[PATCH] D67971: [WIP] Add warning when zero-initializing a union if its first member is not is largest

[Leonard Chan via Phabricator via cfe-commits]

leonardchan created this revision.
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This may result in the entire union not being initialized. For example:

  union U {
    uint32_t x;
    uint64_t y;
  };
  
  void func() {
    U u = {};
    ...
  }

Some might assume that `U u = {}` zero-initializes the whole union whereas only the bits that compose `U.x` are zero-inited.

Oddly enough (mostly from what I've seen personally), this seems to only matter in the case we use `-ftrivial-auto-var-init=pattern` where llvm `undef`s are replaced with non-zero values. In any normal case, it seems that these `undef`s just get lowered to zeros.

See: http://lists.llvm.org/pipermail/cfe-dev/2019-September/063383.html


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D67971

Files:
  clang/include/clang/Basic/DiagnosticSemaKinds.td
  clang/lib/Sema/SemaInit.cpp


Index: clang/lib/Sema/SemaInit.cpp
===================================================================
--- clang/lib/Sema/SemaInit.cpp
+++ clang/lib/Sema/SemaInit.cpp
@@ -2041,8 +2041,16 @@
         CheckEmptyInitializable(
             InitializedEntity::InitializeMember(*Field, &Entity),
             IList->getEndLoc());
-        if (StructuredList)
+        if (StructuredList) {
           StructuredList->setInitializedFieldInUnion(*Field);
+
+          ASTContext &Ctx = SemaRef.Context;
+          unsigned UnionSize = Ctx.getTypeSize(Ctx.getRecordType(RD));
+          unsigned FieldSize = Ctx.getTypeSize(Field->getType());
+          if (UnionSize > FieldSize)
+            SemaRef.Diag(IList->getLBraceLoc(), diag::warn_union_zero_init)
+              << RD->getName() << Field->getName();
+        }
         break;
       }
     }
Index: clang/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -9898,4 +9898,7 @@
   "__builtin_bit_cast %select{source|destination}0 type must be trivially copyable">;
 def err_bit_cast_type_size_mismatch : Error<
   "__builtin_bit_cast source size does not equal destination size (%0 vs %1)">;
+
+def warn_union_zero_init : Warning<
+  "zero-initializing union '%0' whose first non-static named data member '%1' is not the largest in the union will leave uninitialized bits in the rest of the union">;
 } // end of sema component.


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