Making alloca more safe

[Walter Bright]

Denis Koroskin wrote:
> C standard library alloca function has an undefined behavior when 
> requested size is large enough to cause a stack overflow, but many 
> (good) implementations return null instead. So does DMD, for example. I 
> believe it would be even better to go ahead and enforce D implementation 
> to return a GC allocated chunk of memory instead of null in that case. 
> It will not incur any performance hit in 99.9% of the cases and prevent 
> a bug from being happen in the rest. It will also make writing code 
> using it easier (and more safe), since you don't have to worry about 
> possible stack overflow/null-dereference.

I'm a little reluctant to do this because alloca is supposed to be a low 
level routine, not one that has a dependency on the rather large and 
complex gc. A person using alloca is expecting stack allocation, and 
that it goes away after the function exits. Switching arbitrarily to the 
gc will not be detected and may hide a programming error (asking for a 
gigantic piece of memory is not anticipated for alloca, and could be 
caused by an overflow or logic error in calculating its size).

And secondly, I wish to emphasize that a null pointer seg fault is not 
an unsafe thing. It does not lead to memory corruption. It simply stops 
the program.