"not" defined by matching vs "external"

Learning
1

Consider the following 2 definitions of not:

let external_not x = not x

let match_not = function true -> false | false -> true

Godbolt https://godbolt.org/z/IJJAoU says we get different assembly:


camlExample__external_not_8:
        movq    %rax, %rbx
        movq    $4, %rax
        subq    %rbx, %rax
        ret
camlExample__match_not_15:
        cmpq    $1, %rax
        je      .L101
        movq    $1, %rax
        ret
.L101:
        movq    $3, %rax
        ret

Why is this?

2

Try this in godbolt

let t _ = true
let f _ = false

And I guess, you will answer you own question. Now, why booleans (and ints) are represented this way, is another question.

Short answer to differentiate them at runtime from pointers.

3

The not function from the standard library is implemented especially by the compiler backend so that more performant assembly is generated. The match_not function on the other hand is compiled according to the general compilation strategy that applies to any pattern matching (using branches), which results in slightly less efficient assembly.

4

to wit, stdlib.ml has:

external not : bool -> bool = "%boolnot"

The % in the external declaration indicates a compiler primitive.

5

https://github.com/ocaml/ocaml/pull/8547 and the other PRs it links seem related

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