[LLVMdev] optimization assumes malloc return is non-null

[Chris Lattner]

On Wed, 30 Apr 2008, David Vandevoorde wrote:
> Note that more interesting optimizations are possible.  E.g., it's
> perfectly valid to transform:
>
> 	void f(size_t n) {
> 	  char *str = (char*)malloc(n);
> 	  // use str[0 .. 99 ]
> 	  free(str);
> 	}
>
> into
>
> 	void f(size_t n) {
> 	  char *str = (char*)alloca(n);
> 	  // use str[0 .. 99 ]
> 	}
>
> (Can LLVM do that?  Is that maybe why the optimization happens?)

This isn't safe in general unless you can (tightly) bound "n".  You don't 
want to overflow the stack.

We do delete "free(malloc(n))" though.

-Chris

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