Re: eliminate dead stores across functions

[Richard Biener <richard dot guenther at gmail dot com>]

On Tue, Mar 6, 2018 at 4:50 PM, Bin.Cheng <amker.cheng@gmail.com> wrote:
> On Tue, Mar 6, 2018 at 2:28 PM, Richard Biener
> <richard.guenther@gmail.com> wrote:
>> On Tue, Mar 6, 2018 at 1:00 PM, Prathamesh Kulkarni
>> <prathamesh.kulkarni@linaro.org> wrote:
>>> 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.
>>
>> Do you think the situation happens often enough to make this worthwhile?
> There is one probably more useful case.  Program may use global flags
> controlling
> how it does (heavy) computation.  Such flags are only set couple of
> times in execution
> time.  It would be useful if we can (IPA) propagate flags into computation heavy
> functions by versioning (if necessary).  For example:
>
> int flag = 1;
> void foo ()
> {
>   //heavy computation wrto to flag
> }
> void main()
> {
>   flag = 2;
>   foo();
>   flag = 1;
>   foo();
> }

Yeah, libquantum does this.  There's also related example
from some SPEC fortran testcase:

vodi foo()
{
  static int initialized;
  static T data;
  if (!initialized)
    {
       data.x = 1;
       initialized = 1;
    }
...
}

where we want to constant propagate from data.x.  IIRC I tried
to work on this, not sure if I solved it yet...

Richard.

> Of course this may only be useful for LTO.

> Thanks,
> bin
>>
>> ipa-reference doesn't compute must-def, only may-def and may-use IIRC.
>>
>> Richard.
>>
>>> Thanks,
>>> Prathamesh