rust - Is `iter().map().sum()` as fast as `iter().fold()`? -

does compiler generate same code iter().map().sum() , iter().fold()? in end achieve same goal, first code iterate 2 times, once map , once sum.

here example. version faster in total?

pub fn square(s: u32) -> u64 {     match s {         s @ 1...64 => 2u64.pow(s - 1),         _ => panic!("square must between 1 , 64")     } }  pub fn total() -> u64 {     // fold     (0..64).fold(0u64, |r, s| r + square(s + 1))     // or map     (1..64).map(square).sum() } 

what tools @ assembly or benchmark this?

for them generate same code, they'd first have do same thing. 2 examples not:

fn total_fold() -> u64 {     (0..64).fold(0u64, |r, s| r + square(s + 1)) }  fn total_map() -> u64 {     (1..64).map(square).sum() }  fn main() {     println!("{}", total_fold());     println!("{}", total_map()); } 

18446744073709551615 9223372036854775807 

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let's assume meant

fn total_fold() -> u64 {     (1..64).fold(0u64, |r, s| r + square(s + 1)) }  fn total_map() -> u64 {     (1..64).map(|i| square(i + 1)).sum() } 

there few avenues check:

1. the generated llvm ir
2. the generated assembly
3. benchmark

the easiest source ir , assembly 1 of playgrounds (official or alternate). these both have buttons view assembly or ir. can pass --emit=llvm-ir or --emit=asm compiler generate these files.

make sure generate assembly or ir in release mode. attribute #[inline(never)] useful keep functions separate find them easier in output.

benchmarking documented in rust programming language, there's no need repeat valuable information.

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before rust 1.14, these not produce exact same assembly. i'd wait benchmarking / profiling data see if there's meaningful impact on performance before worried.

as of rust 1.14, do produce same assembly! 1 reason love rust. can write clear , idiomatic code , smart people come along , make equally fast.

but first code iterate 2 times, once map , once sum.

this incorrect, , i'd love know source told can go correct @ point , prevent future misunderstandings. an iterator operates on pull basis; 1 element processed @ time. core method next, yields single value, running enough computation produce value.