[DebugInfo] Generate fixups as emitting DWARF .debug_line.

It is necessary to generate fixups in .debug_line as relaxation is
enabled due to the address delta may be changed after relaxation.

DWARF will record the mappings of lines and addresses in
.debug_line section. It will encode the information using special
opcodes, standard opcodes and extended opcodes in Line Number
Program. I use DW_LNS_fixed_advance_pc to encode fixed length
address delta and DW_LNE_set_address to encode absolute address
to make it possible to generate fixups in .debug_line section.

Differential Revision: https://reviews.llvm.org/D46850

llvm-svn: 338477

1 files changed, 51 insertions, 0 deletions

diff --git a/llvm/lib/MC/MCDwarf.cpp b/llvm/lib/MC/MCDwarf.cpp
index a02cddb5a0b..0461c2564cc 100644
--- a/llvm/lib/MC/MCDwarf.cpp
+++ b/llvm/lib/MC/MCDwarf.cpp
@@ -731,6 +731,57 @@ void MCDwarfLineAddr::Encode(MCContext &Context, MCDwarfLineTableParams Params,
   }
 }
 
+bool MCDwarfLineAddr::FixedEncode(MCContext &Context,
+                                  MCDwarfLineTableParams Params,
+                                  int64_t LineDelta, uint64_t AddrDelta,
+                                  raw_ostream &OS,
+                                  uint32_t *Offset, uint32_t *Size) {
+  if (LineDelta != INT64_MAX) {
+    OS << char(dwarf::DW_LNS_advance_line);
+    encodeSLEB128(LineDelta, OS);
+  }
+
+  // Use address delta to adjust address or use absolute address to adjust
+  // address.
+  bool SetDelta;
+  // According to DWARF spec., the DW_LNS_fixed_advance_pc opcode takes a
+  // single uhalf (unencoded) operand. So, the maximum value of AddrDelta
+  // is 65535. We set a conservative upper bound for it for relaxation.
+  if (AddrDelta > 60000) {
+    const MCAsmInfo *asmInfo = Context.getAsmInfo();
+    unsigned AddrSize = asmInfo->getCodePointerSize();
+
+    OS << char(dwarf::DW_LNS_extended_op);
+    encodeULEB128(1 + AddrSize, OS);
+    OS << char(dwarf::DW_LNE_set_address);
+    // Generate fixup for the address.
+    *Offset = OS.tell();
+    *Size = AddrSize;
+    SetDelta = false;
+    std::vector<uint8_t> FillData;
+    FillData.insert(FillData.begin(), AddrSize, 0);
+    OS.write(reinterpret_cast<char *>(FillData.data()), AddrSize);
+  } else {
+    OS << char(dwarf::DW_LNS_fixed_advance_pc);
+    // Generate fixup for 2-bytes address delta.
+    *Offset = OS.tell();
+    *Size = 2;
+    SetDelta = true;
+    OS << char(0);
+    OS << char(0);
+  }
+
+  if (LineDelta == INT64_MAX) {
+    OS << char(dwarf::DW_LNS_extended_op);
+    OS << char(1);
+    OS << char(dwarf::DW_LNE_end_sequence);
+  } else {
+    OS << char(dwarf::DW_LNS_copy);
+  }
+
+  return SetDelta;
+}
+
 // Utility function to write a tuple for .debug_abbrev.
 static void EmitAbbrev(MCStreamer *MCOS, uint64_t Name, uint64_t Form) {
   MCOS->EmitULEB128IntValue(Name);