pub type AttrAtom = Atom<EmptyStaticAtomSet>;Expand description
An attribute’s value, interned so identical values are stored once.
This was a plain String, which meant one separate heap allocation per
attribute per element with no sharing between them. A census over the
application’s own transcript markup
(blitz-tests/tests/attribute_value_duplication.rs) found 777 attribute
values of which 54 were distinct: 14.4x duplication, and 91.3% of the
value bytes were a copy of a string already in the tree. One class string
appeared 24 times. That is what a Tailwind UI looks like in memory, and it
is the shape Blink shares through ElementDataCache for the same reason
(element_data.h:172, “very common for many elements to have duplicate
sets of attributes”).
Atom is the right tool and was already in the dependency graph, because
QualName above is built from it. It is 8 bytes against String’s 24,
stores up to 7 bytes inline with no heap allocation at all, and interns
anything longer in a refcounted global table with per-bucket locks rather
than one global one.
The trade is a hash and a possible lock acquisition per write, against a
heap allocation and a memcpy per write today, and equality becoming a
pointer comparison rather than a memcmp. Reads are unaffected: this derefs
to str, so every &attr.value, .as_str(), .parse() and == call
site continues to compile and mean the same thing.
Atom is generic over a set of strings interned at compile time. We have
none to pre-intern: attribute names are already atoms via QualName, and
values are arbitrary author strings, so every one of ours takes the dynamic
path. EmptyStaticAtomSet is the crate’s own declaration of that case.
Named AttrAtom rather than the more obvious AttrValue, because stylo
already exports an AttrValue enum that document.rs uses in the same
breath as this type. Two different things under one name in one file is how
a later reader loses an afternoon.
Aliased Type§
pub struct AttrAtom { /* private fields */ }