Re: [Caml-list] When can we ignore CAMLparam and CAMLreturn?

[Jianzhou Zhao <jianzhou@seas.upenn.edu>]

On Wed, Oct 27, 2010 at 5:36 AM, Goswin von Brederlow <goswin-v-b@web.de> wrote:
> Jacques Garrigue <garrigue@math.nagoya-u.ac.jp> writes:
>
>> On 2010/10/26, at 1:19, Jianzhou Zhao wrote:
>>
>>> Hi All,
>>>
>>> Here is the code from LLVM-OCaml bindings.
>>>
>>> /////////////////
>>> /* llvalue -> GenericValue.t array -> ExecutionEngine.t -> GenericValue.t */
>>> CAMLprim value llvm_ee_run_function(LLVMValueRef F, value Args,
>>>                                   LLVMExecutionEngineRef EE) {
>>> unsigned NumArgs;
>>> LLVMGenericValueRef Result, *GVArgs;
>>> unsigned I;
>>>
>>> NumArgs = Wosize_val(Args);
>>> GVArgs = (LLVMGenericValueRef*) malloc(NumArgs * sizeof(LLVMGenericValueRef));
>>> for (I = 0; I != NumArgs; ++I)
>>>   GVArgs[I] = Genericvalue_val(Field(Args, I));
>>>
>>> Result = LLVMRunFunction(EE, F, NumArgs, GVArgs);
>>>
>>> free(GVArgs);
>>> return alloc_generic_value(Result);
>>> }
>>> ////////////////////////
>>>
>>> The 'llvm_ee_run_function' does not protect the Args parameter by
>>> CAMLparam with CAMLreturn. Is this safe in this case, because we
>>> allocated a GVArgs? The Ocaml manual suggests to use CAMLparam for any
>>> value parameters (http://caml.inria.fr/pub/docs/manual-ocaml/manual032.html#toc140).
>>
>> The basic rule is that those macros are only needed if some allocation in the caml heap
>> occurs before accessing a caml value. Expressed another way, whenever allocation is
>> used all unprotected pointers to the caml heap should be considered as invalid.
>>
>> Since in the above code there is no allocation in the caml heap before the Genericvalue_val's,
>> Args need not be protected. Similarly for the result, we are just returning the result of another
>> function, with no risk of corruption.
>>
>> When in doubt, it is always safer to use CAMLparam/CAMLreturn, eventhough they will
>> generate a bit more code.
>
> As a special case: If you enter/leave_blocking_section() then some other
> thread might (almost certainly will) do an allocation. So any ocaml
> values you need in enter/leave_blocking_section() need to be copied to C
> values beforehand and values you need after need to be protected.
>
> MfG
>        Goswin
>

Does it imply that, if any Ocaml binding uses
enter/leave_blocking_section, the Ocaml runtime only switch contexts
at enter/leave_blocking_section(). If it allows other threads run at
any point, we might have to protect values in any function.

-- 
Jianzhou