DWARFContext: Return empty data extractors instead of null pointers

[labath]

Summary

There are several reasons for doing this:

• generally, there's no reason to differentiate between a section being absent and it being present, but empty
• it matches more closely what llvm DWARF parser is doing (which also doesn't differentiate the two cases)
• SymbolFileDWARF also doesn't differentiate the two cases, which makes porting the rest of sections easier
• it fixes a bug in how the return-null-if-empty logic was implemented (it returned nullptr only the second time we tried to get the debug_aranges section), which meant that we hit an assert when trying to parse an empty-but-present section

[aprantl]

source/Plugins/SymbolFile/DWARF/DWARFContext.h

26

lgtm

source/Plugins/SymbolFile/DWARF/DWARFDebugAranges.cpp

44–45

lgtm

source/Plugins/SymbolFile/DWARF/DWARFDebugInfo.cpp

49–50

ltgm

lit/SymbolFile/DWARF/debug_aranges-empty-section.s

+# Test that an empty .debug_aranges section doesn't confuse (or crash) us.
+
+# RUN: llvm-mc %s -triple x86_64-pc-linux -filetype=obj >%t
+# RUN: lldb %t -o "breakpoint set -n f" -b | FileCheck %s
+
+# CHECK: Breakpoint 1: where = {{.*}}`f, address = 0x0000000000000047
+
+  .text
+  .globl  f                       # -- Begin function f
+  .type f,@function
+        .rept 0x47
+        nop
+        .endr
+f:                                      # @f
+.Lfunc_begin0:
+  retq
+.Lfunc_end0:
+  .size f, .Lfunc_end0-f
+                                        # -- End function
+  .section  .debug_str,"MS",@progbits,1
+.Linfo_string3:
+  .asciz  "f"                     # string offset=83
+  .section  .debug_abbrev,"",@progbits
+  .byte 1                       # Abbreviation Code
+  .byte 17                      # DW_TAG_compile_unit
+  .byte 1                       # DW_CHILDREN_yes
+  .byte 17                      # DW_AT_low_pc
+  .byte 1                       # DW_FORM_addr
+  .byte 18                      # DW_AT_high_pc
+  .byte 6                       # DW_FORM_data4
+  .byte 0                       # EOM(1)
+  .byte 0                       # EOM(2)
+  .byte 2                       # Abbreviation Code
+  .byte 46                      # DW_TAG_subprogram
+  .byte 0                       # DW_CHILDREN_no
+  .byte 17                      # DW_AT_low_pc
+  .byte 1                       # DW_FORM_addr
+  .byte 18                      # DW_AT_high_pc
+  .byte 6                       # DW_FORM_data4
+  .byte 3                       # DW_AT_name
+  .byte 14                      # DW_FORM_strp
+  .byte 0                       # EOM(1)
+  .byte 0                       # EOM(2)
+  .byte 0                       # EOM(3)
+  .section  .debug_info,"",@progbits
+.Lcu_begin0:
+  .long .Ldebug_info_end0-.Ldebug_info_start0 # Length of Unit
+.Ldebug_info_start0:
+  .short  4                       # DWARF version number
+  .long .debug_abbrev           # Offset Into Abbrev. Section
+  .byte 8                       # Address Size (in bytes)
+  .byte 1                       # Abbrev [1] 0xb:0x35 DW_TAG_compile_unit
+  .quad .Lfunc_begin0           # DW_AT_low_pc
+  .long .Lfunc_end0-.Lfunc_begin0 # DW_AT_high_pc
+  .byte 2                       # Abbrev [2] 0x2a:0x15 DW_TAG_subprogram
+  .quad .Lfunc_begin0           # DW_AT_low_pc
+  .long .Lfunc_end0-.Lfunc_begin0 # DW_AT_high_pc
+  .long .Linfo_string3          # DW_AT_name
+  .byte 0                       # End Of Children Mark
+.Ldebug_info_end0:
+
+  .section  ".note.GNU-stack","",@progbits
+  .section  .debug_aranges,"",@progbits

source/Plugins/SymbolFile/DWARF/DWARFContext.h

 class DWARFContext {
 private:
   Module &m_module;
   llvm::Optional<DWARFDataExtractor> m_data_debug_aranges;
 
 public:
   explicit DWARFContext(Module &module);
 
-  const DWARFDataExtractor *getOrLoadArangesData();
+  const DWARFDataExtractor &getOrLoadArangesData();
 };
 } // namespace lldb_private
 
 #endif

source/Plugins/SymbolFile/DWARF/DWARFContext.cpp

 //===-- DWARFContext.cpp ----------------------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "DWARFContext.h"
 
 #include "lldb/Core/Section.h"
 
 using namespace lldb;
 using namespace lldb_private;
 
-static const DWARFDataExtractor *
-LoadOrGetSection(Module &module, SectionType section_type,
-                 llvm::Optional<DWARFDataExtractor> &extractor) {
-  if (extractor.hasValue())
-    return extractor->GetByteSize() > 0 ? extractor.getPointer() : nullptr;
-
-  // Initialize to an empty extractor so that we always take the fast path going
-  // forward.
-  extractor.emplace();
-
-  const SectionList *section_list = module.GetSectionList();
+static DWARFDataExtractor LoadSection(Module &module,
+                                      SectionType section_type) {
+  SectionList *section_list = module.GetSectionList();
   if (!section_list)
-    return nullptr;
+    return DWARFDataExtractor();
 
   auto section_sp = section_list->FindSectionByType(section_type, true);
   if (!section_sp)
-    return nullptr;
+    return DWARFDataExtractor();
 
-  section_sp->GetSectionData(*extractor);
-  return extractor.getPointer();
+  DWARFDataExtractor data;
+  section_sp->GetSectionData(data);
+  return data;
+}
+
+static const DWARFDataExtractor &
+LoadOrGetSection(Module &module, SectionType section_type,
+                 llvm::Optional<DWARFDataExtractor> &extractor) {
+  if (!extractor)
+    extractor = LoadSection(module, section_type);
+  return *extractor;
 }
 
 DWARFContext::DWARFContext(Module &module) : m_module(module) {}
 
-const DWARFDataExtractor *DWARFContext::getOrLoadArangesData() {
+const DWARFDataExtractor &DWARFContext::getOrLoadArangesData() {
   return LoadOrGetSection(m_module, eSectionTypeDWARFDebugAranges,
                           m_data_debug_aranges);
 }

source/Plugins/SymbolFile/DWARF/DWARFDebugAranges.cpp

public:
   void operator()(const DWARFDebugArangeSet &set) {
     count += set.NumDescriptors();
   }
   uint32_t &count;
 };
 
 // Extract
 llvm::Error
 DWARFDebugAranges::extract(const DWARFDataExtractor &debug_aranges_data) {
-  assert(debug_aranges_data.ValidOffset(0));
-
   lldb::offset_t offset = 0;
 
   DWARFDebugArangeSet set;
   Range range;
   while (debug_aranges_data.ValidOffset(offset)) {
     llvm::Error error = set.extract(debug_aranges_data, &offset);
     if (!error)
       return error;
 
     const uint32_t num_descriptors = set.NumDescriptors();
     if (num_descriptors > 0) {
       const dw_offset_t cu_offset = set.GetHeader().cu_offset;
 
       for (uint32_t i = 0; i < num_descriptors; ++i) {
         const DWARFDebugArangeSet::Descriptor &descriptor =
             set.GetDescriptorRef(i);
         m_aranges.Append(RangeToDIE::Entry(descriptor.address,
                                            descriptor.length, cu_offset));
       }
     }
     set.Clear();
-    }
-    return llvm::ErrorSuccess();
 }
 
 void DWARFDebugAranges::Dump(Log *log) const {
   if (log == NULL)
     return;
 
   const size_t num_entries = m_aranges.GetSize();
   for (size_t i = 0; i < num_entries; ++i) {

source/Plugins/SymbolFile/DWARF/DWARFDebugInfo.cpp

 
 llvm::Expected<DWARFDebugAranges &> DWARFDebugInfo::GetCompileUnitAranges() {
   if (m_cu_aranges_up)
     return *m_cu_aranges_up;
 
   assert(m_dwarf2Data);
 
   m_cu_aranges_up = llvm::make_unique<DWARFDebugAranges>();
-  const DWARFDataExtractor *debug_aranges_data =
+  const DWARFDataExtractor &debug_aranges_data =
       m_context.getOrLoadArangesData();
-  if (debug_aranges_data) {
-    llvm::Error error = m_cu_aranges_up->extract(*debug_aranges_data);
-    if (error)
-      return std::move(error);
-  }
+  if (llvm::Error error = m_cu_aranges_up->extract(debug_aranges_data))
 
   // Make a list of all CUs represented by the arange data in the file.
   std::set<dw_offset_t> cus_with_data;
   for (size_t n = 0; n < m_cu_aranges_up->GetNumRanges(); n++) {
     dw_offset_t offset = m_cu_aranges_up->OffsetAtIndex(n);
     if (offset != DW_INVALID_OFFSET)
       cus_with_data.insert(offset);
   }