UserspaceEmulator: Add lazy caching of debug info for shared libraries

Keep the debug symbols for shared libraries in memory after we opened
them the first time. This dramatically speeds up symbolication of
backtraces when running dynamically linked programs in UE.

DevTools/UserspaceEmulator/Emulator.cpp

@@ -302,24 +302,29 @@ String Emulator::create_backtrace_line(FlatPtr address)
     String lib_name = region->name().substring(0, separator_index.value());
     String lib_path = lib_name;
     if (region->name().contains(".so"))
-        lib_path = String::format("/usr/lib/%s", lib_path.characters());
+        lib_path = String::formatted("/usr/lib/{}", lib_path);
 
-    auto mapped_file = make<MappedFile>(lib_path);
-    if (!mapped_file->is_valid())
-        return minimal;
+    if (!m_dynamic_library_cache.contains(lib_path)) {
+        MappedFile mapped_file { lib_path };
+        if (!mapped_file.is_valid())
+            return minimal;
+
+        auto loader = ELF::Loader::create((const u8*)mapped_file.data(), mapped_file.size());
+        auto debug_info = make<Debug::DebugInfo>(loader);
+        m_dynamic_library_cache.set(lib_path, CachedELF { move(mapped_file), move(loader), move(debug_info) });
+    }
 
-    auto loader = ELF::Loader::create((const u8*)mapped_file->data(), mapped_file->size());
-    String symbol = loader->symbolicate(address - region->base());
+    auto it = m_dynamic_library_cache.find(lib_path);
+    auto& loader = *it->value.elf_loader;
+    String symbol = loader.symbolicate(address - region->base());
 
     auto line_without_source_info = String::format("=={%d}==    %p  [%s]: %s", getpid(), address, lib_name.characters(), symbol.characters());
-    auto debug_info = make<Debug::DebugInfo>(loader);
 
-    auto source_position = debug_info->get_source_position(address - region->base());
+    auto source_position = it->value.debug_info->get_source_position(address - region->base());
     if (source_position.has_value())
         return String::format("=={%d}==    %p  [%s]: %s (\033[34;1m%s\033[0m:%u)", getpid(), address, lib_name.characters(), symbol.characters(), LexicalPath(source_position.value().file_path).basename().characters(), source_position.value().line_number);
-    else {
-        return line_without_source_info;
-    }
+
+    return line_without_source_info;
 }
 
 void Emulator::dump_backtrace(const Vector<FlatPtr>& backtrace)

DevTools/UserspaceEmulator/Emulator.h

@@ -30,6 +30,7 @@
 #include "Report.h"
 #include "SoftCPU.h"
 #include "SoftMMU.h"
+#include <AK/MappedFile.h>
 #include <AK/Types.h>
 #include <LibDebug/DebugInfo.h>
 #include <LibELF/AuxiliaryVector.h>
@@ -200,6 +201,14 @@ private:
     FlatPtr m_signal_trampoline { 0 };
     Optional<FlatPtr> m_loader_text_base;
     Optional<size_t> m_loader_text_size;
+
+    struct CachedELF {
+        MappedFile mapped_file;
+        NonnullRefPtr<ELF::Loader> elf_loader;
+        OwnPtr<Debug::DebugInfo> debug_info;
+    };
+
+    HashMap<String, CachedELF> m_dynamic_library_cache;
 };
 
 ALWAYS_INLINE bool Emulator::is_in_malloc_or_free() const