[clang] [clang] Better bitfield access units (PR #65742)

[Nathan Sidwell via cfe-commits]

================
@@ -394,33 +412,155 @@ void CGRecordLowering::accumulateFields() {
               : getStorageType(*Field),
           *Field));
       ++Field;
-    } else {
-      ++Field;
     }
   }
 }
 
-void
-CGRecordLowering::accumulateBitFields(RecordDecl::field_iterator Field,
-                                      RecordDecl::field_iterator FieldEnd) {
-  // Run stores the first element of the current run of bitfields.  FieldEnd is
-  // used as a special value to note that we don't have a current run.  A
-  // bitfield run is a contiguous collection of bitfields that can be stored in
-  // the same storage block.  Zero-sized bitfields and bitfields that would
-  // cross an alignment boundary break a run and start a new one.
-  RecordDecl::field_iterator Run = FieldEnd;
-  // Tail is the offset of the first bit off the end of the current run.  It's
-  // used to determine if the ASTRecordLayout is treating these two bitfields as
-  // contiguous.  StartBitOffset is offset of the beginning of the Run.
-  uint64_t StartBitOffset, Tail = 0;
+namespace {
+
+// A run of bitfields assigned to the same access unit -- the size of memory
+// loads & stores.
+class BitFieldAccessUnit {
+  RecordDecl::field_iterator Begin; // Field at start of this access unit.
+  RecordDecl::field_iterator End;   // Field just after this access unit.
+
+  CharUnits StartOffset; // Starting offset in the containing record.
+  CharUnits EndOffset;   // Finish offset (exclusive) in the containing record.
+
+  bool ContainsVolatile; // This access unit contains a volatile bitfield.
+
+public:
+  // End barrier constructor.
+  BitFieldAccessUnit(RecordDecl::field_iterator F, CharUnits Offset,
+                     bool Volatile = false)
+      : Begin(F), End(F), StartOffset(Offset), EndOffset(Offset),
+        ContainsVolatile(Volatile) {}
+
+  // Collect contiguous bitfields into an access unit.
+  BitFieldAccessUnit(RecordDecl::field_iterator FieldBegin,
+                     RecordDecl::field_iterator FieldEnd,
+                     const CGRecordLowering &CGRL);
----------------
urnathan wrote:

Updated to add gatherBitFieldAccessUnit and installBitFieldAccessUnit members of CGRecordLowering.  You're right that it makes the structure clearer.

https://github.com/llvm/llvm-project/pull/65742