mirror of
https://github.com/LadybirdBrowser/ladybird
synced 2026-05-13 10:26:37 +02:00
74e2dac929
Short string concatenations are a common allocation churn pattern in JS
execution. Many of these results are immediately observed as flat
strings, so the intended win is to avoid spending GC and flattening work
on an intermediate representation that does not carry its weight.
Microbenchmark:
const n = 10_000_000;
function bench(a, b) {
let total = 0;
for (let i = 0; i < n; ++i)
total += (a + b).length;
if (total !== n * 2)
throw new Error(String(total));
}
bench("a", "b");
Measured with hyperfine against the same build with the fast path
disabled:
baseline: 822.7 ms +/- 25.7 ms
optimized: 385.5 ms +/- 21.5 ms
speedup: 2.13 +/- 0.14 times faster
145 lines
4.2 KiB
C++
145 lines
4.2 KiB
C++
/*
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* Copyright (c) 2020-2025, Andreas Kling <andreas@ladybird.org>
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* Copyright (c) 2022, Linus Groh <linusg@serenityos.org>
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*
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* SPDX-License-Identifier: BSD-2-Clause
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*/
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#pragma once
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#include <AK/Optional.h>
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#include <AK/String.h>
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#include <AK/StringView.h>
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#include <AK/Utf16String.h>
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#include <LibGC/CellAllocator.h>
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#include <LibJS/Export.h>
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#include <LibJS/Forward.h>
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#include <LibJS/Heap/Cell.h>
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#include <LibJS/Runtime/Completion.h>
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#include <LibJS/Runtime/Value.h>
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namespace JS {
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class JS_API PrimitiveString : public Cell {
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GC_CELL(PrimitiveString, Cell);
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GC_DECLARE_ALLOCATOR(PrimitiveString);
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public:
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static constexpr bool OVERRIDES_FINALIZE = true;
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[[nodiscard]] static GC::Ref<PrimitiveString> create(VM&, Utf16String const&);
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[[nodiscard]] static GC::Ref<PrimitiveString> create(VM&, Utf16View const&);
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[[nodiscard]] static GC::Ref<PrimitiveString> create(VM&, Utf16FlyString const&);
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[[nodiscard]] static GC::Ref<PrimitiveString> create(VM&, String const&);
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[[nodiscard]] static GC::Ref<PrimitiveString> create(VM&, StringView);
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[[nodiscard]] static GC::Ref<PrimitiveString> create(VM&, FlyString const&);
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[[nodiscard]] static GC::Ref<PrimitiveString> create(VM&, PrimitiveString&, PrimitiveString&);
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[[nodiscard]] static GC::Ref<PrimitiveString> create(VM&, PrimitiveString const&, size_t code_unit_offset, size_t code_unit_length);
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[[nodiscard]] static GC::Ref<PrimitiveString> create_from_unsigned_integer(VM&, u64);
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virtual ~PrimitiveString() override;
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PrimitiveString(PrimitiveString const&) = delete;
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PrimitiveString& operator=(PrimitiveString const&) = delete;
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bool is_empty() const;
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[[nodiscard]] String utf8_string() const;
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[[nodiscard]] StringView utf8_string_view() const;
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bool has_utf8_string() const { return m_utf8_string.has_value(); }
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[[nodiscard]] Utf16String utf16_string() const;
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[[nodiscard]] Utf16View utf16_string_view() const;
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bool has_utf16_string() const { return m_utf16_string.has_value(); }
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size_t length_in_utf16_code_units() const;
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ThrowCompletionOr<Optional<Value>> get(VM&, PropertyKey const&) const;
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[[nodiscard]] bool operator==(PrimitiveString const&) const;
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protected:
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enum class DeferredKind : u8 {
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None,
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Rope,
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Substring,
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};
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explicit PrimitiveString(DeferredKind deferred_kind)
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: m_deferred_kind(deferred_kind)
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{
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}
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mutable DeferredKind m_deferred_kind { DeferredKind::None };
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mutable Optional<String> m_utf8_string;
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mutable Optional<Utf16String> m_utf16_string;
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bool m_utf8_string_is_in_cache { false };
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bool m_utf16_string_is_in_cache { false };
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enum class EncodingPreference {
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UTF8,
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UTF16,
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};
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private:
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friend class RopeString;
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friend class Substring;
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virtual void finalize() override;
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explicit PrimitiveString(Utf16String);
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explicit PrimitiveString(String);
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void resolve_if_needed(EncodingPreference) const;
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Optional<StringView> short_flat_string_storage_view() const;
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static GC::Ptr<PrimitiveString> try_create_short_flat_concatenated_string(VM&, PrimitiveString const& lhs, PrimitiveString const& rhs);
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};
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class RopeString final : public PrimitiveString {
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GC_CELL(RopeString, PrimitiveString);
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GC_DECLARE_ALLOCATOR(RopeString);
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public:
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virtual ~RopeString() override;
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private:
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friend class PrimitiveString;
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explicit RopeString(GC::Ref<PrimitiveString>, GC::Ref<PrimitiveString>);
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virtual void visit_edges(Visitor&) override;
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void resolve(EncodingPreference) const;
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mutable GC::Ptr<PrimitiveString> m_lhs;
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mutable GC::Ptr<PrimitiveString> m_rhs;
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};
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class Substring final : public PrimitiveString {
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GC_CELL(Substring, PrimitiveString);
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GC_DECLARE_ALLOCATOR(Substring);
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public:
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virtual ~Substring() override;
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private:
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friend class PrimitiveString;
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explicit Substring(GC::Ref<PrimitiveString>, size_t code_unit_offset, size_t code_unit_length);
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virtual void visit_edges(Visitor&) override;
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void resolve(EncodingPreference) const;
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mutable GC::Ptr<PrimitiveString> m_source_string;
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size_t m_code_unit_offset { 0 };
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size_t m_code_unit_length { 0 };
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};
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}
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