Replacing malloc with alloca.

[Florian Weimer]

* Ajit Kumar Agarwal:

> The replacement of malloc with alloca can be done on the following
> analysis.
>
> If the lifetime of an object does not stretch beyond the immediate
> scope. In such cases the malloc can be replaced with alloca.  This
> increases the performance to a great extent.

You also need to make sure that the object is small (less than a page)
and that there is no deep recursion going on.  Otherwise, the program
may no longer work after the transformation with real-world restricted
stack sizes.  It may even end up with additional security issues.