[PATCH] Add __attribute__((malloc) to allocator and remove unused code

[Marc Glisse]

On Thu, 17 May 2018, Jonathan Wakely wrote:

> On 17/05/18 12:54 +0200, Marc Glisse wrote:
>> On Mon, 14 May 2018, Jonathan Wakely wrote:
>> 
>>> As discussed at https://gcc.gnu.org/ml/libstdc++/2018-01/msg00073.html
>>> we can simplify the allocator function for valarray memory. I also
>>> noticed that the _Array(size_t) constructor is never used.
>>>
>>> 	* include/bits/valarray_array.h (__valarray_get_memory): Remove.
>>> 	(__valarray_get_storage): Call operator new directly. Remove ignored
>>> 	top-level restrict qualifier and add malloc attribute instead.
>> 
>> I am trying to understand the point of adding this attribute. The function 
>> is just
>> 
>> { return static_cast<_Tp*>(operator new(__n * sizeof(_Tp))); }
>> 
>> The idea is that it isn't safe (? see PR 23383) to mark operator new with 
>> the attribute, but it is safe for this particular use?
>
> I'd forgotten about that (I was assuming the compiler doesn't need to
> be told about the properties of operator new, because they're defined
> by the language). We can remove the attribute.

I am not necessarily asking to remove it. I don't have a good 
understanding of what would break if we marked operator new with the 
attribute, so I have no idea if those reasons also apply for this use in 
valarray.

>> When optimizing, I certainly hope this trivial function gets inlined, and 
>> then the attribute is lost (should the inliner add 'restrict' when inlining 
>> a function with attribute malloc?) and all that matters is operator new.

If we determine that using the attribute here but not on operator new is 
the right choice, then I believe we need some middle-end tweaks so it 
isn't ignored.

-- 
Marc Glisse