rust - Why are these ASCII methods inconsistent? -

when @ rust ascii operations feels there consistency issue between

is_lowercase/is_uppercase:

pub fn is_uppercase(&self) -> bool {     (self.chr - b'a') < 26 } 

is_alphabetic:

pub fn is_alphabetic(&self) -> bool {     (self.chr >= 0x41 && self.chr <= 0x5a) || (self.chr >= 0x61 && self.chr <= 0x7a) } 

is there reason? 2 methods totally equivalent or missing something? these functions marked stable i'm confused.

edit:

to make clearer, expect decide on best (in terms of performance/readability/common practice) implementation lower/upper have

pub fn is_alphabetic(&self) -> bool {     self.is_lowercase() || self.is_uppercase() } 

since question changed performance, i'll add second answer.

to start, created clone of ascii module (playpen):

pub struct alpha(u8);  impl alpha {     #[inline(never)]     pub fn is_uppercase_sub(&self) -> bool {         (self.0 - b'a') < 26     }      #[inline(never)]     pub fn is_uppercase_range(&self) -> bool {         self.0 >= 0x41 && self.0 <= 0x5a     } }  fn main() {     let yes = alpha(b'a');     let no  = alpha(b'a');      println!("{}, {}", yes.is_uppercase_sub(), yes.is_uppercase_range()); } 

in playpen, make sure optimization set -o2 , click ir. shows llvm intermediate representation. it's higher-level assembly, if you'd like.

there's lots of output, sections fastcc. i've removed various bits make code clearer, can see exact same function called, though our code calls 2 different implementations, 1 subtraction , 1 range:

%3 = call fastcc zeroext i1 @_zn5alpha16is_uppercase_sub20h63aa0b11479803f4laae %5 = call fastcc zeroext i1 @_zn5alpha16is_uppercase_sub20h63aa0b11479803f4laae 

the llvm optimizer can tell these implementations same, it's developers preference. might able commit rust make them consistent, if you'd like! ^_^

asking is_alphabetic harder; inlining come play here. if llvm inlines is_upper , is_lower is_alphabetic, suggested change better. if doesn't, potentially 1 function call 3! could bad.

these types of questions lot harder answer @ level; 1 have looking (edit , profiling!) @ real rust code in large understand optimizer regards inlining.