From 0bf3df28b9479583d045588341221c4bd67edc0b Mon Sep 17 00:00:00 2001
From: Andreas Kling <kling@serenityos.org>
Date: Sun, 23 May 2021 23:01:11 +0200
Subject: [PATCH] LibDebug: Memoize resolved paths in
 DebugInfo::prepare_lines()

When loading debug info, we encounter the same filename over and over
(since files usually have many lines!) and we were wasting a ton of
time re-checking if the filename was part of the Toolchain or libgcc,
along with some other checks.

This patch makes prepare_lines() significantly faster by memoizing
the result of these checks per filename.

This makes "bt 12" ~25% faster (from 850ms to 650ms on my machine.) :^)
---
 Userland/Libraries/LibDebug/DebugInfo.cpp | 32 ++++++++++++++---------
 1 file changed, 20 insertions(+), 12 deletions(-)

diff --git a/Userland/Libraries/LibDebug/DebugInfo.cpp b/Userland/Libraries/LibDebug/DebugInfo.cpp
index a3b03cfebc..4af2ea5c09 100644
--- a/Userland/Libraries/LibDebug/DebugInfo.cpp
+++ b/Userland/Libraries/LibDebug/DebugInfo.cpp
@@ -90,20 +90,28 @@ void DebugInfo::prepare_lines()
         all_lines.append(program.lines());
     }
 
-    String serenity_slash("serenity/");
+    HashMap<FlyString, Optional<String>> memoized_full_paths;
+    auto compute_full_path = [&](FlyString const& file_path) -> Optional<String> {
+        if (file_path.view().contains("Toolchain/"sv) || file_path.view().contains("libgcc"sv))
+            return {};
+        if (file_path.view().starts_with("./"sv) && !m_source_root.is_null())
+            return LexicalPath::join(m_source_root, file_path).string();
+        if (auto index_of_serenity_slash = file_path.view().find("serenity/"sv); index_of_serenity_slash.has_value()) {
+            auto start_index = index_of_serenity_slash.value() + "serenity/"sv.length();
+            return file_path.view().substring_view(start_index, file_path.length() - start_index);
+        }
+        return file_path;
+    };
 
-    for (auto& line_info : all_lines) {
-        String file_path = line_info.file;
-        if (file_path.contains("Toolchain/") || file_path.contains("libgcc"))
+    for (auto const& line_info : all_lines) {
+        auto it = memoized_full_paths.find(line_info.file);
+        if (it == memoized_full_paths.end()) {
+            memoized_full_paths.set(line_info.file, compute_full_path(line_info.file));
+            it = memoized_full_paths.find(line_info.file);
+        }
+        if (!it->value.has_value())
             continue;
-        if (file_path.contains(serenity_slash)) {
-            auto start_index = file_path.index_of(serenity_slash).value() + serenity_slash.length();
-            file_path = file_path.substring(start_index, file_path.length() - start_index);
-        }
-        if (file_path.starts_with("./") && !m_source_root.is_null()) {
-            file_path = LexicalPath::canonicalized_path(String::formatted("{}/{}", m_source_root, file_path));
-        }
-        m_sorted_lines.append({ line_info.address, file_path, line_info.line });
+        m_sorted_lines.append({ line_info.address, it->value.value(), line_info.line });
     }
     quick_sort(m_sorted_lines, [](auto& a, auto& b) {
         return a.address < b.address;