Add an off-by-default warning to complain about MSVC bitfield padding (#117428)

This just adds a warning for bitfields placed next to other bitfields
where the underlying type has different storage. Under the MS struct
bitfield packing ABI such bitfields are not packed.

1 files changed, 25 insertions, 2 deletions

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 152f3f3..a7d59ec2 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -19399,9 +19399,9 @@ void Sema::ActOnFields(Scope *S, SourceLocation RecLoc, Decl *EnclosingDecl,
 
   // Verify that all the fields are okay.
   SmallVector<FieldDecl*, 32> RecFields;
-
+  const FieldDecl *PreviousField = nullptr;
   for (ArrayRef<Decl *>::iterator i = Fields.begin(), end = Fields.end();
-       i != end; ++i) {
+       i != end; PreviousField = cast<FieldDecl>(*i), ++i) {
     FieldDecl *FD = cast<FieldDecl>(*i);
 
     // Get the type for the field.
@@ -19617,6 +19617,29 @@ void Sema::ActOnFields(Scope *S, SourceLocation RecLoc, Decl *EnclosingDecl,
 
     if (Record && FD->getType().isVolatileQualified())
       Record->setHasVolatileMember(true);
+    bool ReportMSBitfieldStoragePacking =
+        Record && PreviousField &&
+        !Diags.isIgnored(diag::warn_ms_bitfield_mismatched_storage_packing,
+                         Record->getLocation());
+    auto IsNonDependentBitField = [](const FieldDecl *FD) {
+      return FD->isBitField() && !FD->getType()->isDependentType();
+    };
+
+    if (ReportMSBitfieldStoragePacking && IsNonDependentBitField(FD) &&
+        IsNonDependentBitField(PreviousField)) {
+      CharUnits FDStorageSize = Context.getTypeSizeInChars(FD->getType());
+      CharUnits PreviousFieldStorageSize =
+          Context.getTypeSizeInChars(PreviousField->getType());
+      if (FDStorageSize != PreviousFieldStorageSize) {
+        Diag(FD->getLocation(),
+             diag::warn_ms_bitfield_mismatched_storage_packing)
+            << FD << FD->getType() << FDStorageSize.getQuantity()
+            << PreviousFieldStorageSize.getQuantity();
+        Diag(PreviousField->getLocation(),
+             diag::note_ms_bitfield_mismatched_storage_size_previous)
+            << PreviousField << PreviousField->getType();
+      }
+    }
     // Keep track of the number of named members.
     if (FD->getIdentifier())
       ++NumNamedMembers;