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.
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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