Make u8::to_ascii_lowercase and to_ascii_uppercase branchless

Author: SimonSapin

src/libcore/benches/ascii_case.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 &
     !(

src/libcore/num/mod.rs

@@ -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