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Auto merge of #37972 - bluss:iter-find-is-on-a-roll, r=sfackler

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Improve the slice iterator's searching methods

Improve all, any, find, position, rposition by explicitly unrolling the loop for the slice iterators.

- Introduce a few extension methods and functions for raw pointers make the new code easy to express
- Introduce helper methods `search_while, rsearch_while` that generalize all the searching methods

LLVM doesn't unroll the loop in `.find()` by default (clang is the same), so performance benefits a lot from explicit unrolling here. An iterator method without conditional exits (like `.fold()`) does not need this on the other hand.

One of the raw pointer extension methods is `fn post_inc(&mut self) -> Self` which is the rustic equivalent of “`ptr++`”, and it is a nice way to express the raw pointer loop (see commit 3).

Specific development notes about `search_while`: I tried both computing an end pointer "rounded" to 4, as well as the `ptrdistance >= 4` loop condition, ptrdistance was better. I tried handling the last 0-3 elements unrolled or with a while loop, the loop was better.
This commit is contained in:
bors 2017-01-17 21:52:03 +00:00
commit c07a6ae77c
1 changed files with 185 additions and 14 deletions
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199
src/libcore/slice.rs
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@ -172,21 +172,35 @@ macro_rules! slice_offset {
($ptr:expr, $by:expr) => {{
let ptr = $ptr;
if size_from_ptr(ptr) == 0 {
::intrinsics::arith_offset(ptr as *mut i8, $by) as *mut _
(ptr as *mut i8).wrapping_offset($by) as _
} else {
ptr.offset($by)
}
}};
}
macro_rules! slice_ref {
// make a &T from a *const T
macro_rules! make_ref {
($ptr:expr) => {{
let ptr = $ptr;
if size_from_ptr(ptr) == 0 {
// Use a non-null pointer value
&*(1 as *mut _)
} else {
&*ptr
}
}};
}
// make a &mut T from a *mut T
macro_rules! make_ref_mut {
($ptr:expr) => {{
let ptr = $ptr;
if size_from_ptr(ptr) == 0 {
// Use a non-null pointer value
&mut *(1 as *mut _)
} else {
mem::transmute(ptr)
&mut *ptr
}
}};
}
@ -963,7 +977,7 @@ fn size_from_ptr<T>(_: *const T) -> usize {
// The shared definition of the `Iter` and `IterMut` iterators
macro_rules! iterator {
(struct $name:ident -> $ptr:ty, $elem:ty) => {
(struct $name:ident -> $ptr:ty, $elem:ty, $mkref:ident) => {
#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T> Iterator for $name<'a, T> {
type Item = $elem;
@ -979,18 +993,14 @@ macro_rules! iterator {
if self.ptr == self.end {
None
} else {
let old = self.ptr;
self.ptr = slice_offset!(self.ptr, 1);
Some(slice_ref!(old))
Some($mkref!(self.ptr.post_inc()))
}
}
}
#[inline]
fn size_hint(&self) -> (usize, Option<usize>) {
let diff = (self.end as usize).wrapping_sub(self.ptr as usize);
let size = mem::size_of::<T>();
let exact = diff / (if size == 0 {1} else {size});
let exact = ptrdistance(self.ptr, self.end);
(exact, Some(exact))
}
@ -1009,6 +1019,64 @@ macro_rules! iterator {
fn last(mut self) -> Option<$elem> {
self.next_back()
}
fn all<F>(&mut self, mut predicate: F) -> bool
where F: FnMut(Self::Item) -> bool,
{
self.search_while(true, move |elt| {
if predicate(elt) {
SearchWhile::Continue
} else {
SearchWhile::Done(false)
}
})
}
fn any<F>(&mut self, mut predicate: F) -> bool
where F: FnMut(Self::Item) -> bool,
{
!self.all(move |elt| !predicate(elt))
}
fn find<F>(&mut self, mut predicate: F) -> Option<Self::Item>
where F: FnMut(&Self::Item) -> bool,
{
self.search_while(None, move |elt| {
if predicate(&elt) {
SearchWhile::Done(Some(elt))
} else {
SearchWhile::Continue
}
})
}
fn position<F>(&mut self, mut predicate: F) -> Option<usize>
where F: FnMut(Self::Item) -> bool,
{
let mut index = 0;
self.search_while(None, move |elt| {
if predicate(elt) {
SearchWhile::Done(Some(index))
} else {
index += 1;
SearchWhile::Continue
}
})
}
fn rposition<F>(&mut self, mut predicate: F) -> Option<usize>
where F: FnMut(Self::Item) -> bool,
{
let mut index = self.len();
self.rsearch_while(None, move |elt| {
index -= 1;
if predicate(elt) {
SearchWhile::Done(Some(index))
} else {
SearchWhile::Continue
}
})
}
}
#[stable(feature = "rust1", since = "1.0.0")]
@ -1024,12 +1092,53 @@ macro_rules! iterator {
if self.end == self.ptr {
None
} else {
self.end = slice_offset!(self.end, -1);
Some(slice_ref!(self.end))
Some($mkref!(self.end.pre_dec()))
}
}
}
}
// search_while is a generalization of the internal iteration methods.
impl<'a, T> $name<'a, T> {
// search through the iterator's element using the closure `g`.
// if no element was found, return `default`.
fn search_while<Acc, G>(&mut self, default: Acc, mut g: G) -> Acc
where Self: Sized,
G: FnMut($elem) -> SearchWhile<Acc>
{
// manual unrolling is needed when there are conditional exits from the loop
unsafe {
while ptrdistance(self.ptr, self.end) >= 4 {
search_while!(g($mkref!(self.ptr.post_inc())));
search_while!(g($mkref!(self.ptr.post_inc())));
search_while!(g($mkref!(self.ptr.post_inc())));
search_while!(g($mkref!(self.ptr.post_inc())));
}
while self.ptr != self.end {
search_while!(g($mkref!(self.ptr.post_inc())));
}
}
default
}
fn rsearch_while<Acc, G>(&mut self, default: Acc, mut g: G) -> Acc
where Self: Sized,
G: FnMut($elem) -> SearchWhile<Acc>
{
unsafe {
while ptrdistance(self.ptr, self.end) >= 4 {
search_while!(g($mkref!(self.end.pre_dec())));
search_while!(g($mkref!(self.end.pre_dec())));
search_while!(g($mkref!(self.end.pre_dec())));
search_while!(g($mkref!(self.end.pre_dec())));
}
while self.ptr != self.end {
search_while!(g($mkref!(self.end.pre_dec())));
}
}
default
}
}
}
}
@ -1061,6 +1170,24 @@ macro_rules! make_mut_slice {
}}
}
// An enum used for controlling the execution of `.search_while()`.
enum SearchWhile<T> {
// Continue searching
Continue,
// Fold is complete and will return this value
Done(T),
}
// helper macro for search while's control flow
macro_rules! search_while {
($e:expr) => {
match $e {
SearchWhile::Continue => { }
SearchWhile::Done(done) => return done,
}
}
}
/// Immutable slice iterator
///
/// This struct is created by the [`iter`] method on [slices].
@ -1147,7 +1274,7 @@ impl<'a, T> Iter<'a, T> {
}
}
iterator!{struct Iter -> *const T, &'a T}
iterator!{struct Iter -> *const T, &'a T, make_ref}
#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T> ExactSizeIterator for Iter<'a, T> {
@ -1275,7 +1402,7 @@ impl<'a, T> IterMut<'a, T> {
}
}
iterator!{struct IterMut -> *mut T, &'a mut T}
iterator!{struct IterMut -> *mut T, &'a mut T, make_ref_mut}
#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T> ExactSizeIterator for IterMut<'a, T> {
@ -1290,6 +1417,50 @@ impl<'a, T> FusedIterator for IterMut<'a, T> {}
#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<'a, T> TrustedLen for IterMut<'a, T> {}
// Return the number of elements of `T` from `start` to `end`.
// Return the arithmetic difference if `T` is zero size.
#[inline(always)]
fn ptrdistance<T>(start: *const T, end: *const T) -> usize {
let diff = (end as usize).wrapping_sub(start as usize);
let size = mem::size_of::<T>();
diff / (if size == 0 { 1 } else { size })
}
// Extension methods for raw pointers, used by the iterators
trait PointerExt : Copy {
unsafe fn slice_offset(self, i: isize) -> Self;
/// Increment self by 1, but return the old value
#[inline(always)]
unsafe fn post_inc(&mut self) -> Self {
let current = *self;
*self = self.slice_offset(1);
current
}
/// Decrement self by 1, and return the new value
#[inline(always)]
unsafe fn pre_dec(&mut self) -> Self {
*self = self.slice_offset(-1);
*self
}
}
impl<T> PointerExt for *const T {
#[inline(always)]
unsafe fn slice_offset(self, i: isize) -> Self {
slice_offset!(self, i)
}
}
impl<T> PointerExt for *mut T {
#[inline(always)]
unsafe fn slice_offset(self, i: isize) -> Self {
slice_offset!(self, i)
}
}
/// An internal abstraction over the splitting iterators, so that
/// splitn, splitn_mut etc can be implemented once.
#[doc(hidden)]