Debug Info: One more bitfield bugfix. While yesterday's r240853 fixed

the DW_AT_bit_offset computation, the byte offset is in fact also
endian-dependent as it needs to point to the storage unit containing the
most-significant bit of the the bitfield.
I'm so looking forward to emitting the endian-agnostic DWARF 3 version
instead.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240890 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/AsmPrinter/DwarfUnit.cpp

@@ -1346,9 +1346,9 @@ void DwarfUnit::constructMemberDIE(DIE &Buffer, const DIDerivedType *DT) {
       addUInt(MemberDie, dwarf::DW_AT_byte_size, None, FieldSize/8);
       addUInt(MemberDie, dwarf::DW_AT_bit_size, None, Size);
       //
-      // The DWARF 2 DW_AT_bit_offset is counting the bits between
-      // the high end of the aligned storage unit containing the bit
-      // field to the high end of the bit field.
+      // The DWARF 2 DW_AT_bit_offset is counting the bits between the most
+      // significant bit of the aligned storage unit containing the bit field to
+      // the most significan bit of the bit field.
       //
       // FIXME: DWARF 4 states that DW_AT_data_bit_offset (which
       // counts from the beginning, regardless of endianness) should
@@ -1361,24 +1361,29 @@ void DwarfUnit::constructMemberDIE(DIE &Buffer, const DIDerivedType *DT) {
       // | ...             |b1|b2|b3|b4|
       // +-----------+-----*-----+-----*-----+--
       // |           |     |<-- Size ->|     |
-      // |<---- Offset --->|           |<--->|
-      // |           |     |              \_ DW_AT_bit_offset (little endian)
-      // |           |<--->|
-      // |<--------->|  \_ StartBitOffset = DW_AT_bit_offset (big endian)
-      //     \                            = DW_AT_data_bit_offset (biendian)
-      //      \_ OffsetInBytes
+      // |<---- Offset --->|     |     |<--->|
+      // |           |     |     |        \_ DW_AT_bit_offset (little endian)
+      // |           |<--->|     |
+      // |<--big-e.->|  \_ StartBitOffset = DW_AT_bit_offset (big endian)
+      // |      ^                |        = DW_AT_data_bit_offset (biendian)
+      // | OffsetInBytes         |
+      // |      v                |
+      // |<----little-endian---->|
       uint64_t Offset = DT->getOffsetInBits();
       uint64_t Align = DT->getAlignInBits() ? DT->getAlignInBits() : FieldSize;
       uint64_t AlignMask = ~(Align - 1);
       // The bits from the start of the storage unit to the start of the field.
       uint64_t StartBitOffset = Offset - (Offset & AlignMask);
-      // The endian-dependent DWARF 2 offset.
-      uint64_t DwarfBitOffset = Asm->getDataLayout().isLittleEndian()
-        ? OffsetToAlignment(Offset + Size, Align)
-        : StartBitOffset;
-
-      // The byte offset of the field's aligned storage unit inside the struct.
-      OffsetInBytes = (Offset - StartBitOffset) / 8;
+      // OffsetInBytes is the byte offset of the field's aligned storage unit
+      // inside the struct.
+      uint64_t DwarfBitOffset;
+      if (Asm->getDataLayout().isLittleEndian()) {
+        DwarfBitOffset = OffsetToAlignment(Offset + Size, Align);
+        OffsetInBytes = ((Offset + Size) & AlignMask) / 8;
+      } else {
+        DwarfBitOffset = StartBitOffset;
+        OffsetInBytes = (Offset - StartBitOffset) / 8;
+      }
       addUInt(MemberDie, dwarf::DW_AT_bit_offset, None, DwarfBitOffset);
     } else
       // This is not a bitfield.

test/DebugInfo/ARM/bitfield.ll

@@ -14,7 +14,7 @@
 ; CHECK:          DW_AT_byte_size  {{.*}} (0x04)
 ; CHECK:          DW_AT_bit_size   {{.*}} (0x1c)
 ; CHECK:          DW_AT_bit_offset {{.*}} (0x18)
-; CHECK:          DW_AT_data_member_location {{.*}}00
+; CHECK:          DW_AT_data_member_location {{.*}}04
 target datalayout = "e-m:o-p:32:32-f64:32:64-v64:32:64-v128:32:128-a:0:32-n32-S32"
 target triple = "thumbv7-apple-ios"