Re: [Caml-list] segfault in simple program with 4.02 native

["Milan Stanojević" <milanst@gmail.com>]

Could you do a dirty trick where you define a record that is the same
as the one you have now except that is has mutable fields, then you do
your parsing like now and then at the end return a record with
immutable fields (using (Obj.magic mutable : immutable)? You just need
to make sure that your mutable record doesn't escape your code.

On Sat, Sep 6, 2014 at 1:51 AM, Martin Jambon
<martin.jambon@ens-lyon.org> wrote:
> On Fri 05 Sep 2014 05:12:44 PM PDT, Jeremy Yallop wrote:
>>
>> On 6 September 2014 00:39, Martin Jambon <martin.jambon@ens-lyon.org>
>> wrote:
>>>
>>> That code is generated by atdgen. What happens is that we have to either
>>> create an empty record when starting to parse a list of unordered JSON
>>> fields, or use a bunch `let <field name> = ref None in` for each field
>>> and
>>> create the record in the end. While the latter approach is not much more
>>> work to implement, the resulting code was found to be significantly
>>> slower.
>>>
>>> The reason why it's using `Obj.magic 0.0` is that it worked in all cases
>>> (and has been for the past 4 years). Obtaining a well-formed constant
>>> value
>>> for any type is not trivial, so this what we have.
>>>
>>> It's very possible that it's now broken with OCaml 4.02. First try a
>>> 'make
>>> test' from atdgen's source directory (https://github.com/mjambon/atdgen)
>>> and
>>> see if it passes.
>>
>>
>> It does seem to be broken, and the change in behaviour with 4.0.2 is
>> apparently due to improved constant propagation
>> (http://caml.inria.fr/mantis/view.php?id=5779).
>>
>> The compiler now takes more advantage of immutability to improve the
>> memory usage and performance of programs.  It's safe (or ought to be
>> safe) to assume that immutable record fields are never updated, so the
>> values used to initialize the fields can be propagated to other parts
>> of the program.  Here's a small example that shows the change in
>> behaviour between 4.01 and 4.02.
>>
>>     type t = { s : string }
>>     let x = { s =  "one" }
>>     let () = Obj.(set_field (repr x) 0 (repr "two"))
>>     let () = print_endline x.s
>>
>> Using OCaml 4.01 the third line overwrites the field 's'  and the
>> fourth line reads the updated field and prints "two".  Using OCaml
>> 4.02 the initial value of the field is propagated past the write to
>> the code in the fourth line, so the program prints "one".
>>
>> The code currently generated by atdgen assumes that it's safe to treat
>> fields as if they were mutable -- that is, it assumes that it's safe
>> to initialize a field with a value of the wrong type, so long as the
>> value is overwritten before the field is first read.  I don't think
>> such tricks were ever explicitly guaranteed to work, but they're now
>> much more likely to fail, leading to the dummy initial value being
>> accessed at an inappropriate type.
>
>
> Thanks for the explanation, Jeremy. I guess atdgen will have to use "option
> refs" after all unless someone has a better idea.
>
> ATD definition:
>
> type t = {
>  ?field0: foo option;
>  ~field1: string;
>  field2: int;
> }
>
> Generated OCaml code:
>
> let field0 = ref None in
> let field1 = ref "" in
> let field2 = ref None in
> ...
> (* parse json fields coming in an unknown order *)
> ...
> {
>  field0 = !field0;
>  field1 = !field1;
>  field2 = (match !field2 with None -> error ... | Some x - >x);
>
> }
>
>
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