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LibJS: Reduce AST memory usage by shrink-wrapping source range info

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Before this change, each AST node had a 64-byte SourceRange member.
This SourceRange had the following layout:

    filename:       StringView (16 bytes)
    start:          Position (24 bytes)
    end:            Position (24 bytes)

The Position structs have { line, column, offset }, all members size_t.

To reduce memory consumption, AST nodes now only store the following:

    source_code:    NonnullRefPtr<SourceCode> (8 bytes)
    start_offset:   u32 (4 bytes)
    end_offset:     u32 (4 bytes)

SourceCode is a new ref-counted data structure that keeps the filename
and original parsed source code in a single location, and all AST nodes
have a pointer to it.

The start_offset and end_offset can be turned into (line, column) when
necessary by calling SourceCode::range_from_offsets(). This will walk
the source code string and compute line/column numbers on the fly, so
it's not necessarily fast, but it should be rare since this information
is primarily used for diagnostics and exception stack traces.

With this, ASTNode shrinks from 80 bytes to 32 bytes. This gives us a
~23% reduction in memory usage when loading twitter.com/awesomekling
(330 MiB before, 253 MiB after!) :^)
This commit is contained in:
Andreas Kling 2022-11-21 17:37:38 +01:00
parent 3d74d72bcb
commit b0b022507b
16 changed files with 315 additions and 179 deletions
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10
Userland/Libraries/LibJS/Runtime/Error.cpp
View file

@ -54,6 +54,8 @@ ThrowCompletionOr<void> Error::install_error_cause(Value options)
void Error::populate_stack()
{
static auto dummy_source_range = SourceRange { .code = SourceCode::create("", ""), .start = {}, .end = {} };
auto& vm = this->vm();
m_traceback.ensure_capacity(vm.execution_context_stack().size());
for (ssize_t i = vm.execution_context_stack().size() - 1; i >= 0; i--) {
@ -67,7 +69,7 @@ void Error::populate_stack()
// reaction jobs (which aren't called anywhere from the source code).
// They're not going to generate any _unhandled_ exceptions though, so a meaningless
// source range is fine.
context->current_node ? context->current_node->source_range() : SourceRange {});
context->current_node ? context->current_node->source_range() : dummy_source_range);
}
}
@ -82,12 +84,12 @@ String Error::stack_string() const
auto const& frame = m_traceback[i];
auto function_name = frame.function_name;
// Note: Since we don't know whether we have a valid SourceRange here we just check for some default values.
if (!frame.source_range.filename.is_null() || frame.source_range.start.offset != 0 || frame.source_range.end.offset != 0) {
if (!frame.source_range.filename().is_null() || frame.source_range.start.offset != 0 || frame.source_range.end.offset != 0) {
if (function_name == "<unknown>"sv)
stack_string_builder.appendff(" at {}:{}:{}\n", frame.source_range.filename, frame.source_range.start.line, frame.source_range.start.column);
stack_string_builder.appendff(" at {}:{}:{}\n", frame.source_range.filename(), frame.source_range.start.line, frame.source_range.start.column);
else
stack_string_builder.appendff(" at {} ({}:{}:{})\n", function_name, frame.source_range.filename, frame.source_range.start.line, frame.source_range.start.column);
stack_string_builder.appendff(" at {} ({}:{}:{})\n", function_name, frame.source_range.filename(), frame.source_range.start.line, frame.source_range.start.column);
} else {
stack_string_builder.appendff(" at {}\n", function_name.is_empty() ? "<unknown>"sv : function_name.view());
}