1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
|
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
//! Applicable declarations management.
use properties::PropertyDeclarationBlock;
use rule_tree::{CascadeLevel, StyleSource};
use servo_arc::Arc;
use shared_lock::Locked;
use smallvec::SmallVec;
use std::fmt::{Debug, self};
use std::mem;
/// List of applicable declarations. This is a transient structure that shuttles
/// declarations between selector matching and inserting into the rule tree, and
/// therefore we want to avoid heap-allocation where possible.
///
/// In measurements on wikipedia, we pretty much never have more than 8 applicable
/// declarations, so we could consider making this 8 entries instead of 16.
/// However, it may depend a lot on workload, and stack space is cheap.
pub type ApplicableDeclarationList = SmallVec<[ApplicableDeclarationBlock; 16]>;
/// Blink uses 18 bits to store source order, and does not check overflow [1].
/// That's a limit that could be reached in realistic webpages, so we use
/// 24 bits and enforce defined behavior in the overflow case.
///
/// Note that the value of 24 is also hard-coded into the level() accessor,
/// which does a byte-aligned load of the 4th byte. If you change this value
/// you'll need to change that as well.
///
/// [1] https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/css/
/// RuleSet.h?l=128&rcl=90140ab80b84d0f889abc253410f44ed54ae04f3
const SOURCE_ORDER_BITS: usize = 24;
const SOURCE_ORDER_MASK: u32 = (1 << SOURCE_ORDER_BITS) - 1;
const SOURCE_ORDER_MAX: u32 = SOURCE_ORDER_MASK;
/// Stores the source order of a block and the cascade level it belongs to.
#[derive(Clone, Copy, Eq, MallocSizeOf, PartialEq)]
struct SourceOrderAndCascadeLevel(u32);
impl SourceOrderAndCascadeLevel {
fn new(source_order: u32, cascade_level: CascadeLevel) -> SourceOrderAndCascadeLevel {
let mut bits = ::std::cmp::min(source_order, SOURCE_ORDER_MAX);
bits |= (cascade_level as u8 as u32) << SOURCE_ORDER_BITS;
SourceOrderAndCascadeLevel(bits)
}
fn order(&self) -> u32 {
self.0 & SOURCE_ORDER_MASK
}
fn level(&self) -> CascadeLevel {
unsafe {
// Transmute rather than shifting so that we're sure the compiler
// emits a simple byte-aligned load.
let as_bytes: [u8; 4] = mem::transmute(self.0);
CascadeLevel::from_byte(as_bytes[3])
}
}
}
impl Debug for SourceOrderAndCascadeLevel {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
f.debug_struct("SourceOrderAndCascadeLevel")
.field("order", &self.order())
.field("level", &self.level())
.finish()
}
}
/// A property declaration together with its precedence among rules of equal
/// specificity so that we can sort them.
///
/// This represents the declarations in a given declaration block for a given
/// importance.
#[derive(Clone, Debug, MallocSizeOf, PartialEq)]
pub struct ApplicableDeclarationBlock {
/// The style source, either a style rule, or a property declaration block.
#[ignore_malloc_size_of = "Arc"]
pub source: StyleSource,
/// The source order of the block, and the cascade level it belongs to.
order_and_level: SourceOrderAndCascadeLevel,
/// The specificity of the selector this block is represented by.
pub specificity: u32,
}
impl ApplicableDeclarationBlock {
/// Constructs an applicable declaration block from a given property
/// declaration block and importance.
#[inline]
pub fn from_declarations(declarations: Arc<Locked<PropertyDeclarationBlock>>,
level: CascadeLevel)
-> Self {
ApplicableDeclarationBlock {
source: StyleSource::Declarations(declarations),
order_and_level: SourceOrderAndCascadeLevel::new(0, level),
specificity: 0,
}
}
/// Constructs an applicable declaration block from the given components
#[inline]
pub fn new(source: StyleSource,
order: u32,
level: CascadeLevel,
specificity: u32) -> Self {
ApplicableDeclarationBlock {
source: source,
order_and_level: SourceOrderAndCascadeLevel::new(order, level),
specificity: specificity,
}
}
/// Returns the source order of the block.
#[inline]
pub fn source_order(&self) -> u32 {
self.order_and_level.order()
}
/// Returns the cascade level of the block.
#[inline]
pub fn level(&self) -> CascadeLevel {
self.order_and_level.level()
}
/// Convenience method to consume self and return the source alongside the
/// level.
#[inline]
pub fn order_and_level(self) -> (StyleSource, CascadeLevel) {
let level = self.level();
(self.source, level)
}
}
|
