[PATCH] A new HeapToStack allocation promotion pass

Marc Glisse marc.glisse at inria.fr
Fri Oct 4 00:08:47 PDT 2013

(I am not on the list)

> This adds a new optimization pass that can 'promote' malloc'd memory to 
> stack-allocated memory when the lifetime of the allocation can be 
> determined to be bounded by the execution of the function.
> To be specific, consider the following three cases:
> void bar(int *x);
> void foo1() {
>   int *x = malloc(16);
>   bar(x);
>   free(x);
> }
> In this case the malloc can be replaced by an alloca, and the free 
> removed. Note that this is true even though the pointer 'x' is definitely 
> captured (and may be recorded in global storage, etc.).


this seems to rely on the fact that 'bar' returns normally, and thus that 
whenever malloc is executed, free will be as well. However, bar could 
never return, or return abnormally by throwing an exception which will 
skip the call to free.

void bar(int *x) {
   throw 42;

will result in calling free on the stack. Now if there was a destructor 
calling free in foo1... Do you actually also consider exceptions when you 
test that all paths from malloc to the exits contain a call to free? That 
would only leave noreturn functions.

Marc Glisse

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