DebugInfo: Explain why DW_LLE_(GNU_)startx_length is used

This isn't the most object-size efficient encoding, but it's the only
one GDB supports for the pre-standard fission format. I've written fixes
for this twice now... - so perhaps this comment will help me remember
why neither of these have been committed and why I shouldn't try to
write a third fix another year from now...

llvm-svn: 345326

1 files changed, 7 insertions, 4 deletions

diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
index 7f9ef3eba90..4a9ed6d03c6 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
@@ -1962,10 +1962,13 @@ void DwarfDebug::emitDebugLocDWO() {
   for (const auto &List : DebugLocs.getLists()) {
     Asm->OutStreamer->EmitLabel(List.Label);
     for (const auto &Entry : DebugLocs.getEntries(List)) {
-      // Just always use start_length for now - at least that's one address
-      // rather than two. We could get fancier and try to, say, reuse an
-      // address we know we've emitted elsewhere (the start of the function?
-      // The start of the CU or CU subrange that encloses this range?)
+      // GDB only supports startx_length in pre-standard split-DWARF.
+      // (in v5 standard loclists, it currently* /only/ supports base_address +
+      // offset_pair, so the implementations can't really share much since they
+      // need to use different representations)
+      // * as of October 2018, at least
+      // Ideally/in v5, this could use SectionLabels to reuse existing addresses
+      // in the address pool to minimize object size/relocations.
       Asm->emitInt8(dwarf::DW_LLE_startx_length);
       unsigned idx = AddrPool.getIndex(Entry.BeginSym);
       Asm->EmitULEB128(idx);