From 5d56e1daed3aab8149ecbf69baf5bc1785f08627 Mon Sep 17 00:00:00 2001 From: Ulrik Sverdrup Date: Tue, 5 Apr 2016 14:06:20 +0200 Subject: [PATCH] Specialize equality for [T] and comparison for [u8] Where T is a type that can be compared for equality bytewise, we can use memcmp. We can also use memcmp for PartialOrd, Ord for [u8] and by extension &str. This is an improvement for example for the comparison [u8] == [u8] that used to emit a loop that compared the slices byte by byte. One worry here could be that this introduces function calls to memcmp in contexts where it should really inline the comparison or even optimize it out, but llvm takes care of recognizing memcmp specifically. --- src/libcore/lib.rs | 1 + src/libcore/slice.rs | 167 +++++++++++++++++++++++++++++++++++------ src/libcore/str/mod.rs | 25 +----- 3 files changed, 147 insertions(+), 46 deletions(-) diff --git a/src/libcore/lib.rs b/src/libcore/lib.rs index 648b652772a..fa5e90562d8 100644 --- a/src/libcore/lib.rs +++ b/src/libcore/lib.rs @@ -75,6 +75,7 @@ #![feature(unwind_attributes)] #![feature(repr_simd, platform_intrinsics)] #![feature(rustc_attrs)] +#![feature(specialization)] #![feature(staged_api)] #![feature(unboxed_closures)] #![feature(question_mark)] diff --git a/src/libcore/slice.rs b/src/libcore/slice.rs index aa555b44e89..e9cf650af70 100644 --- a/src/libcore/slice.rs +++ b/src/libcore/slice.rs @@ -1630,12 +1630,59 @@ pub unsafe fn from_raw_parts_mut<'a, T>(p: *mut T, len: usize) -> &'a mut [T] { } // -// Boilerplate traits +// Comparison traits // +extern { + /// Call implementation provided memcmp + /// + /// Interprets the data as u8. + /// + /// Return 0 for equal, < 0 for less than and > 0 for greater + /// than. + // FIXME(#32610): Return type should be c_int + fn memcmp(s1: *const u8, s2: *const u8, n: usize) -> i32; +} + #[stable(feature = "rust1", since = "1.0.0")] impl PartialEq<[B]> for [A] where A: PartialEq { fn eq(&self, other: &[B]) -> bool { + SlicePartialEq::equal(self, other) + } + + fn ne(&self, other: &[B]) -> bool { + SlicePartialEq::not_equal(self, other) + } +} + +#[stable(feature = "rust1", since = "1.0.0")] +impl Eq for [T] {} + +#[stable(feature = "rust1", since = "1.0.0")] +impl Ord for [T] { + fn cmp(&self, other: &[T]) -> Ordering { + SliceOrd::compare(self, other) + } +} + +#[stable(feature = "rust1", since = "1.0.0")] +impl PartialOrd for [T] { + fn partial_cmp(&self, other: &[T]) -> Option { + SlicePartialOrd::partial_compare(self, other) + } +} + +// intermediate trait for specialization of slice's PartialEq +trait SlicePartialEq { + fn equal(&self, other: &[B]) -> bool; + fn not_equal(&self, other: &[B]) -> bool; +} + +// Generic slice equality +impl SlicePartialEq for [A] + where A: PartialEq +{ + default fn equal(&self, other: &[B]) -> bool { if self.len() != other.len() { return false; } @@ -1648,7 +1695,8 @@ impl PartialEq<[B]> for [A] where A: PartialEq { true } - fn ne(&self, other: &[B]) -> bool { + + default fn not_equal(&self, other: &[B]) -> bool { if self.len() != other.len() { return true; } @@ -1663,12 +1711,69 @@ impl PartialEq<[B]> for [A] where A: PartialEq { } } -#[stable(feature = "rust1", since = "1.0.0")] -impl Eq for [T] {} +// Use memcmp for bytewise equality when the types allow +impl SlicePartialEq for [A] + where A: PartialEq + BytewiseEquality +{ + fn equal(&self, other: &[A]) -> bool { + if self.len() != other.len() { + return false; + } + unsafe { + let size = mem::size_of_val(self); + memcmp(self.as_ptr() as *const u8, + other.as_ptr() as *const u8, size) == 0 + } + } -#[stable(feature = "rust1", since = "1.0.0")] -impl Ord for [T] { - fn cmp(&self, other: &[T]) -> Ordering { + fn not_equal(&self, other: &[A]) -> bool { + !self.equal(other) + } +} + +// intermediate trait for specialization of slice's PartialOrd +trait SlicePartialOrd { + fn partial_compare(&self, other: &[B]) -> Option; +} + +impl SlicePartialOrd for [A] + where A: PartialOrd +{ + default fn partial_compare(&self, other: &[A]) -> Option { + let l = cmp::min(self.len(), other.len()); + + // Slice to the loop iteration range to enable bound check + // elimination in the compiler + let lhs = &self[..l]; + let rhs = &other[..l]; + + for i in 0..l { + match lhs[i].partial_cmp(&rhs[i]) { + Some(Ordering::Equal) => (), + non_eq => return non_eq, + } + } + + self.len().partial_cmp(&other.len()) + } +} + +impl SlicePartialOrd for [u8] { + #[inline] + fn partial_compare(&self, other: &[u8]) -> Option { + Some(SliceOrd::compare(self, other)) + } +} + +// intermediate trait for specialization of slice's Ord +trait SliceOrd { + fn compare(&self, other: &[B]) -> Ordering; +} + +impl SliceOrd for [A] + where A: Ord +{ + default fn compare(&self, other: &[A]) -> Ordering { let l = cmp::min(self.len(), other.len()); // Slice to the loop iteration range to enable bound check @@ -1687,23 +1792,37 @@ impl Ord for [T] { } } -#[stable(feature = "rust1", since = "1.0.0")] -impl PartialOrd for [T] { - fn partial_cmp(&self, other: &[T]) -> Option { - let l = cmp::min(self.len(), other.len()); - - // Slice to the loop iteration range to enable bound check - // elimination in the compiler - let lhs = &self[..l]; - let rhs = &other[..l]; - - for i in 0..l { - match lhs[i].partial_cmp(&rhs[i]) { - Some(Ordering::Equal) => (), - non_eq => return non_eq, - } +// memcmp compares a sequence of unsigned bytes lexicographically. +// this matches the order we want for [u8], but no others (not even [i8]). +impl SliceOrd for [u8] { + #[inline] + fn compare(&self, other: &[u8]) -> Ordering { + let order = unsafe { + memcmp(self.as_ptr(), other.as_ptr(), + cmp::min(self.len(), other.len())) + }; + if order == 0 { + self.len().cmp(&other.len()) + } else if order < 0 { + Less + } else { + Greater } - - self.len().partial_cmp(&other.len()) } } + +/// Trait implemented for types that can be compared for equality using +/// their bytewise representation +trait BytewiseEquality { } + +macro_rules! impl_marker_for { + ($traitname:ident, $($ty:ty)*) => { + $( + impl $traitname for $ty { } + )* + } +} + +impl_marker_for!(BytewiseEquality, + u8 i8 u16 i16 u32 i32 u64 i64 usize isize char bool); + diff --git a/src/libcore/str/mod.rs b/src/libcore/str/mod.rs index d5a5e2b4741..305546df5be 100644 --- a/src/libcore/str/mod.rs +++ b/src/libcore/str/mod.rs @@ -1150,16 +1150,7 @@ Section: Comparing strings #[lang = "str_eq"] #[inline] fn eq_slice(a: &str, b: &str) -> bool { - a.len() == b.len() && unsafe { cmp_slice(a, b, a.len()) == 0 } -} - -/// Bytewise slice comparison. -/// NOTE: This uses the system's memcmp, which is currently dramatically -/// faster than comparing each byte in a loop. -#[inline] -unsafe fn cmp_slice(a: &str, b: &str, len: usize) -> i32 { - extern { fn memcmp(s1: *const i8, s2: *const i8, n: usize) -> i32; } - memcmp(a.as_ptr() as *const i8, b.as_ptr() as *const i8, len) + a.as_bytes() == b.as_bytes() } /* @@ -1328,8 +1319,7 @@ Section: Trait implementations */ mod traits { - use cmp::{self, Ordering, Ord, PartialEq, PartialOrd, Eq}; - use cmp::Ordering::{Less, Greater}; + use cmp::{Ord, Ordering, PartialEq, PartialOrd, Eq}; use iter::Iterator; use option::Option; use option::Option::Some; @@ -1340,16 +1330,7 @@ mod traits { impl Ord for str { #[inline] fn cmp(&self, other: &str) -> Ordering { - let cmp = unsafe { - super::cmp_slice(self, other, cmp::min(self.len(), other.len())) - }; - if cmp == 0 { - self.len().cmp(&other.len()) - } else if cmp < 0 { - Less - } else { - Greater - } + self.as_bytes().cmp(other.as_bytes()) } }