Debug Info: Fix a bug in the DW_AT_bit_offset calculation that would

result in negative offsets and attempt a better job at documenting
the algorithm.

rdar://21082998

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240853 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/CodeGen/AsmPrinter/DwarfUnit.cpp b/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
index f95f79d903cfa7f433fbf00f3c3fb58c08060298..44de3407369818847df1d50adc6a8fffd158ca77 100644 (file)
--- a/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
+++ b/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
@@ -1345,7 +1345,7 @@ void DwarfUnit::constructMemberDIE(DIE &Buffer, const DIDerivedType *DT) {
       // Handle bitfield, assume bytes are 8 bits.
       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.
@@ -1353,21 +1353,36 @@ void DwarfUnit::constructMemberDIE(DIE &Buffer, const DIDerivedType *DT) {
       // FIXME: DWARF 4 states that DW_AT_data_bit_offset (which
       // counts from the beginning, regardless of endianness) should
       // be used instead.
+      //
+      //
+      // 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
       uint64_t Offset = DT->getOffsetInBits();
-      uint64_t AlignMask = ~(DT->getAlignInBits() - 1);
-      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, Offset);
-
-      OffsetInBytes = FieldOffset >> 3;
+      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;
+      addUInt(MemberDie, dwarf::DW_AT_bit_offset, None, DwarfBitOffset);
     } else
       // This is not a bitfield.
-      OffsetInBytes = DT->getOffsetInBits() >> 3;
+      OffsetInBytes = DT->getOffsetInBits() / 8;
 
     if (DD->getDwarfVersion() <= 2) {
       DIELoc *MemLocationDie = new (DIEValueAllocator) DIELoc;

diff --git a/test/DebugInfo/ARM/bitfield.ll b/test/DebugInfo/ARM/bitfield.ll
new file mode 100644 (file)
index 0000000..9b41e4e
--- /dev/null
+++ b/test/DebugInfo/ARM/bitfield.ll
@@ -0,0 +1,45 @@
+; 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_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
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!11, !12, !13, !14, !15}
+!llvm.ident = !{!16}
+
+!0 = !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 3.7.0 (trunk 240548) (llvm/trunk 240554)", isOptimized: false, runtimeVersion: 0, emissionKind: 1, enums: !2, retainedTypes: !2, subprograms: !2, globals: !3, imports: !2)
+!1 = !DIFile(filename: "test.i", directory: "/")
+!2 = !{}
+!3 = !{!4}
+!4 = !DIGlobalVariable(name: "a", scope: !0, file: !1, line: 5, type: !5, isLocal: false, isDefinition: true, variable: %struct.anon* @a)
+!5 = !DICompositeType(tag: DW_TAG_structure_type, file: !1, line: 1, size: 48, align: 8, elements: !6)
+!6 = !{!7, !9}
+!7 = !DIDerivedType(tag: DW_TAG_member, name: "c", scope: !5, file: !1, line: 2, baseType: !8, size: 8, align: 8)
+!8 = !DIBasicType(name: "char", size: 8, align: 8, encoding: DW_ATE_signed_char)
+!9 = !DIDerivedType(tag: DW_TAG_member, name: "reserved", scope: !5, file: !1, line: 4, baseType: !10, size: 28, align: 32, offset: 12)
+!10 = !DIBasicType(name: "int", size: 32, align: 32, encoding: DW_ATE_signed)
+!11 = !{i32 2, !"Dwarf Version", i32 2}
+!12 = !{i32 2, !"Debug Info Version", i32 3}
+!13 = !{i32 1, !"wchar_size", i32 4}
+!14 = !{i32 1, !"min_enum_size", i32 4}
+!15 = !{i32 1, !"PIC Level", i32 2}
+!16 = !{!"clang version 3.7.0 (trunk 240548) (llvm/trunk 240554)"}