[PATCH] Teach libc++ to use __builtin_operator_new/__builtin_operator_delete when available

[Marshall Clow]

LGTM, then.
Please commit.

— Marshall



On Wed, Jun 4, 2014 at 9:42 AM, Richard Smith <richard at metafoo.co.uk> wrote:

> On 4 Jun 2014 08:02, "Marshall Clow" <mclow.lists at gmail.com> wrote:
> >
> >
> > On Jun 3, 2014, at 3:14 PM, Richard Smith <richard at metafoo.co.uk> wrote:
> >
> > > Hi!
> > >
> > > This patch causes libc++ to use Clang's new __builtin_operator_new and
> __builtin_operator_delete builtins when possible. These builtins allow
> optimizations that the standard builtins do not: in particular, if we can
> remove all uses of an allocation, this permits removing the allocation
> itself too.
> > >
> > > In particular, this allows code using std::vector and its kin to be
> optimized away to nothing. Previously it would get optimized to the point
> where only the allocation and deallocation remained.
> > >
> > > This change has been applied to most calls to ::operator new and
> ::operator delete in libc++; I believe such optimizations are permitted by
> the specification of all affected library components.
> > >
> > > The exceptions are:
> > >  * get_temporary_buffer wants to call nothrow new, and we don't
> support that with the builtins (that would require teaching Clang about
> std::nothrow_t)
> > >  * __refstring is not affected; this doesn't seem particularly
> worthwhile since the only purpose for it is ABI interoperability with GCC
> > >
> > > One other caveat: the patch adds an include of <new> to <valarray>.
> <new> is extremely light, so I doubt this is a problem, but I thought it
> was worth calling out.
> >
> > The code here looks quite straightforward.
> > I don’t see any problems - but I do have questions.
> >
> > How does __builtin_operator_new/__builtin_operator_delete work if a user
> provides their own
> > global operator new/delete ?
> >
> > For example, given code like this:
> >
> > $ cat junk1.cpp
> >
> > int main () {
> >         // something that eventually calls __builtin_operator_new
> > }
> >
> > $ cat junk2.cpp
> > #include <new>
> >
> > char buffer[10240];
> > size_t offset = 0;
> >
> > void * operator new(size_t sz) throw(std::bad_alloc) {
> >         void *p = &buffer[offset];
> >         offset += sz;
> >         return p;
> > }
> > void operator delete(void *) {}
> >
> > does the user’s operator new get called?
>
> Yes, if the allocation isn't optimised out. (The builtins behave just like
> the allocation / deallocation in a new-expression or delete-expression.)
>
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