dwarfgen: Add support for generating the debug_str_offsets section, take 2

This recommits r337910 after fixing an "ambiguous call to addAttribute"
error with some compilers (gcc circa 4.9 and MSVC). It seems that these
compilers will consider a "false -> pointer" conversion during overload
resolution. This creates ambiguity because one I added an overload which
takes a MCExpr * as an argument.

I fix this by making the new overload take MCExpr&, which avoids the
conversion. It also documents the fact that we expect a valid MCExpr
object.

Original commit message follows:

The motivation for this is D49493, where we'd like to test details of
debug_str_offsets behavior which is difficult to trigger from a
traditional test.

This adds the plubming necessary for dwarfgen to generate this section.
The more interesting changes are:
- I've moved emitStringOffsetsTableHeader function from DwarfFile to
  DwarfStringPool, so I can generate the section header more easily from
  the unit test.
- added a new addAttribute overload taking an MCExpr*. This is used to
  generate the DW_AT_str_offsets_base, which links a compile unit to the
  offset table.

I've also added a basic test for reading and writing DW_form_strx forms.

Reviewers: dblaikie, JDevlieghere, probinson

Subscribers: llvm-commits, aprantl

Differential Revision: https://reviews.llvm.org/D49670

llvm-svn: 337933

5 files changed, 32 insertions, 27 deletions

diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
index f1e61b488a2..d2f123ce78e 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
@@ -1506,8 +1506,9 @@ void DwarfDebug::emitAbbreviations() {
 
 void DwarfDebug::emitStringOffsetsTableHeader() {
   DwarfFile &Holder = useSplitDwarf() ? SkeletonHolder : InfoHolder;
-  Holder.emitStringOffsetsTableHeader(
-      Asm->getObjFileLowering().getDwarfStrOffSection());
+  Holder.getStringPool().emitStringOffsetsTableHeader(
+      *Asm, Asm->getObjFileLowering().getDwarfStrOffSection(),
+      Holder.getStringOffsetsStartSym());
 }
 
 template <typename AccelTableT>
@@ -2292,8 +2293,9 @@ void DwarfDebug::emitDebugLineDWO() {
 
 void DwarfDebug::emitStringOffsetsTableHeaderDWO() {
   assert(useSplitDwarf() && "No split dwarf?");
-  InfoHolder.emitStringOffsetsTableHeader(
-      Asm->getObjFileLowering().getDwarfStrOffDWOSection());
+  InfoHolder.getStringPool().emitStringOffsetsTableHeader(
+      *Asm, Asm->getObjFileLowering().getDwarfStrOffDWOSection(),
+      InfoHolder.getStringOffsetsStartSym());
 }
 
 // Emit the .debug_str.dwo section for separated dwarf. This contains the
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfFile.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfFile.cpp
index f3a3cbdbc74..c90bd568162 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfFile.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfFile.cpp
@@ -28,26 +28,6 @@ void DwarfFile::addUnit(std::unique_ptr<DwarfCompileUnit> U) {
   CUs.push_back(std::move(U));
 }
 
-void DwarfFile::emitStringOffsetsTableHeader(MCSection *Section) {
-  if (StrPool.empty())
-    return;
-  Asm->OutStreamer->SwitchSection(Section);
-  unsigned EntrySize = 4;
-  // FIXME: DWARF64
-  // We are emitting the header for a contribution to the string offsets
-  // table. The header consists of an entry with the contribution's
-  // size (not including the size of the length field), the DWARF version and
-  // 2 bytes of padding.
-  Asm->emitInt32(StrPool.size() * EntrySize + 4);
-  Asm->emitInt16(Asm->getDwarfVersion());
-  Asm->emitInt16(0);
-  // Define the symbol that marks the start of the contribution. It is
-  // referenced by most unit headers via DW_AT_str_offsets_base.
-  // Split units do not use the attribute.
-  if (StringOffsetsStartSym)
-    Asm->OutStreamer->EmitLabel(StringOffsetsStartSym);
-}
-
 // Emit the various dwarf units to the unit section USection with
 // the abbreviations going into ASection.
 void DwarfFile::emitUnits(bool UseOffsets) {
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfFile.h b/llvm/lib/CodeGen/AsmPrinter/DwarfFile.h
index 272a26851b7..8dfbc4e1c43 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfFile.h
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfFile.h
@@ -91,9 +91,6 @@ public:
   /// Add a unit to the list of CUs.
   void addUnit(std::unique_ptr<DwarfCompileUnit> U);
 
-  /// Emit the string table offsets header.
-  void emitStringOffsetsTableHeader(MCSection *Section);
-
   /// Emit all of the units to the section listed with the given
   /// abbreviation section.
   void emitUnits(bool UseOffsets);
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfStringPool.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfStringPool.cpp
index f2964673a6b..a61fa83cfb0 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfStringPool.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfStringPool.cpp
@@ -39,6 +39,28 @@ DwarfStringPool::EntryRef DwarfStringPool::getEntry(AsmPrinter &Asm,
   return EntryRef(*I.first);
 }
 
+void DwarfStringPool::emitStringOffsetsTableHeader(AsmPrinter &Asm,
+                                                   MCSection *Section,
+                                                   MCSymbol *StartSym) {
+  if (empty())
+    return;
+  Asm.OutStreamer->SwitchSection(Section);
+  unsigned EntrySize = 4;
+  // FIXME: DWARF64
+  // We are emitting the header for a contribution to the string offsets
+  // table. The header consists of an entry with the contribution's
+  // size (not including the size of the length field), the DWARF version and
+  // 2 bytes of padding.
+  Asm.emitInt32(size() * EntrySize + 4);
+  Asm.emitInt16(Asm.getDwarfVersion());
+  Asm.emitInt16(0);
+  // Define the symbol that marks the start of the contribution. It is
+  // referenced by most unit headers via DW_AT_str_offsets_base.
+  // Split units do not use the attribute.
+  if (StartSym)
+    Asm.OutStreamer->EmitLabel(StartSym);
+}
+
 void DwarfStringPool::emit(AsmPrinter &Asm, MCSection *StrSection,
                            MCSection *OffsetSection, bool UseRelativeOffsets) {
   if (Pool.empty())
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfStringPool.h b/llvm/lib/CodeGen/AsmPrinter/DwarfStringPool.h
index 069c124d1a7..6e6988ea4ad 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfStringPool.h
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfStringPool.h
@@ -19,6 +19,7 @@ namespace llvm {
 
 class AsmPrinter;
 class MCSection;
+class MCSymbol;
 
 // Collection of strings for this unit and assorted symbols.
 // A String->Symbol mapping of strings used by indirect
@@ -36,6 +37,9 @@ public:
 
   DwarfStringPool(BumpPtrAllocator &A, AsmPrinter &Asm, StringRef Prefix);
 
+  void emitStringOffsetsTableHeader(AsmPrinter &Asm, MCSection *OffsetSection,
+                                    MCSymbol *StartSym);
+
   void emit(AsmPrinter &Asm, MCSection *StrSection,
             MCSection *OffsetSection = nullptr,
             bool UseRelativeOffsets = false);