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LibJS: Give VM a cache of single-ASCII-character PrimitiveString

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A large number of JS strings are a single ASCII character. This patch
adds a 128-entry cache for those strings to the VM. The cost of the
cache is 1536 byte of GC heap (all in same block) + 2304 bytes malloc.

This avoids a lot of GC heap allocations, and packing all of these
in the same heap block is nice for fragmentation as well.
This commit is contained in:
Andreas Kling 2020-10-22 17:43:48 +02:00
parent 5c2520e6b2
commit 619cd613d0
Notes: sideshowbarker 2024-07-19 01:48:33 +09:00 
Author: https://github.com/awesomekling
Commit: https://github.com/SerenityOS/serenity/commit/619cd613d02
3 changed files with 17 additions and 0 deletions
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4
Libraries/LibJS/Runtime/PrimitiveString.cpp
View file

@ -42,6 +42,10 @@ PrimitiveString* js_string(Heap& heap, String string)
{
if (string.is_empty())
return &heap.vm().empty_string();
if (string.length() == 1 && (u8)string.characters()[0] < 0x80)
return &heap.vm().single_ascii_character_string(string.characters()[0]);
return heap.allocate_without_global_object<PrimitiveString>(move(string));
}

7
Libraries/LibJS/Runtime/VM.cpp
View file

@ -47,6 +47,10 @@ VM::VM()
: m_heap(*this)
{
m_empty_string = m_heap.allocate_without_global_object<PrimitiveString>(String::empty());
for (size_t i = 0; i < 128; ++i) {
m_single_ascii_character_strings[i] = m_heap.allocate_without_global_object<PrimitiveString>(String::format("%c", i));
}
#define __JS_ENUMERATE(SymbolName, snake_name) \
m_well_known_symbol_##snake_name = js_symbol(*this, "Symbol." #SymbolName, false);
JS_ENUMERATE_WELL_KNOWN_SYMBOLS
@ -96,6 +100,9 @@ VM::InterpreterExecutionScope::~InterpreterExecutionScope()
void VM::gather_roots(HashTable<Cell*>& roots)
{
roots.set(m_empty_string);
for (auto* string : m_single_ascii_character_strings)
roots.set(string);
if (m_exception)
roots.set(m_exception);

6
Libraries/LibJS/Runtime/VM.h
View file

@ -108,6 +108,11 @@ public:
Symbol* get_global_symbol(const String& description);
PrimitiveString& empty_string() { return *m_empty_string; }
PrimitiveString& single_ascii_character_string(u8 character)
{
ASSERT(character < 0x80);
return *m_single_ascii_character_strings[character];
}
CallFrame& push_call_frame(bool strict_mode = false)
{
@ -247,6 +252,7 @@ private:
HashMap<String, Symbol*> m_global_symbol_map;
PrimitiveString* m_empty_string { nullptr };
PrimitiveString* m_single_ascii_character_strings[128] {};
#define __JS_ENUMERATE(SymbolName, snake_name) \
Symbol* m_well_known_symbol_##snake_name { nullptr };