Re: [Caml-list] an implicit GC rule?

[Xavier Leroy <Xavier.Leroy@inria.fr>]

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On Sat, May 5, 2018 at 5:25 AM Chet Murthy <murthy.chet@gmail.com> wrote:

> It's been a while since I did this sort of thing, but I suspect if you
> declare CAMLlocal variables for each intermediate expression, and stick in
> the assignments, that should solve your problem (while not making your code
> too ugly).  E.g.
>
> CAMLprim value left_comb(value a, value b, value c)
> {
>     CAMLparam3(a, b, c);
>   CAMLlocal5(l1, l2, l3,  l4, l5);
>     CAMLreturn(l1 = Tree(l2 = Tree((l3 = Leaf(a)), (l4 = Leaf(b)), (l5 =
> Leaf(c))));
> }
>

That's bold C/C++ programming!  It might even work in C++, where assignment
expressions are l-values if I remember correctly.

However, I'm afraid it won't work in C because an assignment expression "lv
= rv" is a r-value equal to the value of rv converted to the type of lv at
the time the assignment is evaluated.  So, if lv is a local variable
registered with the GC, the GC will update lv when needed, but the "lv =
rv" expression will keep its initial value.

There's also the rules concerning sequence points.  I think the code above
respects the C99 rules but I'm less sure about the C11 rules.

>
> Even better, you could linearize the tree of expressions into a sequence,
> and that should solve your problem, also.
>

Yes, that's the robust solution.  Spelling it out:

CAMLprim value left_comb(value a, value b, value c)
{
  CAMLparam3(a, b, c);
  CAMLlocal5(la, lb, lc, tab, t);
  la = Leaf(a);
  lb = Leaf(b);
  lc = Leaf(c);
  tab = Tree(la, lb);
  t = Tree(tab, lc);
  CAMLreturn(t);
}

You can also do "CAMLreturn(Tree(tab, lc))" directly.

- Xavier Leroy


> Uh, I think.  Been a while since I wrote a lotta C/C++ code to interface
> with Ocaml, but this oughta work.
>
> --chet--
>
>
> On Wed, May 2, 2018 at 9:09 AM, Frederic Perriot <fperriot@gmail.com>
> wrote:
>
>> Hello caml-list,
>>
>> I have a GC-related question. To give you some context, I'm writing a
>> tool to parse .cmi files and generate .h and .c files, to facilitate
>> constructing OCaml variants from C bindings.
>>
>> For instance, given the following source:
>>
>> type 'a tree = Leaf of 'a | Tree of 'a tree * 'a tree [@@h_file]
>>
>>
>> the tool produces C functions:
>>
>> CAMLprim value Leaf(value arg1)
>> {
>>     CAMLparam1(arg1);
>>     CAMLlocal1(obj);
>>
>>     obj = caml_alloc_small(1, 0);
>>
>>     Field(obj, 0) = arg1;
>>
>>     CAMLreturn(obj);
>> }
>>
>> CAMLprim value Tree(value arg1, value arg2)
>> {
>>   // similar code here
>> }
>>
>>
>> From there, it's tempting to nest calls to variant constructors from C
>> and write code such as:
>>
>> CAMLprim value left_comb(value a, value b, value c)
>> {
>>     CAMLparam3(a, b, c);
>>     CAMLreturn(Tree(Tree(Leaf(a), Leaf(b)), Leaf(c)));
>> }
>>
>>
>> The problem with the above is the GC root loss due to the nesting of
>> calls to allocating functions.
>>
>> Say Leaf(c) is constructed first, and the resulting value cached in a
>> register, then Leaf(b) triggers a collection, thus invalidating the
>> register contents, and leaving a dangling pointer in the top Tree.
>>
>> Here is an actual ocamlopt output, with Leaf(c) getting cached in rbx:
>>
>>    0x000000000040dbf4 <+149>:    callq  0x40d8fd <Leaf>
>>    0x000000000040dbf9 <+154>:    mov    %rax,%rbx
>>    0x000000000040dbfc <+157>:    mov    -0x90(%rbp),%rax
>>    0x000000000040dc03 <+164>:    mov    %rax,%rdi
>>    0x000000000040dc06 <+167>:    callq  0x40d8fd <Leaf>
>>    0x000000000040dc0b <+172>:    mov    %rax,%r12
>>    0x000000000040dc0e <+175>:    mov    -0x88(%rbp),%rax
>>    0x000000000040dc15 <+182>:    mov    %rax,%rdi
>>    0x000000000040dc18 <+185>:    callq  0x40d8fd <Leaf>
>>    0x000000000040dc1d <+190>:    mov    %r12,%rsi
>>    0x000000000040dc20 <+193>:    mov    %rax,%rdi
>>    0x000000000040dc23 <+196>:    callq  0x40da19 <Tree>
>>    0x000000000040dc28 <+201>:    mov    %rbx,%rsi
>>    0x000000000040dc2b <+204>:    mov    %rax,%rdi
>>    0x000000000040dc2e <+207>:    callq  0x40da19 <Tree>
>>
>>
>> While the C code clearly violates the spirit of the GC rules, I can't
>> help but feel this is still a pitfall.
>>
>> Rule 2 of the manual states: "Local variables of type value must be
>> declared with one of the CAMLlocal macros. [...]"
>>
>> But here, I'm not declaring local variables, unless you count compiler
>> temporaries as local variables?
>>
>> I can see some other people making the same mistake I did. Should
>> there be an explicit warning in the rules? maybe underlining that
>> compiler temps count as variables, or discouraging the kind of nested
>> calls returning values displayed above?
>>
>> thanks,
>> Frédéric Perriot
>>
>> PS: this is also my first time posting to the list, so I take this
>> opportunity to thank you for the great Q's and A's I've read here over
>> the years
>>
>> --
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>
>
>

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