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, 22 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfStringPool.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfStringPool.cpp
index f296467..a61fa83 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())