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Make u8::to_ascii_lowercase and to_ascii_uppercase branchless

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SimonSapin committed Mar 18, 2019
1 parent 8740d5d commit ce933f77c865a15670855ac5941fe200752b739f
Showing with 43 additions and 101 deletions.
  1. +1 −21 src/libcore/benches/ascii_case.rs
  2. +42 −80 src/libcore/num/mod.rs
@@ -1,24 +1,4 @@
// Lower-case ASCII 'a' is the first byte that has its highest bit set after wrap-adding 0x1F:
//
// b'a' + 0x1F == 0x80 == 0b1000_0000
// b'z' + 0x1F == 0x98 == 0b10011000
//
// Lower-case ASCII 'z' is the last byte that has its highest bit unset after wrap-adding 0x05:
//
// b'a' + 0x05 == 0x66 == 0b0110_0110
// b'z' + 0x05 == 0x7F == 0b0111_1111
//
// … except for 0xFB to 0xFF, but those are in the range of bytes that have the highest bit
// unset again after adding 0x1F.
//
// So `(byte + 0x1f) & !(byte + 5)` has its highest bit set
// iff `byte` is a lower-case ASCII letter.
//
// Lower-case ASCII letters all have the 0x20 bit set.
// (Two positions right of 0x80, the highest bit.)
// Unsetting that bit produces the same letter, in upper-case.
//
// Therefore:
// See comments in `u8::to_ascii_uppercase` in `src/libcore/num/mod.rs`.
fn branchless_to_ascii_upper_case(byte: u8) -> u8 {
byte &
!(
@@ -3794,7 +3794,39 @@ impl u8 {
#[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
#[inline]
pub fn to_ascii_uppercase(&self) -> u8 {
ASCII_UPPERCASE_MAP[*self as usize]
// See benchmarks in src/libcore/benches/ascii_case.rs

// Lower-case ASCII 'a' is the first byte that has its highest bit set
// after wrap-adding 0x1F:
//
// b'a' + 0x1F == 0x80 == 0b1000_0000
// b'z' + 0x1F == 0x98 == 0b10011000
//
// Lower-case ASCII 'z' is the last byte that has its highest bit unset
// after wrap-adding 0x05:
//
// b'a' + 0x05 == 0x66 == 0b0110_0110
// b'z' + 0x05 == 0x7F == 0b0111_1111
//
// … except for 0xFB to 0xFF, but those are in the range of bytes
// that have the highest bit unset again after adding 0x1F.
//
// So `(byte + 0x1f) & !(byte + 5)` has its highest bit set
// iff `byte` is a lower-case ASCII letter.
//
// Lower-case ASCII letters all have the 0x20 bit set.
// (Two positions right of 0x80, the highest bit.)
// Unsetting that bit produces the same letter, in upper-case.
//
// Therefore:
*self &
!(
(
self.wrapping_add(0x1f) &
!self.wrapping_add(0x05) &
0x80
) >> 2
)
}

/// Makes a copy of the value in its ASCII lower case equivalent.
@@ -3816,7 +3848,15 @@ impl u8 {
#[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
#[inline]
pub fn to_ascii_lowercase(&self) -> u8 {
ASCII_LOWERCASE_MAP[*self as usize]
// See comments in to_ascii_uppercase above.
*self |
(
(
self.wrapping_add(0x3f) &
!self.wrapping_add(0x25) &
0x80
) >> 2
)
}

/// Checks that two values are an ASCII case-insensitive match.
@@ -4940,84 +4980,6 @@ impl_from! { u32, f64, #[stable(feature = "lossless_float_conv", since = "1.6.0"
// Float -> Float
impl_from! { f32, f64, #[stable(feature = "lossless_float_conv", since = "1.6.0")] }

static ASCII_LOWERCASE_MAP: [u8; 256] = [
0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f,
0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17,
0x18, 0x19, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f,
b' ', b'!', b'"', b'#', b'$', b'%', b'&', b'\'',
b'(', b')', b'*', b'+', b',', b'-', b'.', b'/',
b'0', b'1', b'2', b'3', b'4', b'5', b'6', b'7',
b'8', b'9', b':', b';', b'<', b'=', b'>', b'?',
b'@',

b'a', b'b', b'c', b'd', b'e', b'f', b'g',
b'h', b'i', b'j', b'k', b'l', b'm', b'n', b'o',
b'p', b'q', b'r', b's', b't', b'u', b'v', b'w',
b'x', b'y', b'z',

b'[', b'\\', b']', b'^', b'_',
b'`', b'a', b'b', b'c', b'd', b'e', b'f', b'g',
b'h', b'i', b'j', b'k', b'l', b'm', b'n', b'o',
b'p', b'q', b'r', b's', b't', b'u', b'v', b'w',
b'x', b'y', b'z', b'{', b'|', b'}', b'~', 0x7f,
0x80, 0x81, 0x82, 0x83, 0x84, 0x85, 0x86, 0x87,
0x88, 0x89, 0x8a, 0x8b, 0x8c, 0x8d, 0x8e, 0x8f,
0x90, 0x91, 0x92, 0x93, 0x94, 0x95, 0x96, 0x97,
0x98, 0x99, 0x9a, 0x9b, 0x9c, 0x9d, 0x9e, 0x9f,
0xa0, 0xa1, 0xa2, 0xa3, 0xa4, 0xa5, 0xa6, 0xa7,
0xa8, 0xa9, 0xaa, 0xab, 0xac, 0xad, 0xae, 0xaf,
0xb0, 0xb1, 0xb2, 0xb3, 0xb4, 0xb5, 0xb6, 0xb7,
0xb8, 0xb9, 0xba, 0xbb, 0xbc, 0xbd, 0xbe, 0xbf,
0xc0, 0xc1, 0xc2, 0xc3, 0xc4, 0xc5, 0xc6, 0xc7,
0xc8, 0xc9, 0xca, 0xcb, 0xcc, 0xcd, 0xce, 0xcf,
0xd0, 0xd1, 0xd2, 0xd3, 0xd4, 0xd5, 0xd6, 0xd7,
0xd8, 0xd9, 0xda, 0xdb, 0xdc, 0xdd, 0xde, 0xdf,
0xe0, 0xe1, 0xe2, 0xe3, 0xe4, 0xe5, 0xe6, 0xe7,
0xe8, 0xe9, 0xea, 0xeb, 0xec, 0xed, 0xee, 0xef,
0xf0, 0xf1, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6, 0xf7,
0xf8, 0xf9, 0xfa, 0xfb, 0xfc, 0xfd, 0xfe, 0xff,
];

static ASCII_UPPERCASE_MAP: [u8; 256] = [
0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f,
0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17,
0x18, 0x19, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f,
b' ', b'!', b'"', b'#', b'$', b'%', b'&', b'\'',
b'(', b')', b'*', b'+', b',', b'-', b'.', b'/',
b'0', b'1', b'2', b'3', b'4', b'5', b'6', b'7',
b'8', b'9', b':', b';', b'<', b'=', b'>', b'?',
b'@', b'A', b'B', b'C', b'D', b'E', b'F', b'G',
b'H', b'I', b'J', b'K', b'L', b'M', b'N', b'O',
b'P', b'Q', b'R', b'S', b'T', b'U', b'V', b'W',
b'X', b'Y', b'Z', b'[', b'\\', b']', b'^', b'_',
b'`',

b'A', b'B', b'C', b'D', b'E', b'F', b'G',
b'H', b'I', b'J', b'K', b'L', b'M', b'N', b'O',
b'P', b'Q', b'R', b'S', b'T', b'U', b'V', b'W',
b'X', b'Y', b'Z',

b'{', b'|', b'}', b'~', 0x7f,
0x80, 0x81, 0x82, 0x83, 0x84, 0x85, 0x86, 0x87,
0x88, 0x89, 0x8a, 0x8b, 0x8c, 0x8d, 0x8e, 0x8f,
0x90, 0x91, 0x92, 0x93, 0x94, 0x95, 0x96, 0x97,
0x98, 0x99, 0x9a, 0x9b, 0x9c, 0x9d, 0x9e, 0x9f,
0xa0, 0xa1, 0xa2, 0xa3, 0xa4, 0xa5, 0xa6, 0xa7,
0xa8, 0xa9, 0xaa, 0xab, 0xac, 0xad, 0xae, 0xaf,
0xb0, 0xb1, 0xb2, 0xb3, 0xb4, 0xb5, 0xb6, 0xb7,
0xb8, 0xb9, 0xba, 0xbb, 0xbc, 0xbd, 0xbe, 0xbf,
0xc0, 0xc1, 0xc2, 0xc3, 0xc4, 0xc5, 0xc6, 0xc7,
0xc8, 0xc9, 0xca, 0xcb, 0xcc, 0xcd, 0xce, 0xcf,
0xd0, 0xd1, 0xd2, 0xd3, 0xd4, 0xd5, 0xd6, 0xd7,
0xd8, 0xd9, 0xda, 0xdb, 0xdc, 0xdd, 0xde, 0xdf,
0xe0, 0xe1, 0xe2, 0xe3, 0xe4, 0xe5, 0xe6, 0xe7,
0xe8, 0xe9, 0xea, 0xeb, 0xec, 0xed, 0xee, 0xef,
0xf0, 0xf1, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6, 0xf7,
0xf8, 0xf9, 0xfa, 0xfb, 0xfc, 0xfd, 0xfe, 0xff,
];

enum AsciiCharacterClass {
C, // control
Cw, // control whitespace

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