[Lldb-commits] [lldb] Improve debug names index fetching global variables performance (PR #70231)

[via lldb-commits]

https://github.com/jeffreytan81 created https://github.com/llvm/llvm-project/pull/70231

While using dwarf5 `.debug_names` in internal large targets, we noticed a performance issue (around 10 seconds delay) while `lldb-vscode` tries to show `scopes` for a compile unit. Profiling shows the bottleneck is inside `DebugNamesDWARFIndex::GetGlobalVariables` which linearly search all index entries belongs to a compile unit. 

This patch improves the performance by using the compile units list to filter first before checking index entries. This significantly improves the performance (drops from 10 seconds => under 1 second) in the split dwarf situation because each compile unit has its own named index. 




>From c8d9a1f7387e3e4944d92ccb33699b6a9d0dcf89 Mon Sep 17 00:00:00 2001
From: jeffreytan81 <jeffreytan at fb.com>
Date: Wed, 25 Oct 2023 10:21:06 -0700
Subject: [PATCH] Improve debug names index fetching global variables
 performance

---
 .../Plugins/SymbolFile/DWARF/DebugNamesDWARFIndex.cpp | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/lldb/source/Plugins/SymbolFile/DWARF/DebugNamesDWARFIndex.cpp b/lldb/source/Plugins/SymbolFile/DWARF/DebugNamesDWARFIndex.cpp
index 292ea2806c59dc7..f94491b29bdbe14 100644
--- a/lldb/source/Plugins/SymbolFile/DWARF/DebugNamesDWARFIndex.cpp
+++ b/lldb/source/Plugins/SymbolFile/DWARF/DebugNamesDWARFIndex.cpp
@@ -130,6 +130,17 @@ void DebugNamesDWARFIndex::GetGlobalVariables(
   uint64_t cu_offset = cu.GetOffset();
   bool found_entry_for_cu = false;
   for (const DebugNames::NameIndex &ni: *m_debug_names_up) {
+    // Check if this name index contains an entry for the given CU.
+    bool has_match_cu = false;
+    for (uint32_t i = 0;i < ni.getCUCount();++i) {
+      if (ni.getCUOffset(i) == cu_offset) {
+        has_match_cu = true;
+        break;
+      }
+    }
+    if (!has_match_cu)
+      continue;
+
     for (DebugNames::NameTableEntry nte: ni) {
       uint64_t entry_offset = nte.getEntryOffset();
       llvm::Expected<DebugNames::Entry> entry_or = ni.getEntry(&entry_offset);