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

[Richard Smith]

Thanks, r210211.

On Wed, Jun 4, 2014 at 12:08 PM, Marshall Clow <mclow.lists at gmail.com>
wrote:

> 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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