On Wed, Sep 25, 2013 at 2:43 PM, Marshall Clow <[email protected]>wrote:
> On Sep 25, 2013, at 2:37 PM, Marshall Clow <[email protected]> wrote: > > > On Sep 25, 2013, at 2:14 PM, Benjamin Kramer <[email protected]> > 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. This only applies to calls made as part of a new expression -- an explicit call cannot be transformed, it is allowed to have observed side effects according to as-if.
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