Fix the computation of alignment for fields of packed+aligned structs.

Part of the fix for PR8413.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@123904 91177308-0d34-0410-b5e6-96231b3b80d8

3 files changed, 48 insertions, 14 deletions

diff --git a/lib/AST/ASTContext.cpp b/lib/AST/ASTContext.cpp
index d1ac00871a..043d56a98d 100644
--- a/lib/AST/ASTContext.cpp
+++ b/lib/AST/ASTContext.cpp
@@ -590,8 +590,11 @@ CharUnits ASTContext::getDeclAlign(const Decl *D, bool RefAsPointee) const {
     }
   }
 
+  // If we're using the align attribute only, just ignore everything
+  // else about the declaration and its type.
   if (UseAlignAttrOnly) {
-    // ignore type of value
+    // do nothing
+
   } else if (const ValueDecl *VD = dyn_cast<ValueDecl>(D)) {
     QualType T = VD->getType();
     if (const ReferenceType* RT = T->getAs<ReferenceType>()) {
@@ -617,11 +620,30 @@ CharUnits ASTContext::getDeclAlign(const Decl *D, bool RefAsPointee) const {
       }
       Align = std::max(Align, getPreferredTypeAlign(T.getTypePtr()));
     }
-    if (const FieldDecl *FD = dyn_cast<FieldDecl>(VD)) {
-      // In the case of a field in a packed struct, we want the minimum
-      // of the alignment of the field and the alignment of the struct.
-      Align = std::min(Align,
-        getPreferredTypeAlign(FD->getParent()->getTypeForDecl()));
+
+    // Fields can be subject to extra alignment constraints, like if
+    // the field is packed, the struct is packed, or the struct has a
+    // a max-field-alignment constraint (#pragma pack).  So calculate
+    // the actual alignment of the field within the struct, and then
+    // (as we're expected to) constrain that by the alignment of the type.
+    if (const FieldDecl *field = dyn_cast<FieldDecl>(VD)) {
+      // So calculate the alignment of the field.
+      const ASTRecordLayout &layout = getASTRecordLayout(field->getParent());
+
+      // Start with the record's overall alignment.
+      unsigned fieldAlign = layout.getAlignment();
+
+      // Use the GCD of that and the offset within the record.
+      uint64_t offset = layout.getFieldOffset(field->getFieldIndex());
+      if (offset > 0) {
+        // Alignment is always a power of 2, so the GCD will be a power of 2,
+        // which means we get to do this crazy thing instead of Euclid's.
+        uint64_t lowBitOfOffset = offset & (~offset + 1);
+        if (lowBitOfOffset < fieldAlign)
+          fieldAlign = static_cast<unsigned>(lowBitOfOffset);
+      }
+
+      Align = std::min(Align, fieldAlign);
     }
   }
 
diff --git a/lib/AST/Decl.cpp b/lib/AST/Decl.cpp
index 9e2e8763f8..b1da849ae1 100644
--- a/lib/AST/Decl.cpp
+++ b/lib/AST/Decl.cpp
@@ -1881,6 +1881,25 @@ bool FieldDecl::isAnonymousStructOrUnion() const {
   return false;
 }
 
+unsigned FieldDecl::getFieldIndex() const {
+  if (CachedFieldIndex) return CachedFieldIndex - 1;
+
+  unsigned index = 0;
+  RecordDecl::field_iterator
+    i = getParent()->field_begin(), e = getParent()->field_end();
+  while (true) {
+    assert(i != e && "failed to find field in parent!");
+    if (*i == this)
+      break;
+
+    ++i;
+    ++index;
+  }
+
+  CachedFieldIndex = index + 1;
+  return index;
+}
+
 //===----------------------------------------------------------------------===//
 // TagDecl Implementation
 //===----------------------------------------------------------------------===//
diff --git a/lib/AST/ExprConstant.cpp b/lib/AST/ExprConstant.cpp
index f262a4acff..afce24e625 100644
--- a/lib/AST/ExprConstant.cpp
+++ b/lib/AST/ExprConstant.cpp
@@ -1565,14 +1565,7 @@ bool IntExprEvaluator::VisitOffsetOfExpr(const OffsetOfExpr *E) {
         return false;
       RecordDecl *RD = RT->getDecl();
       const ASTRecordLayout &RL = Info.Ctx.getASTRecordLayout(RD);
-      unsigned i = 0;
-      // FIXME: It would be nice if we didn't have to loop here!
-      for (RecordDecl::field_iterator Field = RD->field_begin(),
-                                      FieldEnd = RD->field_end();
-           Field != FieldEnd; (void)++Field, ++i) {
-        if (*Field == MemberDecl)
-          break;
-      }
+      unsigned i = MemberDecl->getFieldIndex();
       assert(i < RL.getFieldCount() && "offsetof field in wrong type");
       Result += Info.Ctx.toCharUnitsFromBits(RL.getFieldOffset(i));
       CurrentType = MemberDecl->getType().getNonReferenceType();