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LibJS+AK: Make String.prototype.repeat() way faster

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Instead of using a StringBuilder, add a String::repeated(String, N)
overload that takes advantage of knowing it's already all UTF-8.

This makes the following microbenchmark go 4x faster:

    "foo".repeat(100_000_000)

And for single character strings, we can even go 10x faster:

    "x".repeat(100_000_000)
This commit is contained in:
Andreas Kling 2023-12-29 13:20:11 +01:00
parent 9ce267944c
commit a285e36041
Notes: sideshowbarker 2024-07-17 21:11:12 +09:00 
Author: https://github.com/awesomekling
Commit: https://github.com/SerenityOS/serenity/commit/a285e36041
Pull-request: https://github.com/SerenityOS/serenity/pull/22496
Reviewed-by: https://github.com/LucasChollet
3 changed files with 21 additions and 4 deletions
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17
AK/String.cpp
View file

@ -636,4 +636,21 @@ bool String::equals_ignoring_ascii_case(StringView other) const
return StringUtils::equals_ignoring_ascii_case(bytes_as_string_view(), other);
}
String String::repeated(String const& input, size_t count)
{
VERIFY(!Checked<size_t>::multiplication_would_overflow(count, input.bytes().size()));
u8* buffer = nullptr;
auto data = MUST(Detail::StringData::create_uninitialized(count * input.bytes().size(), buffer));
if (input.bytes().size() == 1) {
memset(buffer, input.bytes().first(), count);
return String { move(data) };
}
for (size_t i = 0; i < count; ++i) {
memcpy(buffer + (i * input.bytes().size()), input.bytes().data(), input.bytes().size());
}
return String { data };
}
}

3
AK/String.h
View file

@ -92,6 +92,9 @@ public:
// Creates a new String with a single code point repeated N times.
static ErrorOr<String> repeated(u32 code_point, size_t count);
// Creates a new String from another string, repeated N times.
static String repeated(String const&, size_t count);
// Creates a new String by case-transforming this String. Using these methods require linking LibUnicode into your application.
ErrorOr<String> to_lowercase(Optional<StringView> const& locale = {}) const;
ErrorOr<String> to_uppercase(Optional<StringView> const& locale = {}) const;

5
Userland/Libraries/LibJS/Runtime/StringPrototype.cpp
View file

@ -777,10 +777,7 @@ JS_DEFINE_NATIVE_FUNCTION(StringPrototype::repeat)
return PrimitiveString::create(vm, String {});
// 6. Return the String value that is made from n copies of S appended together.
StringBuilder builder;
for (size_t i = 0; i < n; ++i)
builder.append(string);
return PrimitiveString::create(vm, MUST(builder.to_string()));
return PrimitiveString::create(vm, String::repeated(string, n));
}
// 22.1.3.19 String.prototype.replace ( searchValue, replaceValue ), https://tc39.es/ecma262/#sec-string.prototype.replace