Re: operator new returns nonzero

[Marc Glisse <marc dot glisse at inria dot fr>]

On Sat, 7 Sep 2013, Mike Stump wrote:

> On Sep 7, 2013, at 3:33 AM, Marc Glisse <marc.glisse@inria.fr> wrote:
> > this patch teaches the compiler that operator new, when it can throw, isn't allowed to return a null pointer.
>
> You sure:
>
> @item -fcheck-new
> @opindex fcheck-new
> Check that the pointer returned by @code{operator new} is non-null
> before attempting to modify the storage allocated.  This check is
> normally unnecessary because the C++ standard specifies that
> @code{operator new} only returns @code{0} if it is declared
> @samp{throw()}, in which case the compiler always checks the
> return value even without this option.  In all other cases, when
> @code{operator new} has a non-empty exception specification, memory
> exhaustion is signalled by throwing @code{std::bad_alloc}.  See also
> @samp{new (nothrow)}.
>
> ?

Thanks, I didn't know that option. But it doesn't do the same. -fcheck-new 
is about the front-end inserting a test !=0 between malloc and the 
constructor. My patch is about teaching the middle-end that the value is 
not zero (so even user-coded comparisons with 0 can be simplified).

Now flag_check_new should probably disable this optimization... The 3 
-fcheck-new testcases in the testsuite probably only pass because they 
don't have -O2.

--
Marc Glisse