[lld][ELF] Add --debug-names to create merged .debug_names. (#86508)

`clang -g -gpubnames` (with optional -gsplit-dwarf) creates the
`.debug_names` section ("per-CU" index). By default lld concatenates
input `.debug_names` sections into an output `.debug_names` section.
LLDB can consume the concatenated section but the lookup performance is
not good.

This patch adds --debug-names to create a per-module index by combining
the per-CU indexes into a single index that covers the entire load
module. The produced `.debug_names` is a replacement for `.gdb_index`.

Type units (-fdebug-types-section) are not handled yet.

Co-authored-by: Fangrui Song <[email protected]>

---------

Co-authored-by: Fangrui Song <[email protected]>

Author: cmtice

lld/ELF/Config.h

@@ -229,6 +229,7 @@ struct Config {
   bool cref;
   llvm::SmallVector<std::pair<llvm::GlobPattern, uint64_t>, 0>
       deadRelocInNonAlloc;
+  bool debugNames;
   bool demangle = true;
   bool dependentLibraries;
   bool disableVerify;

lld/ELF/DWARF.cpp

@@ -40,6 +40,7 @@ template <class ELFT> LLDDwarfObj<ELFT>::LLDDwarfObj(ObjFile<ELFT> *obj) {
                 .Case(".debug_gnu_pubtypes", &gnuPubtypesSection)
                 .Case(".debug_line", &lineSection)
                 .Case(".debug_loclists", &loclistsSection)
+                .Case(".debug_names", &namesSection)
                 .Case(".debug_ranges", &rangesSection)
                 .Case(".debug_rnglists", &rnglistsSection)
                 .Case(".debug_str_offsets", &strOffsetsSection)

lld/ELF/DWARF.h

@@ -62,6 +62,9 @@ template <class ELFT> class LLDDwarfObj final : public llvm::DWARFObject {
   const LLDDWARFSection &getGnuPubtypesSection() const override {
     return gnuPubtypesSection;
   }
+  const LLDDWARFSection &getNamesSection() const override {
+    return namesSection;
+  }
 
   StringRef getFileName() const override { return ""; }
   StringRef getAbbrevSection() const override { return abbrevSection; }
@@ -87,6 +90,7 @@ template <class ELFT> class LLDDwarfObj final : public llvm::DWARFObject {
   LLDDWARFSection infoSection;
   LLDDWARFSection lineSection;
   LLDDWARFSection loclistsSection;
+  LLDDWARFSection namesSection;
   LLDDWARFSection rangesSection;
   LLDDWARFSection rnglistsSection;
   LLDDWARFSection strOffsetsSection;

lld/ELF/Driver.cpp

@@ -442,6 +442,8 @@ static void checkOptions() {
       error("-r and -pie may not be used together");
     if (config->exportDynamic)
       error("-r and --export-dynamic may not be used together");
+    if (config->debugNames)
+      error("-r and --debug-names may not be used together");
   }
 
   if (config->executeOnly) {
@@ -1234,6 +1236,7 @@ static void readConfigs(opt::InputArgList &args) {
   config->cref = args.hasArg(OPT_cref);
   config->optimizeBBJumps =
       args.hasFlag(OPT_optimize_bb_jumps, OPT_no_optimize_bb_jumps, false);
+  config->debugNames = args.hasFlag(OPT_debug_names, OPT_no_debug_names, false);
   config->demangle = args.hasFlag(OPT_demangle, OPT_no_demangle, true);
   config->dependencyFile = args.getLastArgValue(OPT_dependency_file);
   config->dependentLibraries = args.hasFlag(OPT_dependent_libraries, OPT_no_dependent_libraries, true);

lld/ELF/Options.td

@@ -153,6 +153,10 @@ def : Flag<["--"], "no-color-diagnostics">, Alias<color_diagnostics>, AliasArgs<
 def cref: FF<"cref">,
   HelpText<"Output cross reference table. If -Map is specified, print to the map file">;
 
+defm debug_names: BB<"debug-names",
+    "Generate a merged .debug_names section",
+    "Do not generate a merged .debug_names section (default)">;
+
 defm demangle: B<"demangle",
     "Demangle symbol names (default)",
     "Do not demangle symbol names">;