Debug Info: Restore the pre-r240853 behavior for DWARF 2 bitfields.

[aprantl]

Summary

This is a spiritual revert of r240853.

The DWARF 2 specification of DW_AT_bit_offset is written from the perspective of a big-endian machine and is ambiguous for little-endian machines.

The way that LLVM pre-r240853 interprets it can result in negative field offsets, which I attempted to fix in r240853. However, it turns out that debuggers actually expect this format using negative offsets and other popular compilers also generate them this way. I therefor suggest to just revert back to the old behavior.

Note that D19630 implements the unambiguous and more space-efficient DWARF4 version using the new DW_AT_data_bit_offset attribute.

[aprantl]

This landed long ago as r267896.

lib/CodeGen/AsmPrinter/DwarfUnit.cpp

if (FieldSize && Size != FieldSize) {
       // The bits from the start of the storage unit to the start of the field.
       uint64_t StartBitOffset = Offset - (Offset & AlignMask);
       // The byte offset of the field's aligned storage unit inside the struct.
       OffsetInBytes = (Offset - StartBitOffset) / 8;
 
       if (DD->getDwarfVersion() >= 4)
         addUInt(MemberDie, dwarf::DW_AT_data_bit_offset, None, Offset);
       else {
-        //
-        // 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.
-        //
-        // Struct      Align       Align       Align
-        // v           v           v           v
-        // +-----------+-----*-----+-----*-----+--
-        // | ...             |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
-        // The endian-dependent DWARF 2 offset.
-        uint64_t DwarfBitOffset = Asm->getDataLayout().isLittleEndian()
-                                      ? OffsetToAlignment(Offset + Size, Align)
-                                      : StartBitOffset;
+        uint64_t HiMark = (Offset + FieldSize) & AlignMask;
+        uint64_t FieldOffset = (HiMark - FieldSize);
+        Offset -= FieldOffset;
+
+        // Maybe we need to work from the other end.
+        if (Asm->getDataLayout().isLittleEndian())
+          Offset = FieldSize - (Offset + Size);
 
-        addUInt(MemberDie, dwarf::DW_AT_bit_offset, None, DwarfBitOffset);
+        addUInt(MemberDie, dwarf::DW_AT_bit_offset, None, Offset);
+        OffsetInBytes = FieldOffset >> 3;
       }
     } else
       // This is not a bitfield.
       OffsetInBytes = DT->getOffsetInBits() / 8;
 
     if (DD->getDwarfVersion() <= 2) {
       DIELoc *MemLocationDie = new (DIEValueAllocator) DIELoc;
       addUInt(*MemLocationDie, dwarf::DW_FORM_data1, dwarf::DW_OP_plus_uconst);

test/DebugInfo/ARM/bitfield.ll

 ; RUN: %llc_dwarf -O0 -filetype=obj -o %t.o %s
 ; RUN: llvm-dwarfdump -debug-dump=info %t.o | FileCheck %s
 ; REQUIRES: object-emission
 ;
 ; Generated from:
 ; struct {
 ;   char c;
 ;   int : 4;
 ;   int reserved : 28;
 ; } a;
 ;
 ; CHECK:      DW_TAG_member
 ; CHECK:          DW_AT_name {{.*}} "reserved"
 ; CHECK:          DW_AT_byte_size  {{.*}} (0x04)
 ; CHECK:          DW_AT_bit_size   {{.*}} (0x1c)
-; CHECK:          DW_AT_bit_offset {{.*}} (0x18)
+; CHECK:          DW_AT_bit_offset {{.*}} (0xfffffffffffffff8)
 ; CHECK:          DW_AT_data_member_location {{.*}}00
 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"
 
 %struct.anon = type { i8, [5 x i8] }
 
 @a = common global %struct.anon zeroinitializer, align 1