[libcxx] Patch/RFC: Avoid using ::operator new in the default allocator

[Marshall Clow]

On Sep 25, 2013, at 2:37 PM, Marshall Clow <mclow.lists at gmail.com> wrote:

> On Sep 25, 2013, at 2:14 PM, Benjamin Kramer <benny.kra at gmail.com> wrote:
> 
>> Since N3664 was implemented in Clang (r186799) it can't optimize unused pairs of ::operator new and ::operator delete anymore. Calls generated by a new/delete expression are still foldable with the updated wording. This affects optimizing away unnecessary code that would be really nice to get right. For example
>> 
>> #include <vector>
>> 
>> int main() {
>> std::vector<int> v;
>> v.push_back(1);
>> 
>> return v[0];
>> }
>> 
>> This should fold down to "return 1;" with no allocations. The example is of course oversimplified but situations like this easily occur in real world code through inlining.
>> 
>> The proposed patch replaces "::operator new(x)" with "new char[x]" and adds the necessary casts in the allocator class, as suggested by Richard Smith. This is sufficient to constant fold code like my test case again.
> 
> Is there some technical reason that clang cannot optimize away unused pairs of ::operator new/delete? (as opposed to "just doesn't do it any more" - not a real quote).

Added in N3664:
	An implementation is allowed to omit a call to a replaceable global allocation function (18.6.1.1, 18.6.1.2). 

To me, "replaceable global allocation function" --> ::operator new.

-- Marshall

Marshall Clow     Idio Software   <mailto:mclow.lists at gmail.com>

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