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
diff --git a/lib/AST/ASTContext.cpp b/lib/AST/ASTContext.cpp
index 572e003..e8d7978 100644
--- a/lib/AST/ASTContext.cpp
+++ b/lib/AST/ASTContext.cpp
@@ -371,12 +371,17 @@
/// 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 @@
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 @@
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 @@
}
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 @@
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 @@
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 @@
// 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