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LibJS: Pack members of the AST Identifier class better

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A bit of creative structure packing brings this from 80 to 56 bytes.
This is hugely impactful on x.com where we have roughly ~2.3 million
Identifier objects after loading the home feed.

In other words, this reduces memory usage on that page by up to 55 MiB.

We should eventually discard most of the AST after parsing, but that
will require more architectural work so this is a nice stopgap
improvement before then.
This commit is contained in:
Andreas Kling
2025-12-20 13:49:14 -06:00
committed by Andreas Kling
parent e7e3c21123
commit 3610a31279
Notes: github-actions[bot] 2025-12-21 21:14:39 +00:00 
Author: https://github.com/awesomekling
Commit: https://github.com/LadybirdBrowser/ladybird/commit/3610a31279d
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/7215
2 changed files with 22 additions and 11 deletions
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2
Libraries/LibJS/AST.cpp
View File

@@ -1002,7 +1002,7 @@ void Identifier::dump(int indent) const
{
print_indent(indent);
if (is_local()) {
outln("Identifier \"{}\" is_local=(true) index=({})", m_string, m_local_index->index);
outln("Identifier \"{}\" is_local=(true) index=({})", m_string, m_local_index);
} else if (is_global()) {
outln("Identifier \"{}\" is_global=(true)", m_string);
} else {

31
Libraries/LibJS/AST.h
View File

@@ -679,28 +679,37 @@ public:
Utf16FlyString const& string() const { return m_string; }
struct Local {
enum Type {
enum Type : u8 {
None,
Argument,
Variable,
};
Type type;
size_t index;
u32 index;
bool is_argument() const { return type == Argument; }
bool is_variable() const { return type == Variable; }
static Local variable(size_t index) { return { Variable, index }; }
static Local argument(size_t index) { return { Argument, index }; }
static Local variable(u32 index) { return { Variable, index }; }
static Local argument(u32 index) { return { Argument, index }; }
};
bool is_local() const { return m_local_index.has_value(); }
bool is_local() const { return m_local_type != Local::Type::None; }
Local local_index() const
{
VERIFY(m_local_index.has_value());
return m_local_index.value();
VERIFY(m_local_type != Local::Type::None);
return Local { m_local_type, m_local_index };
}
void set_local_variable_index(u32 index)
{
m_local_type = Local::Type::Variable;
m_local_index = index;
}
void set_argument_index(u32 index)
{
m_local_type = Local::Type::Argument;
m_local_index = index;
}
void set_local_variable_index(size_t index) { m_local_index = Local::variable(index); }
void set_argument_index(size_t index) { m_local_index = Local::argument(index); }
bool is_global() const { return m_is_global; }
void set_is_global() { m_is_global = true; }
@@ -714,9 +723,11 @@ public:
private:
virtual bool is_identifier() const override { return true; }
u32 m_local_index;
Utf16FlyString m_string;
Optional<Local> m_local_index;
Local::Type m_local_type { Local::Type::None };
bool m_is_global { false };
DeclarationKind m_declaration_kind { DeclarationKind::None };
};