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

[John McCall via cfe-commits]

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@@ -376,33 +377,41 @@ void CGRecordLowering::lowerUnion(bool isNoUniqueAddress) {
 }
 
 void CGRecordLowering::accumulateFields() {
-  for (RecordDecl::field_iterator Field = D->field_begin(),
-                                  FieldEnd = D->field_end();
-    Field != FieldEnd;) {
+  RecordDecl::field_iterator FieldEnd = D->field_end();
+  RecordDecl::field_iterator BitField = FieldEnd;
+  for (RecordDecl::field_iterator Field = D->field_begin(); Field != FieldEnd;
+       ++Field) {
     if (Field->isBitField()) {
-      RecordDecl::field_iterator Start = Field;
-      // Iterate to gather the list of bitfields.
-      for (++Field; Field != FieldEnd && Field->isBitField(); ++Field);
-      accumulateBitFields(Start, Field);
+      if (BitField == FieldEnd)
+        // Start gathering bitfields
+        BitField = Field;
     } else if (!Field->isZeroSize(Context)) {
       // Use base subobject layout for the potentially-overlapping field,
       // as it is done in RecordLayoutBuilder
+      CharUnits Offset = bitsToCharUnits(getFieldBitOffset(*Field));
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rjmccall wrote:

This is permitting accesses to cross a regular field if it has zero size.  That is not allowed; bit-fields are only part of the same memory location if they are part of a consecutive sequence of non-zero-width bit-fields, and bit-fields in different memory locations must never interfere.

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