[Lldb-commits] [lldb] Improve type and namespace lookup using parent chain (PR #108907)

[Felipe de Azevedo Piovezan via lldb-commits]

================
@@ -374,25 +377,40 @@ void DebugNamesDWARFIndex::GetFullyQualifiedType(
   m_fallback.GetFullyQualifiedType(context, callback);
 }
 
+bool DebugNamesDWARFIndex::SameAsEntryATName(
+    llvm::StringRef query_name, const DebugNames::Entry &entry) const {
+  auto maybe_dieoffset = entry.getDIEUnitOffset();
+  if (!maybe_dieoffset)
+    return false;
+
+  // [Optimization] instead of parsing the entry from dwo file, we simply
+  // check if the query_name can point to an entry of the same DIE offset.
+  // This greatly reduced number of dwo file parsed and thus improved the
+  // performance.
+  for (const DebugNames::Entry &query_entry :
+       entry.getNameIndex()->equal_range(query_name)) {
+    auto query_dieoffset = query_entry.getDIEUnitOffset();
+    if (!query_dieoffset)
+      continue;
+
+    if (*query_dieoffset == *maybe_dieoffset) {
+      return true;
+    } else if (*query_dieoffset > *maybe_dieoffset) {
+      // The pool entries of the same name are sequentially cluttered together
+      // so if the query name from `query_name` is after the target entry, this
+      // is definitely not the correct one, we can stop searching.
+      return false;
+    }
+  }
+  return false;
+}
+
 bool DebugNamesDWARFIndex::SameParentChain(
     llvm::ArrayRef<llvm::StringRef> parent_names,
     llvm::ArrayRef<DebugNames::Entry> parent_entries) const {
-
   if (parent_entries.size() != parent_names.size())
     return false;
 
-  auto SameAsEntryATName = [this](llvm::StringRef name,
-                                  const DebugNames::Entry &entry) {
-    // Peek at the AT_name of `entry` and test equality to `name`.
-    auto maybe_dieoffset = entry.getDIEUnitOffset();
-    if (!maybe_dieoffset)
-      return false;
-    DWARFUnit *unit = GetNonSkeletonUnit(entry);
-    if (!unit)
-      return false;
-    return name == unit->PeekDIEName(unit->GetOffset() + *maybe_dieoffset);
----------------
felipepiovezan wrote:

I think this is the core of your proposal, right? In the original implementation that I did, the vast majority of the savings was thanks to being able to do `PeekDIEName`. I would strongly recommend we benchmark this new implementation is the non-dwo case as well.

One way to check is to build with regular object files (the non dwo case) and repeat the experiments I described in this reply: 
https://discourse.llvm.org/t/rfc-improve-dwarf-5-debug-names-type-lookup-parsing-speed/74151/38?u=felipe
And instrumenting this portion of the code: https://github.com/llvm/llvm-project/pull/75365/commits/837bc0f476820f9ee5695223420105dedfea8d79

https://github.com/llvm/llvm-project/pull/108907