Replacing malloc with alloca.

[Jeff Law]

On 09/13/2015 12:28 PM, Florian Weimer wrote:
> * 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.
You also have to make sure you're not inside a loop.  Even a small 
allocation inside a loop is problematical from a security standpoint.

You also need to look at what other objects might be on the stack and 
you have to look at the functional scope, not the immediate scope as 
alloca space isn't returned until the end of a function.

jeff