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

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