LibELF: Don't recompute the same ELF hashes over and over

When performing a global symbol lookup, we were recomputing the symbol
hashes once for every dynamic object searched. The hash function was
at the very top of a profile (15%) of program startup.

With this change, the hash function is no longer visible among the top
stacks in the profile. :^)

Userland/Libraries/LibELF/DynamicLinker.cpp

@@ -38,6 +38,7 @@
 #include <LibELF/DynamicLinker.h>
 #include <LibELF/DynamicLoader.h>
 #include <LibELF/DynamicObject.h>
+#include <LibELF/Hashes.h>
 #include <dlfcn.h>
 #include <fcntl.h>
 #include <sys/types.h>
@@ -64,8 +65,12 @@ bool g_do_breakpoint_trap_before_entry { false };
 Optional<DynamicObject::SymbolLookupResult> DynamicLinker::lookup_global_symbol(const StringView& symbol)
 {
     Optional<DynamicObject::SymbolLookupResult> weak_result;
+
+    auto gnu_hash = compute_gnu_hash(symbol);
+    auto sysv_hash = compute_sysv_hash(symbol);
+
     for (auto& lib : g_global_objects) {
-        auto res = lib->lookup_symbol(symbol);
+        auto res = lib->lookup_symbol(symbol, gnu_hash, sysv_hash);
         if (!res.has_value())
             continue;
         if (res.value().bind == STB_GLOBAL)

Userland/Libraries/LibELF/DynamicLoader.cpp

@@ -31,6 +31,7 @@
 #include <AK/StringBuilder.h>
 #include <LibELF/DynamicLinker.h>
 #include <LibELF/DynamicLoader.h>
+#include <LibELF/Hashes.h>
 #include <LibELF/Validation.h>
 #include <assert.h>
 #include <dlfcn.h>
@@ -142,7 +143,7 @@ bool DynamicLoader::validate()
 
 void* DynamicLoader::symbol_for_name(const StringView& name)
 {
-    auto result = m_dynamic_object->hash_section().lookup_symbol(name);
+    auto result = m_dynamic_object->hash_section().lookup_symbol(name, compute_gnu_hash(name), compute_sysv_hash(name));
     if (!result.has_value())
         return nullptr;
     auto symbol = result.value();

Userland/Libraries/LibELF/DynamicObject.cpp

@@ -249,14 +249,8 @@ DynamicObject::RelocationSection DynamicObject::plt_relocation_section() const
     return RelocationSection(Section(*this, m_plt_relocation_offset_location, m_size_of_plt_relocation_entry_list, m_size_of_relocation_entry, "DT_JMPREL"));
 }
 
-auto DynamicObject::HashSection::lookup_symbol(const StringView& name) const -> Optional<Symbol>
+auto DynamicObject::HashSection::lookup_elf_symbol(const StringView& name, u32 hash_value) const -> Optional<Symbol>
 {
-    return (this->*(m_lookup_function))(name);
-}
-
-auto DynamicObject::HashSection::lookup_elf_symbol(const StringView& name) const -> Optional<Symbol>
-{
-    u32 hash_value = compute_sysv_hash(name);
     u32* hash_table_begin = (u32*)address().as_ptr();
     size_t num_buckets = hash_table_begin[0];
 
@@ -279,7 +273,7 @@ auto DynamicObject::HashSection::lookup_elf_symbol(const StringView& name) const
     return {};
 }
 
-auto DynamicObject::HashSection::lookup_gnu_symbol(const StringView& name) const -> Optional<Symbol>
+auto DynamicObject::HashSection::lookup_gnu_symbol(const StringView& name, u32 hash_value) const -> Optional<Symbol>
 {
     // Algorithm reference: https://ent-voy.blogspot.com/2011/02/
     // TODO: Handle 64bit bloomwords for ELF_CLASS64
@@ -299,7 +293,7 @@ auto DynamicObject::HashSection::lookup_gnu_symbol(const StringView& name) const
     const u32* const buckets = &bloom_words[num_maskwords];
     const u32* const chains = &buckets[num_buckets];
 
-    BloomWord hash1 = compute_gnu_hash(name);
+    BloomWord hash1 = hash_value;
     BloomWord hash2 = hash1 >> shift2;
     const BloomWord bitmask = (1 << (hash1 % bloom_word_size)) | (1 << (hash2 % bloom_word_size));
 
@@ -431,7 +425,12 @@ static const char* name_for_dtag(Elf32_Sword d_tag)
 
 auto DynamicObject::lookup_symbol(const StringView& name) const -> Optional<SymbolLookupResult>
 {
-    auto result = hash_section().lookup_symbol(name);
+    return lookup_symbol(name, compute_gnu_hash(name), compute_sysv_hash(name));
+}
+
+auto DynamicObject::lookup_symbol(const StringView& name, u32 gnu_hash, u32 sysv_hash) const -> Optional<SymbolLookupResult>
+{
+    auto result = hash_section().lookup_symbol(name, gnu_hash, sysv_hash);
     if (!result.has_value())
         return {};
     auto symbol = result.value();

Userland/Libraries/LibELF/DynamicObject.h

@@ -184,27 +184,22 @@ public:
     public:
         HashSection(const Section& section, HashType hash_type)
             : Section(section.m_dynamic, section.m_section_offset, section.m_section_size_bytes, section.m_entry_size, section.m_name)
+            , m_hash_type(hash_type)
         {
-            switch (hash_type) {
-            case HashType::SYSV:
-                m_lookup_function = &HashSection::lookup_elf_symbol;
-                break;
-            case HashType::GNU:
-                m_lookup_function = &HashSection::lookup_gnu_symbol;
-                break;
-            default:
-                ASSERT_NOT_REACHED();
-            }
         }
 
-        Optional<Symbol> lookup_symbol(const StringView& name) const;
+        Optional<Symbol> lookup_symbol(const StringView& name, u32 gnu_hash, u32 sysv_hash) const
+        {
+            if (m_hash_type == HashType::SYSV)
+                return lookup_elf_symbol(name, sysv_hash);
+            return lookup_gnu_symbol(name, gnu_hash);
+        }
 
     private:
-        Optional<Symbol> lookup_elf_symbol(const StringView& name) const;
-        Optional<Symbol> lookup_gnu_symbol(const StringView& name) const;
+        Optional<Symbol> lookup_elf_symbol(const StringView& name, u32 hash) const;
+        Optional<Symbol> lookup_gnu_symbol(const StringView& name, u32 hash) const;
 
-        typedef Optional<Symbol> (HashSection::*LookupFunction)(const StringView&) const;
-        LookupFunction m_lookup_function {};
+        HashType m_hash_type {};
     };
 
     unsigned symbol_count() const { return m_symbol_count; }
@@ -256,7 +251,9 @@ public:
         unsigned bind { STB_LOCAL };
         const ELF::DynamicObject* dynamic_object { nullptr }; // The object in which the symbol is defined
     };
+
     Optional<SymbolLookupResult> lookup_symbol(const StringView& name) const;
+    Optional<SymbolLookupResult> lookup_symbol(const StringView& name, u32 gnu_hash, u32 sysv_hash) const;
 
     // Will be called from _fixup_plt_entry, as part of the PLT trampoline
     VirtualAddress patch_plt_entry(u32 relocation_offset);