[PATCH] D54497: [ELF] --gdb-index: extract entities from .debug_info when .debug_gnu_pubnames is absent

[Fangrui Song via Phabricator via llvm-commits]

MaskRay created this revision.
MaskRay added reviewers: ruiu, dblaikie, saugustine, echristo.
Herald added subscribers: llvm-commits, arphaman, arichardson, emaste.
Herald added a reviewer: espindola.

When .debug_gnu_pubnames is absent, gold traverses .debug_info to
extract entities to make up symbol table entries. This patch implements
a subset of it to improve .gdb_index coverage.


Repository:
  rLLD LLVM Linker

https://reviews.llvm.org/D54497

Files:
  ELF/SyntheticSections.cpp
  test/ELF/gdb-index-debug-info.s


Index: test/ELF/gdb-index-debug-info.s
===================================================================
--- /dev/null
+++ test/ELF/gdb-index-debug-info.s
@@ -0,0 +1,33 @@
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
+# RUN: ld.lld --gdb-index %t.o -o %t
+# RUN: llvm-dwarfdump -gdb-index %t | FileCheck %s
+
+# CHECK: String name: __libc_csu_init
+
+.section .debug_abbrev,"", at progbits
+	.byte	1              # Abbreviation Code
+	.byte	17             # DW_TAG_compile_unit
+	.byte	1              # DW_CHILDREN_yes
+	.byte	0              # EOM(1)
+	.byte	0              # EOM(2)
+	.byte	2              # Abbreviation Code
+	.byte	46             # DW_TAG_subprogram
+	.byte	0              # DW_CHILDREN_no
+	.byte	3              # DW_AT_name
+	.byte	8              # DW_FORM_string
+	.byte	0              # EOM(1)
+	.byte	0              # EOM(2)
+	.byte	0
+
+.section .debug_info,"", at progbits
+.Lcu_begin0:
+	.long	.Lcu_end0 - .Lcu_begin0 - 4
+	.short	4              # DWARF version number
+	.long	0              # Offset Into Abbrev. Section
+	.byte	4              # Address Size
+.Ldie:
+	.byte	1              # Abbrev [1] DW_TAG_compile_unit
+	.byte	2              # Abbrev [2] DW_TAG_subprogram
+	.asciz	"__libc_csu_init"  # DW_AT_name
+	.byte	0
+.Lcu_end0:
Index: ELF/SyntheticSections.cpp
===================================================================
--- ELF/SyntheticSections.cpp
+++ ELF/SyntheticSections.cpp
@@ -2412,6 +2412,34 @@
   return Ret;
 }
 
+// gold/gdb-index.cc Gdb_index_info_reader::visit_die traverses .debug_info to
+// extract symbols when .debug_gnu_pubnames is absent. This function implements
+// a subset of it to improve coverage of the .gdb_index symbol table.
+static void extractSymbol(DWARFDie D, uint32_t Idx,
+                          std::vector<GdbIndexSection::NameAttrEntry> &Ret) {
+  switch (D.getTag()) {
+  case DW_TAG_constant:
+  case DW_TAG_enumerator:
+  case DW_TAG_subprogram:
+  case DW_TAG_variable: {
+    // Ignore mangled names for now.
+    if (StringRef(D.getName(DINameKind::LinkageName)).startswith("_Z"))
+      break;
+    // As of 2018-11, gold uses 0 as the attributes part. We follow it here.
+    if (const char *Name = D.getName(DINameKind::ShortName))
+      Ret.push_back({{Name, computeGdbHash(Name)}, Idx});
+    break;
+  }
+  default:
+    break;
+  }
+  DWARFDie Child = D.getFirstChild();
+  while (Child) {
+    extractSymbol(Child, Idx, Ret);
+    Child = Child.getSibling();
+  }
+}
+
 template <class ELFT>
 static std::vector<GdbIndexSection::NameAttrEntry>
 readPubNamesAndTypes(const LLDDwarfObj<ELFT> &Obj,
@@ -2544,6 +2572,13 @@
     NameAttrs[I] = readPubNamesAndTypes<ELFT>(
         static_cast<const LLDDwarfObj<ELFT> &>(Dwarf.getDWARFObj()),
         Chunks[I].CompilationUnits);
+    if (NameAttrs[I].empty()) {
+      uint32_t Idx = 0;
+      for (const auto &U : Dwarf.info_section_units())
+        if (DWARFDie CUDie = U->getUnitDIE(false))
+          extractSymbol(CUDie, Idx, NameAttrs[I]);
+      ++Idx;
+    }
   });
 
   auto *Ret = make<GdbIndexSection>();


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