Re: [RFC, PATCH] Split pool_allocator and create a new object_allocator

["Martin Liška" <mliska@suse.cz>]

On 07/03/2015 10:55 AM, Richard Sandiford wrote:
> Trevor Saunders <tbsaunde@tbsaunde.org> writes:
>> On Thu, Jul 02, 2015 at 09:09:31PM +0100, Richard Sandiford wrote:
>>> Martin Liška <mliska@suse.cz> writes:
>>>> diff --git a/gcc/asan.c b/gcc/asan.c
>>>> index e89817e..dabd6f1 100644
>>>> --- a/gcc/asan.c
>>>> +++ b/gcc/asan.c
>>>> @@ -362,20 +362,20 @@ struct asan_mem_ref
>>>>    /* Pool allocation new operator.  */
>>>>    inline void *operator new (size_t)
>>>>    {
>>>> -    return pool.allocate ();
>>>> +    return ::new (pool.allocate ()) asan_mem_ref ();
>>>>    }
>>>>  
>>>>    /* Delete operator utilizing pool allocation.  */
>>>>    inline void operator delete (void *ptr)
>>>>    {
>>>> -    pool.remove ((asan_mem_ref *) ptr);
>>>> +    pool.remove (ptr);
>>>>    }
>>>>  
>>>>    /* Memory allocation pool.  */
>>>> -  static pool_allocator<asan_mem_ref> pool;
>>>> +  static pool_allocator pool;
>>>>  };
>>>
>>> I'm probably going over old ground/wounds, sorry, but what's the benefit
>>> of having this sort of pattern?  Why not simply have object_allocators
>>> and make callers use pool.allocate () and pool.remove (x) (with pool.remove
>>> calling the destructor) instead of new and delete?  It feels wrong to me
>>> to tie the data type to a particular allocation object like this.
>>
>> Well the big question is what does allocate() do about construction?  if
>> it seems wierd for it to not call the ctor, but I'm not sure we can do a
>> good job of forwarding args to allocate() with C++98.
> 
> If you need non-default constructors then:
> 
>   new (pool) type (aaa, bbb)...;
> 
> doesn't seem too bad.  I agree object_allocator's allocate () should call
> the constructor.

Hello.

I do not insist on having a new/delete operator for aforementioned class.
However, I don't know a different approach that will do an object construction
in the allocate method w/o utilizing placement new?

> 
>> However it seems kind of wierd the operator new here is calling the
>> placement new on the object it allocates.
> 
> Yeah.
> 
>>> And using the pool allocator functions directly has the nice property
>>> that you can tell when a delete/remove isn't necessary because the pool
>>> itself is being cleared.
>>
>> Well, all these cases involve a pool with static storage lifetime right?
>> so actually if you don't delete things in these pool they are
>> effectively leaked.
> 
> They might have a static storage lifetime now, but it doesn't seem like
> a good idea to hard-bake that into the interface (by saying that for
> these types you should use new and delete, but for other pool-allocated
> types you should use object_allocators).  Maybe I just have bad memories
> from doing the SWITCHABLE_TARGET stuff, but there I was changing a lot
> of state that was "obviously" static in the old days, but that needed
> to become non-static to support vaguely-efficient switching between
> different subtargets.  The same kind of thing is likely to happen again.
> I assume things like the jit would prefer not to have new global state
> with load-time construction.

Agree with that it's a global state. But even before my transformation the code
utilized static variables that are similar problem from e.g. JIT perspective. Best
approach would be to encapsulate these static allocators to a class (a pass?). It's
quite a lot of work.

Thanks,
Martin

> 
> Thanks,
> Richard
>