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LibELF: Apply some minor optimizations to symbol lookup

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Optimizations:

- Make sure `DT_SYMTAB` is a string view literal, instead of string.

- DynamicObject::HashSection::lookup_sysv_symbol should be using
  raw_name() from symbol comparison to avoid needlessly calling
  `strlen`, when the StrinView::operator= walks the cstring without
  calling `strlen` first.

- DynamicObject::HashSection::lookup_gnu_symbol shouldn't create a
  symbol unless we know the hashes match first.

In order to test these changes I enabled Undefined behavior sanitizer
which creates a huge amount of relocations, and then ran the browser
with the help argument 100 times. The browser is a fairly big app with
a few different libraries being loaded, so it seemed liked a good
target.

Command: `time -n 100 br --help`

Before:
```
Timing report:
==============
Command:         br --help
Average time:    3897.679931 ms
Excluding first: 3901.242431 ms
```

After:
```
Timing report:
==============
Command:         br --help
Average time:    3612.860107 ms
Excluding first: 3613.54541 ms
```
This commit is contained in:
Brian Gianforcaro 2021-08-28 05:13:42 -07:00 • committed by Andreas Kling
parent ad1051089c
commit 2038d2c49e
1 changed files with 8 additions and 5 deletions
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13
Userland/Libraries/LibELF/DynamicObject.cpp
View file

@ -204,7 +204,7 @@ DynamicObject::Relocation DynamicObject::RelocationSection::relocation_at_offset
DynamicObject::Symbol DynamicObject::symbol(unsigned index) const
{
auto symbol_section = Section(*this, m_symbol_table_offset, (m_symbol_count * m_size_of_symbol_table_entry), m_size_of_symbol_table_entry, "DT_SYMTAB");
auto symbol_section = Section(*this, m_symbol_table_offset, (m_symbol_count * m_size_of_symbol_table_entry), m_size_of_symbol_table_entry, "DT_SYMTAB"sv);
auto symbol_entry = (ElfW(Sym)*)symbol_section.address().offset(index * symbol_section.entry_size()).as_ptr();
return Symbol(*this, index, *symbol_entry);
}
@ -267,7 +267,7 @@ auto DynamicObject::HashSection::lookup_sysv_symbol(const StringView& name, u32
for (u32 i = buckets[hash_value % num_buckets]; i; i = chains[i]) {
auto symbol = m_dynamic.symbol(i);
if (name == symbol.name()) {
if (name == symbol.raw_name()) {
dbgln_if(DYNAMIC_LOAD_DEBUG, "Returning SYSV dynamic symbol with index {} for {}: {}", i, symbol.name(), symbol.address().as_ptr());
return symbol;
}
@ -308,9 +308,12 @@ auto DynamicObject::HashSection::lookup_gnu_symbol(const StringView& name, u32 h
for (hash1 &= ~1;; ++current_sym) {
hash2 = *(current_chain++);
auto symbol = m_dynamic.symbol(current_sym);
if ((hash1 == (hash2 & ~1)) && name == symbol.raw_name())
return symbol;
if (hash1 == (hash2 & ~1)) {
auto symbol = m_dynamic.symbol(current_sym);
if (name == symbol.raw_name())
return symbol;
}
if (hash2 & 1)
break;
}