Re: Integer overflow in operator new

[Florian Weimer <fw at deneb dot enyo dot de>]

* Joe Buck:

> Consider an implementation that, when given
>
> 	 Foo* array_of_foo = new Foo[n_elements];
>
> passes __compute_size(elements, sizeof Foo) instead of n_elements*sizeof Foo
> to operator new, where __compute_size is
>
> inline size_t __compute_size(size_t num, size_t size) {
>     size_t product = num * size;
>     return product >= num ? product : ~size_t(0);
> }

I don't think this check is correct.  Consider num = 0x33333334 and
size = 6.  It seems that the check is difficult to perform efficiently
unless the architecture provides unsigned multiplication with overflow
detection, or an instruction to implement __builtin_clz.