Invert the order of each compound selector.

MozReview-Commit-ID: JkBjqyYgbrB

components/selectors/builder.rs

@@ -2,15 +2,208 @@
  * 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/. */
 
-use parser::{Component, SelectorImpl, SelectorIter};
+//! Helper module to build up a selector safely and efficiently.
+//!
+//! Our selector representation is designed to optimize matching, and has
+//! several requirements:
+//! * All simple selectors and combinators are stored inline in the same buffer
+//!   as Component instances.
+//! * We store the top-level compound selectors from right to left, i.e. in
+//!   matching order.
+//! * We store the simple selectors for each combinator from left to right, so
+//!   that we match the cheaper simple selectors first.
+//!
+//! Meeting all these constraints without extra memmove traffic during parsing
+//! is non-trivial. This module encapsulates those details and presents an
+//! easy-to-use API for the parser.
+
+use parser::{Combinator, Component, SelectorImpl};
+use servo_arc::{Arc, HeaderWithLength, ThinArc};
+use sink::Push;
+use smallvec::SmallVec;
 use std::cmp;
 use std::ops::Add;
+use std::ptr;
+use std::slice;
+
+/// Top-level SelectorBuilder struct. This should be stack-allocated by the
+/// consumer and never moved (because it contains a lot of inline data that
+/// would be slow to memmov).
+///
+/// After instantation, callers may call the push_simple_selector() and
+/// push_combinator() methods to append selector data as it is encountered
+/// (from left to right). Once the process is complete, callers should invoke
+/// build(), which transforms the contents of the SelectorBuilder into a heap-
+/// allocated Selector and leaves the builder in a drained state.
+pub struct SelectorBuilder<Impl: SelectorImpl> {
+    /// The entire sequence of simple selectors, from left to right, without combinators.
+    ///
+    /// We make this large because the result of parsing a selector is fed into a new
+    /// Arc-ed allocation, so any spilled vec would be a wasted allocation. Also,
+    /// Components are large enough that we don't have much cache locality benefit
+    /// from reserving stack space for fewer of them.
+    simple_selectors: SmallVec<[Component<Impl>; 32]>,
+    /// The combinators, and the length of the compound selector to their left.
+    combinators: SmallVec<[(Combinator, usize); 16]>,
+    /// The length of the current compount selector.
+    current_len: usize,
+}
+
+impl<Impl: SelectorImpl> Default for SelectorBuilder<Impl> {
+    #[inline(always)]
+    fn default() -> Self {
+        SelectorBuilder {
+            simple_selectors: SmallVec::new(),
+            combinators: SmallVec::new(),
+            current_len: 0,
+        }
+    }
+}
+
+impl<Impl: SelectorImpl> Push<Component<Impl>> for SelectorBuilder<Impl> {
+    fn push(&mut self, value: Component<Impl>) {
+        self.push_simple_selector(value);
+    }
+}
+
+impl<Impl: SelectorImpl> SelectorBuilder<Impl> {
+    /// Pushes a simple selector onto the current compound selector.
+    #[inline(always)]
+    pub fn push_simple_selector(&mut self, ss: Component<Impl>) {
+        debug_assert!(!ss.is_combinator());
+        self.simple_selectors.push(ss);
+        self.current_len += 1;
+    }
+
+    /// Completes the current compound selector and starts a new one, delimited
+    /// by the given combinator.
+    #[inline(always)]
+    pub fn push_combinator(&mut self, c: Combinator) {
+        self.combinators.push((c, self.current_len));
+        self.current_len = 0;
+    }
+
+    /// Returns true if no simple selectors have ever been pushed to this builder.
+    #[inline(always)]
+    pub fn is_empty(&self) -> bool {
+        self.simple_selectors.is_empty()
+    }
+
+    /// Consumes the builder, producing a Selector.
+    #[inline(always)]
+    pub fn build(&mut self, parsed_pseudo: bool) -> ThinArc<SpecificityAndFlags, Component<Impl>> {
+        // Compute the specificity and flags.
+        let mut spec = SpecificityAndFlags(specificity(self.simple_selectors.iter()));
+        if parsed_pseudo {
+            spec.0 |= HAS_PSEUDO_BIT;
+        }
+
+        self.build_with_specificity_and_flags(spec)
+    }
+
+
+    /// Builds with an explicit SpecificityAndFlags. This is separated from build() so
+    /// that unit tests can pass an explicit specificity.
+    #[inline(always)]
+    pub fn build_with_specificity_and_flags(&mut self, spec: SpecificityAndFlags)
+                                            -> ThinArc<SpecificityAndFlags, Component<Impl>> {
+        // First, compute the total number of Components we'll need to allocate
+        // space for.
+        let full_len = self.simple_selectors.len() + self.combinators.len();
+
+        // Create the header.
+        let header = HeaderWithLength::new(spec, full_len);
+
+        // Create the Arc using an iterator that drains our buffers.
+        let iter = SelectorBuilderIter::new(self, full_len);
+        Arc::into_thin(Arc::from_header_and_iter(header, iter))
+    }
+}
+
+struct SelectorBuilderIter<'a, Impl: SelectorImpl> {
+    builder: &'a mut SelectorBuilder<Impl>,
+    end: *const Component<Impl>,
+    base: *const Component<Impl>,
+    ptr: *const Component<Impl>,
+    full_len: usize,
+}
+
+impl<'a, Impl: SelectorImpl> SelectorBuilderIter<'a, Impl> {
+    fn new(builder: &'a mut SelectorBuilder<Impl>, full_len: usize) -> Self {
+        // Store a pointer to the end of the array of simple selectors,
+        // and set ourselves up to iterate the rightmost compound selector.
+        let sequence_base = &*builder.simple_selectors as *const [Component<Impl>] as *const Component<Impl>;
+        let end = unsafe { sequence_base.offset(builder.simple_selectors.len() as isize) };
+        let base = unsafe { end.offset(-(builder.current_len as isize)) };
+        let ptr = base;
+
+        // Next, tell the SmallVec to forget about its entries so that they
+        // won't be dropped when it frees its buffer. We're transferring
+        // ownership into the selector.
+        unsafe { builder.simple_selectors.set_len(0); }
+
+        SelectorBuilderIter {
+            builder: builder,
+            end: end,
+            base: base,
+            ptr: ptr,
+            full_len: full_len,
+        }
+    }
+}
+
+impl<'a, Impl: SelectorImpl> ExactSizeIterator for SelectorBuilderIter<'a, Impl> {
+    fn len(&self) -> usize {
+        self.full_len
+    }
+}
+
+impl<'a, Impl: SelectorImpl> Iterator for SelectorBuilderIter<'a, Impl> {
+    type Item = Component<Impl>;
+    #[inline(always)]
+    fn next(&mut self) -> Option<Self::Item> {
+        // If ptr is below end, continue walking this compound selector.
+        if self.ptr != self.end {
+            debug_assert!(self.ptr < self.end);
+            let result = unsafe { Some(ptr::read(self.ptr)) };
+            self.ptr = unsafe { self.ptr.offset(1) };
+            return result;
+        }
+
+        if let Some((combinator, len)) = self.builder.combinators.pop() {
+            // There's another compound selector. Reset the pointers to iterate it,
+            // and then return the combinator.
+            self.end = self.base;
+            self.base = unsafe { self.end.offset(-(len as isize)) };
+            self.ptr = self.base;
+            Some(Component::Combinator(combinator))
+        } else {
+            // All done.
+            None
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (self.len(), Some(self.len()))
+    }
+}
+
+#[cfg(debug_assertions)]
+impl<'a, Impl: SelectorImpl> Drop for SelectorBuilderIter<'a, Impl> {
+    fn drop(&mut self) {
+        // Failing to iterate the entire builder would cause us to leak (but not
+        // crash). Verify that this doesn't happen.
+        debug_assert!(self.builder.simple_selectors.len() == 0);
+        debug_assert!(self.builder.combinators.len() == 0);
+        debug_assert!(self.ptr == self.end);
+    }
+}
+
+pub const HAS_PSEUDO_BIT: u32 = 1 << 30;
 
 #[derive(Copy, Clone, Debug, Eq, PartialEq)]
 pub struct SpecificityAndFlags(pub u32);
 
-pub const HAS_PSEUDO_BIT: u32 = 1 << 30;
-
 impl SpecificityAndFlags {
     pub fn specificity(&self) -> u32 {
         self.0 & !HAS_PSEUDO_BIT
@@ -73,13 +266,13 @@ impl From<Specificity> for u32 {
     }
 }
 
-pub fn specificity<Impl>(iter: SelectorIter<Impl>) -> u32
+fn specificity<Impl>(iter: slice::Iter<Component<Impl>>) -> u32
     where Impl: SelectorImpl
 {
     complex_selector_specificity(iter).into()
 }
 
-fn complex_selector_specificity<Impl>(mut iter: SelectorIter<Impl>)
+fn complex_selector_specificity<Impl>(mut iter: slice::Iter<Component<Impl>>)
                                       -> Specificity
     where Impl: SelectorImpl
 {
@@ -129,13 +322,8 @@ fn complex_selector_specificity<Impl>(mut iter: SelectorIter<Impl>)
     }
 
     let mut specificity = Default::default();
-    loop {
     for simple_selector in &mut iter {
         simple_selector_specificity(&simple_selector, &mut specificity);
     }
-        if iter.next_sequence().is_none() {
-            break;
-        }
-    }
     specificity
 }

components/selectors/matching.rs

@@ -460,25 +460,28 @@ pub fn matches_complex_selector<E, F>(mut iter: SelectorIter<E::Impl>,
         }
     }
 
+    // If this is the special pseudo-element mode, consume the ::pseudo-element
+    // before proceeding, since the caller has already handled that part.
     if context.shared.matching_mode == MatchingMode::ForStatelessPseudoElement {
-        match *iter.next().unwrap() {
+        // Consume the pseudo.
-            // Stateful pseudo, just don't match.
+        let pseudo = iter.next().unwrap();
-            Component::NonTSPseudoClass(..) => return false,
+        debug_assert!(matches!(*pseudo, Component::PseudoElement(..)),
-            Component::PseudoElement(..) => {
+                      "Used MatchingMode::ForStatelessPseudoElement in a non-pseudo selector");
-                // Pseudo, just eat the whole sequence.
-                let next = iter.next();
-                debug_assert!(next.is_none(),
-                              "Someone messed up pseudo-element parsing?");
 
+        // The only other parser-allowed Component in this sequence is a state
+        // class. We just don't match in that case.
+        if let Some(s) = iter.next() {
+            debug_assert!(matches!(*s, Component::NonTSPseudoClass(..)),
+                          "Someone messed up pseudo-element parsing");
+            return false;
+        }
+
+        // Advance to the non-pseudo-element part of the selector, and inform the context.
         if iter.next_sequence().is_none() {
             return true;
         }
-                // Inform the context that the we've advanced to the next compound selector.
         context.note_next_sequence(&mut iter);
     }
-            _ => panic!("Used MatchingMode::ForStatelessPseudoElement in a non-pseudo selector"),
-        }
-    }
 
     match matches_complex_selector_internal(iter,
                                             element,

components/selectors/parser.rs

@@ -5,12 +5,11 @@
 use attr::{AttrSelectorWithNamespace, ParsedAttrSelectorOperation, AttrSelectorOperator};
 use attr::{ParsedCaseSensitivity, SELECTOR_WHITESPACE, NamespaceConstraint};
 use bloom::BLOOM_HASH_MASK;
-use builder::{HAS_PSEUDO_BIT, SpecificityAndFlags};
+use builder::{SelectorBuilder, SpecificityAndFlags};
-use builder::specificity;
 use cssparser::{ParseError, BasicParseError, CompactCowStr};
 use cssparser::{Token, Parser as CssParser, parse_nth, ToCss, serialize_identifier, CssStringWriter};
 use precomputed_hash::PrecomputedHash;
-use servo_arc::{Arc, HeaderWithLength, ThinArc};
+use servo_arc::ThinArc;
 use sink::Push;
 use smallvec::SmallVec;
 use std::ascii::AsciiExt;
@@ -374,6 +373,14 @@ pub fn namespace_empty_string<Impl: SelectorImpl>() -> Impl::NamespaceUrl {
 /// callers want the higher-level iterator.
 ///
 /// We store compound selectors internally right-to-left (in matching order).
+/// Additionally, we invert the order of top-level compound selectors so that
+/// each one matches left-to-right. This is because matching namespace, local name,
+/// id, and class are all relatively cheap, whereas matching pseudo-classes might
+/// be expensive (depending on the pseudo-class). Since authors tend to put the
+/// pseudo-classes on the right, it's faster to start matching on the left.
+///
+/// This reordering doesn't change the semantics of selector matching, and we
+/// handle it in to_css to make it invisible to serialization.
 #[derive(Clone, Eq, PartialEq)]
 pub struct Selector<Impl: SelectorImpl>(ThinArc<SpecificityAndFlags, Component<Impl>>);
 
@@ -454,12 +461,6 @@ impl<Impl: SelectorImpl> Selector<Impl> {
         self.0.slice.iter()
     }
 
-    /// Returns an iterator over the entire sequence of simple selectors and
-    /// combinators, in parse order (from left to right).
-    pub fn iter_raw_parse_order(&self) -> Rev<slice::Iter<Component<Impl>>> {
-        self.0.slice.iter().rev()
-    }
-
     /// Returns an iterator over the sequence of simple selectors and
     /// combinators, in parse order (from left to right), _starting_
     /// 'offset_from_right' entries from the past-the-end sentinel on
@@ -472,11 +473,18 @@ impl<Impl: SelectorImpl> Selector<Impl> {
         self.0.slice[..offset_from_right].iter().rev()
     }
 
-    /// Creates a Selector from a vec of Components. Used in tests.
+    /// Creates a Selector from a vec of Components, specified in parse order. Used in tests.
-    pub fn from_vec(mut vec: Vec<Component<Impl>>, specificity_and_flags: u32) -> Self {
+    pub fn from_vec(vec: Vec<Component<Impl>>, specificity_and_flags: u32) -> Self {
-        vec.reverse(); // FIXME(bholley): This goes away in the next patch.
+        let mut builder = SelectorBuilder::default();
-        let header = HeaderWithLength::new(SpecificityAndFlags(specificity_and_flags), vec.len());
+        for component in vec.into_iter() {
-        Selector(Arc::into_thin(Arc::from_header_and_iter(header, vec.into_iter())))
+            if let Some(combinator) = component.as_combinator() {
+                builder.push_combinator(combinator);
+            } else {
+                builder.push_simple_selector(component);
+            }
+        }
+        let spec = SpecificityAndFlags(specificity_and_flags);
+        Selector(builder.build_with_specificity_and_flags(spec))
     }
 
     /// Returns count of simple selectors and combinators in the Selector.
@@ -751,9 +759,31 @@ impl<Impl: SelectorImpl> ToCss for SelectorList<Impl> {
 
 impl<Impl: SelectorImpl> ToCss for Selector<Impl> {
     fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-       for item in self.iter_raw_parse_order() {
+        // Compound selectors invert the order of their contents, so we need to
+        // undo that during serialization.
+        //
+        // This two-iterator strategy involves walking over the selector twice.
+        // We could do something more clever, but selector serialization probably
+        // isn't hot enough to justify it, and the stringification likely
+        // dominates anyway.
+        //
+        // NB: A parse-order iterator is a Rev<>, which doesn't expose as_slice(),
+        // which we need for |split|. So we split by combinators on a match-order
+        // sequence and then reverse.
+        let mut combinators = self.iter_raw_match_order().rev().filter(|x| x.is_combinator());
+        let compound_selectors = self.iter_raw_match_order().as_slice().split(|x| x.is_combinator()).rev();
+
+        let mut combinators_exhausted = false;
+        for compound in compound_selectors {
+            debug_assert!(!combinators_exhausted);
+            for item in compound.iter() {
                 item.to_css(dest)?;
             }
+            match combinators.next() {
+                Some(c) => c.to_css(dest)?,
+                None => combinators_exhausted = true,
+            };
+        }
 
        Ok(())
     }
@@ -905,12 +935,6 @@ fn display_to_css_identifier<T: Display, W: fmt::Write>(x: &T, dest: &mut W) ->
     serialize_identifier(&string, dest)
 }
 
-/// We make this large because the result of parsing a selector is fed into a new
-/// Arc-ed allocation, so any spilled vec would be a wasted allocation. Also,
-/// Components are large enough that we don't have much cache locality benefit
-/// from reserving stack space for fewer of them.
-type ParseVec<Impl> = SmallVec<[Component<Impl>; 32]>;
-
 /// Build up a Selector.
 /// selector : simple_selector_sequence [ combinator simple_selector_sequence ]* ;
 ///
@@ -921,12 +945,12 @@ fn parse_selector<'i, 't, P, E, Impl>(
         -> Result<Selector<Impl>, ParseError<'i, SelectorParseError<'i, E>>>
     where P: Parser<'i, Impl=Impl, Error=E>, Impl: SelectorImpl
 {
-    let mut sequence = ParseVec::new();
+    let mut builder = SelectorBuilder::default();
+
     let mut parsed_pseudo_element;
     'outer_loop: loop {
         // Parse a sequence of simple selectors.
-        parsed_pseudo_element =
+        parsed_pseudo_element = parse_compound_selector(parser, input, &mut builder)?;
-            parse_compound_selector(parser, input, &mut sequence)?;
         if parsed_pseudo_element {
             break;
         }
@@ -962,23 +986,10 @@ fn parse_selector<'i, 't, P, E, Impl>(
                 }
             }
         }
-        sequence.push(Component::Combinator(combinator));
+        builder.push_combinator(combinator);
     }
 
-    let mut spec = SpecificityAndFlags(specificity(SelectorIter {
+    Ok(Selector(builder.build(parsed_pseudo_element)))
-        iter: sequence.iter(),
-        next_combinator: None,
-    }));
-    if parsed_pseudo_element {
-        spec.0 |= HAS_PSEUDO_BIT;
-    }
-
-    // FIXME(bholley): This is just temporary to maintain the semantics of this patch.
-    sequence.reverse();
-
-    let header = HeaderWithLength::new(spec, sequence.len());
-    let complex = Selector(Arc::into_thin(Arc::from_header_and_iter(header, sequence.into_iter())));
-    Ok(complex)
 }
 
 impl<Impl: SelectorImpl> Selector<Impl> {
@@ -1342,7 +1353,7 @@ fn parse_negation<'i, 't, P, E, Impl>(parser: &P,
 fn parse_compound_selector<'i, 't, P, E, Impl>(
     parser: &P,
     input: &mut CssParser<'i, 't>,
-    mut sequence: &mut ParseVec<Impl>)
+    mut builder: &mut SelectorBuilder<Impl>)
     -> Result<bool, ParseError<'i, SelectorParseError<'i, E>>>
     where P: Parser<'i, Impl=Impl, Error=E>, Impl: SelectorImpl
 {
@@ -1355,12 +1366,12 @@ fn parse_compound_selector<'i, 't, P, E, Impl>(
         }
     }
     let mut empty = true;
-    if !parse_type_selector(parser, input, sequence)? {
+    if !parse_type_selector(parser, input, builder)? {
         if let Some(url) = parser.default_namespace() {
             // If there was no explicit type selector, but there is a
             // default namespace, there is an implicit "<defaultns>|*" type
             // selector.
-            sequence.push(Component::DefaultNamespace(url))
+            builder.push_simple_selector(Component::DefaultNamespace(url))
         }
     } else {
         empty = false;
@@ -1371,7 +1382,7 @@ fn parse_compound_selector<'i, 't, P, E, Impl>(
         match parse_one_simple_selector(parser, input, /* inside_negation = */ false)? {
             None => break,
             Some(SimpleSelectorParseResult::SimpleSelector(s)) => {
-                sequence.push(s);
+                builder.push_simple_selector(s);
                 empty = false
             }
             Some(SimpleSelectorParseResult::PseudoElement(p)) => {
@@ -1401,13 +1412,13 @@ fn parse_compound_selector<'i, 't, P, E, Impl>(
                     state_selectors.push(Component::NonTSPseudoClass(pseudo_class));
                 }
 
-                if !sequence.is_empty() {
+                if !builder.is_empty() {
-                    sequence.push(Component::Combinator(Combinator::PseudoElement));
+                    builder.push_combinator(Combinator::PseudoElement);
                 }
 
-                sequence.push(Component::PseudoElement(p));
+                builder.push_simple_selector(Component::PseudoElement(p));
-                for state_selector in state_selectors {
+                for state_selector in state_selectors.into_iter() {
-                    sequence.push(state_selector);
+                    builder.push_simple_selector(state_selector);
                 }
 
                 pseudo = true;
@@ -1558,6 +1569,7 @@ fn parse_simple_pseudo_class<'i, P, E, Impl>(parser: &P, name: CompactCowStr<'i>
 // NB: pub module in order to access the DummyParser
 #[cfg(test)]
 pub mod tests {
+    use builder::HAS_PSEUDO_BIT;
     use cssparser::{Parser as CssParser, ToCss, serialize_identifier, ParserInput};
     use parser;
     use std::borrow::Cow;