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LibJS: Get rid of unnecessary work from canonical_numeric_index_string

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The spec version of canonical_numeric_index_string is absurdly complex,
and ends up converting from a string to a number, and then back again
which is both slow and also requires a few allocations and a string
compare.

Instead this patch moves away from using Values to represent canonical
a canonical index. In most cases all we need to know is whether a
PropertyKey is an integer between 0 and 2^^32-2, which we already
compute when we construct a PropertyKey so the existing is_number()
check is sufficient.

The more expensive case is handling strings containing numbers that
don't roundtrip through string conversion. In most cases these turn
into regular string properties, but for TypedArray access these
property names are not treated as normal named properties.
TypedArrays treat these numeric properties as magic indexes that are
ignored on read and are not stored (but are evaluated) on assignment.

For that reason there's now a mode flag on canonical_numeric_index_string
so that only TypedArrays take the cost of the ToString round trip test.
In order to improve the performance of this path this patch includes
some early returns to avoid conversion in cases where we can quickly
know whether a property can round trip.
This commit is contained in:
Anonymous 2022-02-14 01:17:07 -08:00 • committed by Linus Groh
parent 7c4d42279d
commit 745b998774
10 changed files with 207 additions and 96 deletions
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52
Userland/Libraries/LibJS/Runtime/AbstractOperations.cpp
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@ -1031,7 +1031,7 @@ Object* create_mapped_arguments_object(GlobalObject& global_object, FunctionObje
}
// 7.1.21 CanonicalNumericIndexString ( argument ), https://tc39.es/ecma262/#sec-canonicalnumericindexstring
Value canonical_numeric_index_string(GlobalObject& global_object, PropertyKey const& property_key)
CanonicalIndex canonical_numeric_index_string(PropertyKey const& property_key, CanonicalIndexMode mode)
{
// NOTE: If the property name is a number type (An implementation-defined optimized
// property key type), it can be treated as a string property that has already been
@ -1040,24 +1040,56 @@ Value canonical_numeric_index_string(GlobalObject& global_object, PropertyKey co
VERIFY(property_key.is_string() || property_key.is_number());
if (property_key.is_number())
return Value(property_key.as_number());
return CanonicalIndex(CanonicalIndex::Type::Index, property_key.as_number());
if (mode != CanonicalIndexMode::DetectNumericRoundtrip)
return CanonicalIndex(CanonicalIndex::Type::Undefined, 0);
// 1. Assert: Type(argument) is String.
auto argument = Value(js_string(global_object.vm(), property_key.as_string()));
auto& argument = property_key.as_string();
// 2. If argument is "-0", return -0𝔽.
if (argument.as_string().string() == "-0")
return Value(-0.0);
// Handle trivial cases without a full round trip test
// We do not need to check for argument == "0" at this point because we
// already covered it with the is_number() == true path.
if (argument.is_empty())
return CanonicalIndex(CanonicalIndex::Type::Undefined, 0);
u32 current_index = 0;
if (argument.characters()[current_index] == '-') {
current_index++;
if (current_index == argument.length())
return CanonicalIndex(CanonicalIndex::Type::Undefined, 0);
}
if (argument.characters()[current_index] == '0') {
current_index++;
if (current_index == argument.length())
return CanonicalIndex(CanonicalIndex::Type::Numeric, 0);
if (argument.characters()[current_index] != '.')
return CanonicalIndex(CanonicalIndex::Type::Undefined, 0);
current_index++;
if (current_index == argument.length())
return CanonicalIndex(CanonicalIndex::Type::Undefined, 0);
}
// Short circuit a few common cases
if (argument == "Infinity" || argument == "-Infinity" || argument == "NaN")
return CanonicalIndex(CanonicalIndex::Type::Numeric, 0);
// Short circuit any string that doesn't start with digits
if (char first_non_zero = argument.characters()[current_index]; first_non_zero < '0' || first_non_zero > '9')
return CanonicalIndex(CanonicalIndex::Type::Undefined, 0);
// 3. Let n be ! ToNumber(argument).
auto n = MUST(argument.to_number(global_object));
char* endptr;
auto n = Value(strtod(argument.characters(), &endptr));
if (endptr != argument.characters() + argument.length())
return CanonicalIndex(CanonicalIndex::Type::Undefined, 0);
// 4. If SameValue(! ToString(n), argument) is false, return undefined.
if (!same_value(MUST(n.to_primitive_string(global_object)), argument))
return js_undefined();
if (n.to_string_without_side_effects() != argument)
return CanonicalIndex(CanonicalIndex::Type::Undefined, 0);
// 5. Return n.
return n;
return CanonicalIndex(CanonicalIndex::Type::Numeric, 0);
}
// 22.1.3.17.1 GetSubstitution ( matched, str, position, captures, namedCaptures, replacement ), https://tc39.es/ecma262/#sec-getsubstitution