4 ms·

Idiomatic: https://godbolt.org/z/6ndYYxhx3 https://godbolt.org/z/6ndYYxhx3 Original: https://godbolt.org/z/rPfEWPGc1 https://godbolt.org/z/rPfEWPGc1 It’s cool

• Animats 3y ago
  It's surprisingly difficult to write something where the compiler doesn't detect that it's working on a constant. I had to code up a loop that pushed values onto the vector before the compiler generated the general case. "black_box" is not, apparently, opaque to the optimizer. Ideomatic form, inner loop: [1] LBB4_3: cmp byte ptr [rax + rcx], 0 jne .LBB4_4 inc rcx cmp rdx, rcx jne .LBB4_3 C-like form, inner loop: [2] .LBB4_3: cmp byte ptr [rax + rcx], 0 jne .LBB4_4 inc rcx cmp rdx, rcx jne .LBB4_3 Both ways, the compiler found the optimal solution. In the C-like loop, it was able to hoist and then elide the subscript check. In the ideomatic form, it's implicit in the iterator that you won't go off the end of the array. The lambda function was expanded inline and optimized. There's zero call overhead from using a lambda function there. Very nice work by the Rust compiler people and LLVM. [1] https://godbolt.org/z/5hn7Y9Mf9 https://godbolt.org/z/5hn7Y9Mf9 [2] https://godbolt.org/z/rW4MG4v3c https://godbolt.org/z/rW4MG4v3c