +58,11 @@ namespace Casts {
   /// Just make sure this doesn't crash.
   float PR9558 = reinterpret_cast<const float&>("asd");
 }
+
+
+/// This used to crash in collectBlock().
+struct S {
+};
+S s;
+S *sp[2] = {&s, &s};
+S *&spp = sp[1];