eliminate dead stores across functions

[Prathamesh Kulkarni <prathamesh dot kulkarni at linaro dot org>]

Hi,
For the following test-case,

int a;

__attribute__((noinline))
static void foo()
{
  a = 3;
}

int main()
{
  a = 4;
  foo ();
  return a;
}

I assume it's safe to remove "a = 4"  since 'a' would be overwritten
by call to foo ?
IIUC, ipa-reference pass does mod/ref analysis to compute side-effects
of function call,
so could we perhaps use ipa_reference_get_not_written_global() in dse
pass to check if a global variable will be killed on call to a
function ? If not, I suppose we could write a similar ipa pass that
computes the set of killed global variables per function but I am not
sure if that's the correct approach.

Thanks,
Prathamesh