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

1 files changed, 6 insertions, 4 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