[SystemZ][z/OS] Ignore leading zero width bitfield alignment on z/OS target

Zero length bitfield alignment is not respected if they are leading members on z/OS target.

Reviewed By: abhina.sreeskantharajan

Differential Revision: https://reviews.llvm.org/D98890

clang/include/clang/Basic/TargetInfo.h

@@ -155,6 +155,10 @@ protected:
   /// zero-length bitfield.
   unsigned UseZeroLengthBitfieldAlignment : 1;
 
+  /// Whether zero length bitfield alignment is respected if they are the
+  /// leading members.
+  unsigned UseLeadingZeroLengthBitfield : 1;
+
   ///  Whether explicit bit field alignment attributes are honored.
   unsigned UseExplicitBitFieldAlignment : 1;
 
@@ -768,6 +772,12 @@ public:
     return UseZeroLengthBitfieldAlignment;
   }
 
+  /// Check whether zero length bitfield alignment is respected if they are
+  /// leading members.
+  bool useLeadingZeroLengthBitfield() const {
+    return UseLeadingZeroLengthBitfield;
+  }
+
   /// Get the fixed alignment value in bits for a member that follows
   /// a zero length bitfield.
   unsigned getZeroLengthBitfieldBoundary() const {

clang/lib/AST/RecordLayoutBuilder.cpp

@@ -1627,12 +1627,17 @@ void ItaniumRecordLayoutBuilder::LayoutBitField(const FieldDecl *D) {
     // Some such targets do honor it on zero-width bitfields.
     if (FieldSize == 0 &&
         Context.getTargetInfo().useZeroLengthBitfieldAlignment()) {
-      // The alignment to round up to is the max of the field's natural
-      // alignment and a target-specific fixed value (sometimes zero).
-      unsigned ZeroLengthBitfieldBoundary =
-        Context.getTargetInfo().getZeroLengthBitfieldBoundary();
-      FieldAlign = std::max(FieldAlign, ZeroLengthBitfieldBoundary);
-
+      // Some targets don't honor leading zero-width bitfield.
+      if (!IsUnion && FieldOffset == 0 &&
+          !Context.getTargetInfo().useLeadingZeroLengthBitfield())
+        FieldAlign = 1;
+      else {
+        // The alignment to round up to is the max of the field's natural
+        // alignment and a target-specific fixed value (sometimes zero).
+        unsigned ZeroLengthBitfieldBoundary =
+            Context.getTargetInfo().getZeroLengthBitfieldBoundary();
+        FieldAlign = std::max(FieldAlign, ZeroLengthBitfieldBoundary);
+      }
     // If that doesn't apply, just ignore the field alignment.
     } else {
       FieldAlign = 1;

clang/lib/Basic/TargetInfo.cpp

@@ -104,6 +104,7 @@ TargetInfo::TargetInfo(const llvm::Triple &T) : TargetOpts(), Triple(T) {
   UseSignedCharForObjCBool = true;
   UseBitFieldTypeAlignment = true;
   UseZeroLengthBitfieldAlignment = false;
+  UseLeadingZeroLengthBitfield = true;
   UseExplicitBitFieldAlignment = true;
   ZeroLengthBitfieldBoundary = 0;
   HalfFormat = &llvm::APFloat::IEEEhalf();

clang/lib/Basic/Targets/OSTargets.h

@@ -796,6 +796,7 @@ public:
     this->WCharType = TargetInfo::UnsignedInt;
     this->UseBitFieldTypeAlignment = false;
     this->UseZeroLengthBitfieldAlignment = true;
+    this->UseLeadingZeroLengthBitfield = false;
     this->ZeroLengthBitfieldBoundary = 32;
     this->MinGlobalAlign = 0;
     this->DefaultAlignForAttributeAligned = 128;

clang/test/CodeGen/SystemZ/zos-alignment.c

@@ -83,6 +83,35 @@ struct s6 {
 // CHECK-NEXT:         8 |   char *[] b
 // CHECK-NEXT:           | [sizeof=8, align=8]
 
+struct s7 {
+  long  :0;
+  short a;
+} S7;
+// CHECK:              0 | struct s7
+// CHECK-NEXT:       0:- |   long
+// CHECK-NEXT:         0 |   short a
+// CHECK-NEXT:           | [sizeof=2, align=2]
+
+#pragma pack(2)
+struct s8 {
+  unsigned long       :0;
+  long long           a;
+} S8;
+#pragma pack()
+// CHECK:              0 | struct s8
+// CHECK-NEXT:       0:- |   unsigned long
+// CHECK-NEXT:         0 |   long long a
+// CHECK-NEXT:           | [sizeof=8, align=2]
+
+struct s9 {
+  unsigned int   :0;
+  unsigned short :0;
+} S9;
+// CHECK:              0 | struct s9
+// CHECK-NEXT:       0:- |   unsigned int
+// CHECK-NEXT:       0:- |   unsigned short
+// CHECK-NEXT:           | [sizeof=0, align=1]
+
 struct s10 {
  unsigned int __attribute__((aligned)) a;
 } S10;