[llvm] r345326 - DebugInfo: Explain why DW_LLE_(GNU_)startx_length is used

[David Blaikie via llvm-commits]

Author: dblaikie
Date: Thu Oct 25 15:26:25 2018
New Revision: 345326

URL: http://llvm.org/viewvc/llvm-project?rev=345326&view=rev
Log:
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...

Modified:
    llvm/trunk/lib/CodeGen/AsmPrinter/DwarfDebug.cpp

Modified: llvm/trunk/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/AsmPrinter/DwarfDebug.cpp?rev=345326&r1=345325&r2=345326&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/AsmPrinter/DwarfDebug.cpp (original)
+++ llvm/trunk/lib/CodeGen/AsmPrinter/DwarfDebug.cpp Thu Oct 25 15:26:25 2018
@@ -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);