[lldb][NFCI] Use llvm's libDebugInfo for DebugRanges

[fdeazeve]

Summary

In an effort to unify the different dwarf parsers available in the codebase,
this commit removes LLDB's custom parsing for the .debug_ranges DWARF section,
instead calling into LLVM's parser.

Subsequent work should look into unifying llvm::DWARDebugRangeList (whose
entries are pairs of (start, end) addresses) with lldb::DWARFRangeList (whose
entries are pairs of (start, length)). The lists themselves are also different
data structures, but functionally equivalent.

Depends on D150363

[bulbazord]

Ok, this looks like it's doing the same thing to me which is good. My understanding of this change is that you're changing lldb::DWARFDebugRanges::Extract to use llvm::DWARFDebugRangeList instead of lldb::DWARFRangeList.

Out of curiosity, do you have an idea of the change to performance (if any)? I wouldn't expect it to be very different if at all because I don't think the algorithms between lldb and llvm are different but it would be nice to make sure.

[fdeazeve]

In D150366#4336168, @bulbazord wrote:

Ok, this looks like it's doing the same thing to me which is good. My understanding of this change is that you're changing lldb::DWARFDebugRanges::Extract to use llvm::DWARFDebugRangeList instead of lldb::DWARFRangeList.

Out of curiosity, do you have an idea of the change to performance (if any)? I wouldn't expect it to be very different if at all because I don't think the algorithms between lldb and llvm are different but it would be nice to make sure.

I ran an experiment with a C++ project that forced LLDB to parse roughly 30,000 range entries and no significant differences were detected:

// LLDB's implementation
 Time (mean ± σ):     695.2 ms ±   7.9 ms    [User: 157.8 ms, System: 69.0 ms]
 Range (min … max):   678.7 ms … 703.1 ms    10 runs
// LLVM's implementation
 Time (mean ± σ):     694.9 ms ±   9.7 ms    [User: 158.8 ms, System: 70.5 ms]
 Range (min … max):   675.2 ms … 711.7 ms    10 runs

[JDevlieghere]

LGTM

lldb/source/Plugins/SymbolFile/DWARF/DWARFDebugRanges.h

 public:
   DWARFDebugRanges();
 
   void Extract(lldb_private::DWARFContext &context);
   DWARFRangeList FindRanges(const DWARFUnit *cu,
                             dw_offset_t debug_ranges_offset) const;
 
 protected:
-  bool Extract(lldb_private::DWARFContext &context, lldb::offset_t *offset_ptr,
-               DWARFRangeList &range_list);
-
   std::map<dw_offset_t, DWARFRangeList> m_range_map;
 };
 
 #endif // LLDB_SOURCE_PLUGINS_SYMBOLFILE_DWARF_DWARFDEBUGRANGES_H

lldb/source/Plugins/SymbolFile/DWARF/DWARFDebugRanges.cpp

 //===-- DWARFDebugRanges.cpp ----------------------------------------------===//
 //
 // 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 "DWARFDebugRanges.h"
 #include "DWARFUnit.h"
+#include "llvm/DebugInfo/DWARF/DWARFDebugRangeList.h"
 
 using namespace lldb_private;
 
-static dw_addr_t GetBaseAddressMarker(uint32_t addr_size) {
-  switch(addr_size) {
-    case 2:
-      return 0xffff;
-    case 4:
-      return 0xffffffff;
-    case 8:
-      return 0xffffffffffffffff;
-  }
-  llvm_unreachable("GetBaseAddressMarker unsupported address size.");
-}
-
 DWARFDebugRanges::DWARFDebugRanges() : m_range_map() {}
 
 void DWARFDebugRanges::Extract(DWARFContext &context) {
-  DWARFRangeList range_list;
-  lldb::offset_t offset = 0;
-  dw_offset_t debug_ranges_offset = offset;
-  while (Extract(context, &offset, range_list)) {
-    range_list.Sort();
-    m_range_map[debug_ranges_offset] = range_list;
-    debug_ranges_offset = offset;
-  }
+  llvm::DWARFDataExtractor extractor =
+      context.getOrLoadRangesData().GetAsLLVM();
+  llvm::DWARFDebugRangeList extracted_list;
+  uint64_t current_offset = 0;
+  auto extract_next_list = [&] {
+    if (auto error = extracted_list.extract(extractor, &current_offset)) {
+      consumeError(std::move(error));
+      return false;
     }
-
-bool DWARFDebugRanges::Extract(DWARFContext &context,
-                               lldb::offset_t *offset_ptr,
-                               DWARFRangeList &range_list) {
-  range_list.Clear();
-
-  lldb::offset_t range_offset = *offset_ptr;
-  const DWARFDataExtractor &debug_ranges_data = context.getOrLoadRangesData();
-  uint32_t addr_size = debug_ranges_data.GetAddressByteSize();
-  dw_addr_t base_addr = 0;
-  dw_addr_t base_addr_marker = GetBaseAddressMarker(addr_size);
-
-  while (
-      debug_ranges_data.ValidOffsetForDataOfSize(*offset_ptr, 2 * addr_size)) {
-    dw_addr_t begin = debug_ranges_data.GetMaxU64(offset_ptr, addr_size);
-    dw_addr_t end = debug_ranges_data.GetMaxU64(offset_ptr, addr_size);
-
-    if (!begin && !end) {
-      // End of range list
-      break;
-    }
-
-    if (begin == base_addr_marker) {
-      base_addr = end;
-      continue;
-    }
-
-    // Filter out empty ranges
-    if (begin < end)
-      range_list.Append(DWARFRangeList::Entry(begin + base_addr, end - begin));
-  }
-
-  // Make sure we consumed at least something
-  return range_offset != *offset_ptr;
+    return true;
+  };
+
+  uint64_t previous_offset = current_offset;
+  while (extractor.isValidOffset(current_offset) && extract_next_list()) {
+    DWARFRangeList &lldb_range_list = m_range_map[previous_offset];
+    lldb_range_list.Reserve(extracted_list.getEntries().size());
+    for (auto &range : extracted_list.getEntries())
+      lldb_range_list.Append(range.StartAddress,
+                             range.EndAddress - range.StartAddress);
+    lldb_range_list.Sort();
+    previous_offset = current_offset;
 }
 
 DWARFRangeList
 DWARFDebugRanges::FindRanges(const DWARFUnit *cu,
                              dw_offset_t debug_ranges_offset) const {
   dw_addr_t debug_ranges_address = cu->GetRangesBase() + debug_ranges_offset;
   auto pos = m_range_map.find(debug_ranges_address);
   DWARFRangeList ans =
       pos == m_range_map.end() ? DWARFRangeList() : pos->second;
 
   // All DW_AT_ranges are relative to the base address of the compile
   // unit. We add the compile unit base address to make sure all the
   // addresses are properly fixed up.
   ans.Slide(cu->GetBaseAddress());
   return ans;
 }