Implement #pragma pack use in structure packing. The general approach

is to encode the state of the #pragma pack stack as an attribute when
the structure is declared. 

 - Extend PackedAttr to take an alignment (in bits), and reuse for
   both __attribute__((packed)) (which takes no argument, instead
   packing tightly (to "minimize the memory required") and for #pragma
   pack (which allows specification of the maximum alignment in
   bytes). __attribute__((packed)) is just encoded as Alignment=1.

   This conflates two related but different mechanisms, but it didn't
   seem worth another attribute.

 - I have attempted to follow the MSVC semantics as opposed to the gcc
   ones, since if I understand correctly #pragma pack originated with
   MSVC. The semantics are generally equivalent except when the stack
   is altered during the definition of a structure; its not clear if
   anyone does this in practice. See testcase if curious.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@57623 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 32 insertions, 10 deletions

diff --git a/lib/AST/ASTContext.cpp b/lib/AST/ASTContext.cpp
index 572e003dd7..e8d7978bca 100644
--- a/lib/AST/ASTContext.cpp
+++ b/lib/AST/ASTContext.cpp
@@ -371,12 +371,17 @@ ASTContext::getTypeInfo(QualType T) {
 
 /// LayoutField - Field layout.
 void ASTRecordLayout::LayoutField(const FieldDecl *FD, unsigned FieldNo,
-                                  bool IsUnion, bool StructIsPacked,
+                                  bool IsUnion, unsigned StructPacking,
                                   ASTContext &Context) {
-  bool FieldIsPacked = StructIsPacked || FD->getAttr<PackedAttr>();
+  unsigned FieldPacking = StructPacking;
   uint64_t FieldOffset = IsUnion ? 0 : Size;
   uint64_t FieldSize;
   unsigned FieldAlign;
+
+  // FIXME: Should this override struct packing? Probably we want to
+  // take the minimum?
+  if (const PackedAttr *PA = FD->getAttr<PackedAttr>())
+    FieldPacking = PA->getAlignment();
   
   if (const Expr *BitWidthExpr = FD->getBitWidth()) {
     // TODO: Need to check this algorithm on other targets!
@@ -388,9 +393,14 @@ void ASTRecordLayout::LayoutField(const FieldDecl *FD, unsigned FieldNo,
       Context.getTypeInfo(FD->getType());
     uint64_t TypeSize = FieldInfo.first;
     
+    // Determine the alignment of this bitfield. The packing
+    // attributes define a maximum and the alignment attribute defines
+    // a minimum.
+    // FIXME: What is the right behavior when the specified alignment
+    // is smaller than the specified packing?
     FieldAlign = FieldInfo.second;
-    if (FieldIsPacked)
-      FieldAlign = 1;
+    if (FieldPacking)
+      FieldAlign = std::min(FieldAlign, FieldPacking);
     if (const AlignedAttr *AA = FD->getAttr<AlignedAttr>())
       FieldAlign = std::max(FieldAlign, AA->getAlignment());
     
@@ -418,8 +428,15 @@ void ASTRecordLayout::LayoutField(const FieldDecl *FD, unsigned FieldNo,
       FieldAlign = FieldInfo.second;
     }
     
-    if (FieldIsPacked)
-      FieldAlign = 8;
+    // Determine the alignment of this bitfield. The packing
+    // attributes define a maximum and the alignment attribute defines
+    // a minimum. Additionally, the packing alignment must be at least
+    // a byte for non-bitfields.
+    //
+    // FIXME: What is the right behavior when the specified alignment
+    // is smaller than the specified packing?
+    if (FieldPacking)
+      FieldAlign = std::min(FieldAlign, std::max(8U, FieldPacking));
     if (const AlignedAttr *AA = FD->getAttr<AlignedAttr>())
       FieldAlign = std::max(FieldAlign, AA->getAlignment());
     
@@ -470,7 +487,9 @@ ASTContext::getASTObjCInterfaceLayout(const ObjCInterfaceDecl *D) {
   }
   Entry = NewEntry;
 
-  bool IsPacked = D->getAttr<PackedAttr>();
+  unsigned StructPacking = 0;
+  if (const PackedAttr *PA = D->getAttr<PackedAttr>())
+    StructPacking = PA->getAlignment();
 
   if (const AlignedAttr *AA = D->getAttr<AlignedAttr>())
     NewEntry->SetAlignment(std::max(NewEntry->getAlignment(), 
@@ -481,7 +500,7 @@ ASTContext::getASTObjCInterfaceLayout(const ObjCInterfaceDecl *D) {
   for (ObjCInterfaceDecl::ivar_iterator IVI = D->ivar_begin(), 
        IVE = D->ivar_end(); IVI != IVE; ++IVI) {
     const ObjCIvarDecl* Ivar = (*IVI);
-    NewEntry->LayoutField(Ivar, i++, false, IsPacked, *this);
+    NewEntry->LayoutField(Ivar, i++, false, StructPacking, *this);
   }
 
   // Finally, round the size of the total struct up to the alignment of the
@@ -507,9 +526,12 @@ const ASTRecordLayout &ASTContext::getASTRecordLayout(const RecordDecl *D) {
   Entry = NewEntry;
 
   NewEntry->InitializeLayout(D->getNumMembers());
-  bool StructIsPacked = D->getAttr<PackedAttr>();
   bool IsUnion = D->isUnion();
 
+  unsigned StructPacking = 0;
+  if (const PackedAttr *PA = D->getAttr<PackedAttr>())
+    StructPacking = PA->getAlignment();
+
   if (const AlignedAttr *AA = D->getAttr<AlignedAttr>())
     NewEntry->SetAlignment(std::max(NewEntry->getAlignment(), 
                                     AA->getAlignment()));
@@ -518,7 +540,7 @@ const ASTRecordLayout &ASTContext::getASTRecordLayout(const RecordDecl *D) {
   // the future, this will need to be tweakable by targets.
   for (unsigned i = 0, e = D->getNumMembers(); i != e; ++i) {
     const FieldDecl *FD = D->getMember(i);
-    NewEntry->LayoutField(FD, i, IsUnion, StructIsPacked, *this);
+    NewEntry->LayoutField(FD, i, IsUnion, StructPacking, *this);
   }
 
   // Finally, round the size of the total struct up to the alignment of the