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, 14 insertions, 0 deletions

diff --git a/lib/Sema/SemaDecl.cpp b/lib/Sema/SemaDecl.cpp
index fee732f2f5..e32da244de 100644
--- a/lib/Sema/SemaDecl.cpp
+++ b/lib/Sema/SemaDecl.cpp
@@ -1939,6 +1939,20 @@ Sema::DeclTy *Sema::ActOnTagStruct(Scope *S, TagDecl::TagKind Kind, TagKind TK,
     // Add it to the decl chain.
     PushOnScopeChains(New, S);
   }
+
+  // Handle #pragma pack: if the #pragma pack stack has non-default
+  // alignment, make up a packed attribute for this decl. These
+  // attributes are checked when the ASTContext lays out the
+  // structure.
+  //
+  // It is important for implementing the correct semantics that this
+  // happen here (in act on tag decl). The #pragma pack stack is
+  // maintained as a result of parser callbacks which can occur at
+  // many points during the parsing of a struct declaration (because
+  // the #pragma tokens are effectively skipped over during the
+  // parsing of the struct).
+  if (unsigned Alignment = PackContext.getAlignment())
+    New->addAttr(new PackedAttr(Alignment * 8));
   
   if (Attr)
     ProcessDeclAttributeList(New, Attr);