[Caml-list] Data representation of records

[Caml-list] Data representation of records

[Ryohei Tokuda]

Dear the compiler developers,

I am just curious about memory representation of records.
When I read dump of `clambda`, I felt the representation of records a
little bit strange.
In the following program, functions just create the variants and records.

a.ml

  type t = A of int | B of float
  type r1 = {x:int; y:float}

  let make_a x = A x
  let make_b x = B x
  let make_r1 x y = {x;y}

  type r2 = {a:int; b:int}

  let make_r2 a b = {a; b}

The result of `ocamlopt -dclambda -c a.ml` show the following.

(seq
  (let
    (make_a/1014
       (closure  (fun camlA__make_a_1014 1  x/1015 (makeblock 0 x/1015)) ))
    (setfield_imm 0 (global camlA!) make_a/1014))
  (let
    (make_b/1016
       (closure  (fun camlA__make_b_1016 1  x/1017 (makeblock 1 x/1017)) ))
    (setfield_imm 1 (global camlA!) make_b/1016))
  (let
    (make_r/1018
       (closure
         (fun camlA__make_r_1018 2  x/1019 y/1020 (makeblock 0 x/1019
y/1020)) ))
    (setfield_imm 3 (global camlA!) make_r/1018))
  (let
    (make_r/1024
       (closure
         (fun camlA__make_r_1024 2  a/1025 b/1026 (makeblock 0 a/1025
b/1026)) ))
    (setfield_imm 2 (global camlA!) make_r/1024))
  0a)

This dump of IL is likely to show:
- `makeblock` seems like `alloc`, which first argument is the label.
- `makeblock` creates  variants, and labels are different in A and B
(0 for A, and 1 for B).
- `makeblock` also creates records, and the label seems always 0.

I checked the above understanding by reading output assemblies,
I believe it is right comprehension.

My question is, why records need the labels.
In my comprehension, there is no chance that we check the label of records.

Thanks.

-- 
Ryohei Tokuda

Re: [Caml-list] Data representation of records

[Leo White]

> My question is, why records need the labels.
> In my comprehension, there is no chance that we check the label of records.

The "label" is called the tag. Every allocated object in OCaml has a tag. They are stored in the header word and are part of the information used by the GC to traverse OCaml values (they are also used by the non-parametric polymorphic functions like "=" and "Hashtbl.hash"). Tags above a certain number have special meaning for the GC, all the rest are used for ordinary values (records, variants etc.). Records have to be given one of these ordinary tags so it might as well be 0.

Regards,

Leo