Add partial support for using anonymous bitfields (e.g., int : 0) to enforce

alignment.  This fixes cases where the anonymous bitfield is followed by a 
non-bitfield member.  E.g.,

struct t4
{
  int foo : 1;
  long : 0;
  char bar;
};

Part of rdar://9859156

llvm-svn: 136858

3 files changed, 42 insertions, 11 deletions

diff --git a/clang/lib/AST/RecordLayoutBuilder.cpp b/clang/lib/AST/RecordLayoutBuilder.cpp
index bb5b33406f4..19a762c30bc 100644
--- a/clang/lib/AST/RecordLayoutBuilder.cpp
+++ b/clang/lib/AST/RecordLayoutBuilder.cpp
@@ -1348,6 +1348,13 @@ void RecordLayoutBuilder::LayoutFields(const RecordDecl *D) {
       }
       LastFD = FD;
     }
+    else if (Context.Target.useZeroLengthBitfieldAlignment() &&
+             !Context.Target.useBitFieldTypeAlignment()) {
+      FieldDecl *FD =  (*Field);
+      if (Context.ZeroBitfieldFollowsBitfield(FD, LastFD))
+        ZeroLengthBitfield = FD;
+      LastFD = FD;
+    }
     LayoutField(*Field);
   }
   if (IsMsStruct && RemainingInAlignment &&
@@ -1442,7 +1449,7 @@ void RecordLayoutBuilder::LayoutBitField(const FieldDecl *D) {
   // This check is needed for 'long long' in -m32 mode.
   if (IsMsStruct && (TypeSize > FieldAlign))
     FieldAlign = TypeSize;
-  
+
   if (ZeroLengthBitfield) {
     // If a zero-length bitfield is inserted after a bitfield,
     // and the alignment of the zero-length bitfield is
@@ -1559,17 +1566,29 @@ void RecordLayoutBuilder::LayoutField(const FieldDecl *D) {
       Context.getTypeInfoInChars(D->getType());
     FieldSize = FieldInfo.first;
     FieldAlign = FieldInfo.second;
-    
+
     if (ZeroLengthBitfield) {
-      // If a zero-length bitfield is inserted after a bitfield,
-      // and the alignment of the zero-length bitfield is
-      // greater than the member that follows it, `bar', `bar' 
-      // will be aligned as the type of the zero-length bitfield.
-      std::pair<CharUnits, CharUnits> FieldInfo = 
-        Context.getTypeInfoInChars(ZeroLengthBitfield->getType());
-      CharUnits ZeroLengthBitfieldAlignment = FieldInfo.second;
-      if (ZeroLengthBitfieldAlignment > FieldAlign)
-        FieldAlign = ZeroLengthBitfieldAlignment;
+      CharUnits ZeroLengthBitfieldBoundary = 
+        Context.toCharUnitsFromBits(
+          Context.Target.getZeroLengthBitfieldBoundary());
+      if (ZeroLengthBitfieldBoundary == CharUnits::Zero()) {
+        // If a zero-length bitfield is inserted after a bitfield,
+        // and the alignment of the zero-length bitfield is
+        // greater than the member that follows it, `bar', `bar' 
+        // will be aligned as the type of the zero-length bitfield.
+        std::pair<CharUnits, CharUnits> FieldInfo = 
+          Context.getTypeInfoInChars(ZeroLengthBitfield->getType());
+        CharUnits ZeroLengthBitfieldAlignment = FieldInfo.second;        
+        if (ZeroLengthBitfieldAlignment > FieldAlign)
+          FieldAlign = ZeroLengthBitfieldAlignment;
+      }
+      else if (ZeroLengthBitfieldBoundary > FieldAlign) {
+        // Align 'bar' based on a fixed alignment specified by the target.
+        assert (Context.Target.useZeroLengthBitfieldAlignment() &&
+                "ZeroLengthBitfieldBoundary should only be used in conjunction"
+                "with useZeroLengthBitfieldAlignment.");
+        FieldAlign = ZeroLengthBitfieldBoundary;
+      }
       ZeroLengthBitfield = 0;
     }
 
diff --git a/clang/lib/Basic/TargetInfo.cpp b/clang/lib/Basic/TargetInfo.cpp
index 7ea51467c18..91b16d57788 100644
--- a/clang/lib/Basic/TargetInfo.cpp
+++ b/clang/lib/Basic/TargetInfo.cpp
@@ -53,6 +53,8 @@ TargetInfo::TargetInfo(const std::string &T) : Triple(T) {
   Int64Type = SignedLongLong;
   SigAtomicType = SignedInt;
   UseBitFieldTypeAlignment = true;
+  UseZeroLengthBitfieldAlignment = false;
+  ZeroLengthBitfieldBoundary = 0;
   FloatFormat = &llvm::APFloat::IEEEsingle;
   DoubleFormat = &llvm::APFloat::IEEEdouble;
   LongDoubleFormat = &llvm::APFloat::IEEEdouble;
diff --git a/clang/lib/Basic/Targets.cpp b/clang/lib/Basic/Targets.cpp
index ec45a87fa02..89b6ce960b0 100644
--- a/clang/lib/Basic/Targets.cpp
+++ b/clang/lib/Basic/Targets.cpp
@@ -1963,6 +1963,16 @@ public:
       // structures. This corresponds to PCC_BITFIELD_TYPE_MATTERS in gcc.
       UseBitFieldTypeAlignment = false;
 
+      /// Do force alignment of members that follow zero length bitfields.  If
+      /// the alignment of the zero-length bitfield is greater than the member 
+      /// that follows it, `bar', `bar' will be aligned as the  type of the 
+      /// zero length bitfield.
+      UseZeroLengthBitfieldAlignment = true;
+
+      /// gcc forces the alignment to 4 bytes, regardless of the type of the
+      /// zero length bitfield.
+      ZeroLengthBitfieldBoundary = 32;
+
       if (IsThumb) {
         // Thumb1 add sp, #imm requires the immediate value be multiple of 4,
         // so set preferred for small types to 32.