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LibELF: Cache consecutive lookups for the same symbol

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This reduces the startup time of LibWeb by 10%, and eliminates 156'000
of the total 481'000 global symbol lookups during a self-test run.
This commit is contained in:
Daniel Bertalan 2023-08-17 08:00:08 +02:00 • committed by Andreas Kling
parent 9a07ac0b6a
commit 1adf06c9f0
3 changed files with 31 additions and 6 deletions
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25
Userland/Libraries/LibELF/DynamicLoader.cpp
View file

@ -209,8 +209,9 @@ void DynamicLoader::do_main_relocations()
{
do_relr_relocations();
Optional<DynamicLoader::CachedLookupResult> cached_result;
m_dynamic_object->relocation_section().for_each_relocation([&](DynamicObject::Relocation const& relocation) {
switch (do_direct_relocation(relocation, ShouldInitializeWeak::No, ShouldCallIfuncResolver::No)) {
switch (do_direct_relocation(relocation, cached_result, ShouldInitializeWeak::No, ShouldCallIfuncResolver::No)) {
case RelocationResult::Failed:
dbgln("Loader.so: {} unresolved symbol '{}'", m_filepath, relocation.symbol().name());
VERIFY_NOT_REACHED();
@ -281,8 +282,9 @@ Result<NonnullRefPtr<DynamicObject>, DlErrorMessage> DynamicLoader::load_stage_3
VERIFY(result == RelocationResult::Success);
}
Optional<DynamicLoader::CachedLookupResult> cached_result;
for (auto const& relocation : m_direct_ifunc_relocations) {
auto result = do_direct_relocation(relocation, ShouldInitializeWeak::No, ShouldCallIfuncResolver::Yes);
auto result = do_direct_relocation(relocation, cached_result, ShouldInitializeWeak::No, ShouldCallIfuncResolver::Yes);
VERIFY(result == RelocationResult::Success);
}
@ -321,8 +323,9 @@ void DynamicLoader::load_stage_4()
void DynamicLoader::do_lazy_relocations()
{
Optional<DynamicLoader::CachedLookupResult> cached_result;
for (auto const& relocation : m_unresolved_relocations) {
if (auto res = do_direct_relocation(relocation, ShouldInitializeWeak::Yes, ShouldCallIfuncResolver::Yes); res != RelocationResult::Success) {
if (auto res = do_direct_relocation(relocation, cached_result, ShouldInitializeWeak::Yes, ShouldCallIfuncResolver::Yes); res != RelocationResult::Success) {
dbgln("Loader.so: {} unresolved symbol '{}'", m_filepath, relocation.symbol().name());
VERIFY_NOT_REACHED();
}
@ -522,7 +525,10 @@ void DynamicLoader::load_program_headers()
}
}
DynamicLoader::RelocationResult DynamicLoader::do_direct_relocation(DynamicObject::Relocation const& relocation, ShouldInitializeWeak should_initialize_weak, ShouldCallIfuncResolver should_call_ifunc_resolver)
DynamicLoader::RelocationResult DynamicLoader::do_direct_relocation(DynamicObject::Relocation const& relocation,
Optional<DynamicLoader::CachedLookupResult>& cached_result,
ShouldInitializeWeak should_initialize_weak,
ShouldCallIfuncResolver should_call_ifunc_resolver)
{
FlatPtr* patch_ptr = nullptr;
if (is_dynamic())
@ -534,12 +540,21 @@ DynamicLoader::RelocationResult DynamicLoader::do_direct_relocation(DynamicObjec
return VirtualAddress { reinterpret_cast<DynamicObject::IfuncResolver>(address.get())() };
};
auto lookup_symbol = [&](DynamicObject::Symbol const& symbol) {
// The static linker sorts relocations by the referenced symbol. Especially when vtables
// in large inheritance hierarchies are involved, there might be tens of references to
// the same symbol. We can avoid redundant lookups by keeping track of the previous result.
if (!cached_result.has_value() || !cached_result.value().symbol.definitely_equals(symbol))
cached_result = DynamicLoader::CachedLookupResult { symbol, DynamicLoader::lookup_symbol(symbol) };
return cached_result.value().result;
};
struct ResolvedTLSSymbol {
DynamicObject const& dynamic_object;
FlatPtr value;
};
auto resolve_tls_symbol = [](DynamicObject::Relocation const& relocation) -> Optional<ResolvedTLSSymbol> {
auto resolve_tls_symbol = [&](DynamicObject::Relocation const& relocation) -> Optional<ResolvedTLSSymbol> {
if (relocation.symbol_index() == 0)
return ResolvedTLSSymbol { relocation.dynamic_object(), 0 };

6
Userland/Libraries/LibELF/DynamicLoader.h
View file

@ -143,7 +143,11 @@ private:
ResolveLater = 2,
CallIfuncResolver = 3,
};
RelocationResult do_direct_relocation(DynamicObject::Relocation const&, ShouldInitializeWeak, ShouldCallIfuncResolver);
struct CachedLookupResult {
DynamicObject::Symbol symbol;
Optional<DynamicObject::SymbolLookupResult> result;
};
RelocationResult do_direct_relocation(DynamicObject::Relocation const&, Optional<CachedLookupResult>&, ShouldInitializeWeak, ShouldCallIfuncResolver);
// Will be called from _fixup_plt_entry, as part of the PLT trampoline
static RelocationResult do_plt_relocation(DynamicObject::Relocation const&, ShouldCallIfuncResolver);
void do_relr_relocations();

6
Userland/Libraries/LibELF/DynamicObject.h
View file

@ -84,6 +84,12 @@ public:
}
DynamicObject const& object() const { return m_dynamic; }
// This might return false even if the two Symbol objects resolve to the same thing.
bool definitely_equals(Symbol const& other) const
{
return &m_dynamic == &other.m_dynamic && &m_sym == &other.m_sym && m_index == other.m_index;
}
private:
DynamicObject const& m_dynamic;
const ElfW(Sym) & m_sym;