Optimize next_code_point and next_code_point_reverse

By reordering some operations, we can expose some opportunites for CSE. Also convert the series of nested `if` branches to early return, which IMO makes the code clearer. Comparison of assembly before and after for `next_code_point`: https://godbolt.org/z/9Te84YzhK Comparison of assembly before and after for `next_code_point_reverse`: https://godbolt.org/z/fTx1a7oz1

Author: Kmeakin

library/core/src/str/validations.rs

@@ -1,22 +1,9 @@
 //! Operations related to UTF-8 validation.
 
 use super::Utf8Error;
+use crate::hint::assert_unchecked;
 use crate::intrinsics::const_eval_select;
 
-/// Returns the initial codepoint accumulator for the first byte.
-/// The first byte is special, only want bottom 5 bits for width 2, 4 bits
-/// for width 3, and 3 bits for width 4.
-#[inline]
-const fn utf8_first_byte(byte: u8, width: u32) -> u32 {
- (byte & (0x7F >> width)) as u32
-}
-
-/// Returns the value of `ch` updated with continuation byte `byte`.
-#[inline]
-const fn utf8_acc_cont_byte(ch: u32, byte: u8) -> u32 {
- (ch << 6) | (byte & CONT_MASK) as u32
-}
-
 /// Checks whether the byte is a UTF-8 continuation byte (i.e., starts with the
 /// bits `10`).
 #[inline]
@@ -33,39 +20,49 @@ pub(super) const fn utf8_is_cont_byte(byte: u8) -> bool {
 #[unstable(feature = "str_internals", issue = "none")]
 #[inline]
 pub unsafe fn next_code_point<'a, I: Iterator<Item = &'a u8>>(bytes: &mut I) -> Option<u32> {
- // Decode UTF-8
- let x = *bytes.next()?;
- if x < 128 {
- return Some(x as u32);
+ let b1 = *bytes.next()?;
+ if b1 < 0x80 {
+ // 1 byte case (U+0000 ..= U+007F):
+ // c = b1
+ return Some(u32::from(b1));
  }
 
- // Multibyte case follows
- // Decode from a byte combination out of: [[[x y] z] w]
- // NOTE: Performance is sensitive to the exact formulation here
- let init = utf8_first_byte(x, 2);
- // SAFETY: `bytes` produces an UTF-8-like string,
- // so the iterator must produce a value here.
- let y = unsafe { *bytes.next().unwrap_unchecked() };
- let mut ch = utf8_acc_cont_byte(init, y);
- if x >= 0xE0 {
- // [[x y z] w] case
- // 5th bit in 0xE0 .. 0xEF is always clear, so `init` is still valid
- // SAFETY: `bytes` produces an UTF-8-like string,
- // so the iterator must produce a value here.
- let z = unsafe { *bytes.next().unwrap_unchecked() };
- let y_z = utf8_acc_cont_byte((y & CONT_MASK) as u32, z);
- ch = init << 12 | y_z;
- if x >= 0xF0 {
- // [x y z w] case
- // use only the lower 3 bits of `init`
- // SAFETY: `bytes` produces an UTF-8-like string,
- // so the iterator must produce a value here.
- let w = unsafe { *bytes.next().unwrap_unchecked() };
- ch = (init & 7) << 18 | utf8_acc_cont_byte(y_z, w);
- }
+ // SAFETY: `bytes` produces a UTF-8-like string
+ let mut next_byte = || unsafe {
+ let b = *bytes.next().unwrap_unchecked();
+ assert_unchecked(utf8_is_cont_byte(b));
+ b
+ };
+ let combine = |c: u32, byte: u8| c << 6 | u32::from(byte & CONT_MASK);
+
+ let b2 = next_byte();
+ let c = u32::from(b1 & 0x1F);
+ let c = combine(c, b2);
+ if b1 < 0xE0 {
+ // 2 byte case (U+0080 ..= U+07FF):
+ // c = (b1 & 0x1F) << 6
+ // | (b2 & 0x3F) << 0
+ return Some(c);
+ }
+
+ let b3 = next_byte();
+ let c = combine(c, b3);
+ if b1 < 0xF0 {
+ // 3 byte case (U+0800 ..= U+FFFF):
+ // c = (b1 & 0x1F) << 12
+ // | (b2 & 0x3F) << 6
+ // | (b3 & 0x3F) << 0
+ return Some(c);
  }
 
- Some(ch)
+ let b4 = next_byte();
+ let c = combine(c, b4);
+ // 4 byte case (U+01_0000 ..= U+10_FFFF):
+ // c = ((b1 & 0x1F) << 18
+ // | (b2 & 0x3F) << 12
+ // | (b3 & 0x3F) << 6
+ // | (b4 & 0x3F) << 0) & 0x1F_FFFF
+ Some(c & 0x1F_FFFF)
 }
 
 /// Reads the last code point out of a byte iterator (assuming a
@@ -80,36 +77,49 @@ pub unsafe fn next_code_point_reverse<'a, I>(bytes: &mut I) -> Option<u32>
 where
  I: DoubleEndedIterator<Item = &'a u8>,
 {
- // Decode UTF-8
- let w = match *bytes.next_back()? {
- next_byte if next_byte < 128 => return Some(next_byte as u32),
- back_byte => back_byte,
+ let b1 = *bytes.next_back()?;
+ if b1 < 0x80 {
+ // 1 byte case (U+0000 ..= U+007F):
+ // c = b1
+ return Some(u32::from(b1));
+ }
+
+ // SAFETY: `bytes` produces a UTF-8-like string
+ let mut next_byte = || unsafe {
+ let b = *bytes.next_back().unwrap_unchecked();
+ assert_unchecked(!b.is_ascii());
+ b
  };
+ let combine = |c: u32, byte: u8, shift| c | u32::from(byte & CONT_MASK) << shift;
 
- // Multibyte case follows
- // Decode from a byte combination out of: [x [y [z w]]]
- let mut ch;
- // SAFETY: `bytes` produces an UTF-8-like string,
- // so the iterator must produce a value here.
- let z = unsafe { *bytes.next_back().unwrap_unchecked() };
- ch = utf8_first_byte(z, 2);
- if utf8_is_cont_byte(z) {
- // SAFETY: `bytes` produces an UTF-8-like string,
- // so the iterator must produce a value here.
- let y = unsafe { *bytes.next_back().unwrap_unchecked() };
- ch = utf8_first_byte(y, 3);
- if utf8_is_cont_byte(y) {
- // SAFETY: `bytes` produces an UTF-8-like string,
- // so the iterator must produce a value here.
- let x = unsafe { *bytes.next_back().unwrap_unchecked() };
- ch = utf8_first_byte(x, 4);
- ch = utf8_acc_cont_byte(ch, y);
- }
- ch = utf8_acc_cont_byte(ch, z);
+ let b2 = next_byte();
+ let c = u32::from(b1 & CONT_MASK);
+ let c = combine(c, b2, 6);
+ if !utf8_is_cont_byte(b2) {
+ // 2 byte case (U+0080 ..= U+07FF):
+ // c = (b2 & 0x3F) << 6
+ // | (b1 & 0x3F) << 0
+ return Some(c);
+ }
+
+ let b3 = next_byte();
+ let c = combine(c, b3, 12);
+ if !utf8_is_cont_byte(b3) {
+ // 3 byte case (U+0800 ..= U+FFFF):
+ // c = ((b3 & 0x3F) << 12
+ // | (b2 & 0x3F) << 6
+ // | (b1 & 0x3F) << 0) & 0xFFFF
+ return Some(c & 0xFFFF);
  }
- ch = utf8_acc_cont_byte(ch, w);
 
- Some(ch)
+ let b4 = next_byte();
+ let c = combine(c, b4, 18);
+ // 4 byte case (U+01_0000 ..= U+10_FFFF):
+ // c = ((b4 & 0x3F) << 18
+ // | (b3 & 0x3F) << 12
+ // | (b2 & 0x3F) << 6
+ // | (b1 & 0x3F) << 0) & 0x1F_FFFF
+ Some(c & 0x1F_FFFF)
 }
 
 const NONASCII_MASK: usize = usize::repeat_u8(0x80);
@@ -280,5 +290,5 @@ pub const fn utf8_char_width(b: u8) -> usize {
  UTF8_CHAR_WIDTH[b as usize] as usize
 }
 
-/// Mask of the value bits of a continuation byte.
+/// Mask of the value bits of a continuation byte (ie the lowest 6 bits).
 const CONT_MASK: u8 = 0b0011_1111;