[clang] [analyzer] Check the correct first and last elements in cstring.UninitializedRead (PR #95408)

[Donát Nagy via cfe-commits]

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@@ -393,6 +401,173 @@ ProgramStateRef CStringChecker::checkNonNull(CheckerContext &C,
   return stateNonNull;
 }
 
+static std::optional<NonLoc> getIndex(ProgramStateRef State,
+                                      const ElementRegion *ER, CharKind CK) {
+  SValBuilder &SValBuilder = State->getStateManager().getSValBuilder();
+  ASTContext &Ctx = SValBuilder.getContext();
+
+  if (CK == CharKind::Regular) {
+    if (ER->getValueType() != Ctx.CharTy)
+      return {};
+    return ER->getIndex();
+  }
+
+  if (ER->getValueType() != Ctx.WideCharTy)
+    return {};
+
+  QualType SizeTy = Ctx.getSizeType();
+  NonLoc WideSize =
+      SValBuilder
+          .makeIntVal(Ctx.getTypeSizeInChars(Ctx.WideCharTy).getQuantity(),
+                      SizeTy)
+          .castAs<NonLoc>();
+  SVal Offset =
+      SValBuilder.evalBinOpNN(State, BO_Mul, ER->getIndex(), WideSize, SizeTy);
+  if (Offset.isUnknown())
+    return {};
+  return Offset.castAs<NonLoc>();
+}
+
+// Try to get hold of the origin regin (e.g. the actual array region from an
----------------
NagyDonat wrote:

```suggestion
// Try to get hold of the origin region (e.g. the actual array region from an
```

https://github.com/llvm/llvm-project/pull/95408