On Jun 3, 2014, at 3:14 PM, Richard Smith <[email protected]> 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?
— Marshall
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