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Re: predicated code motion (in lim)


Got it, thanks.

(Now there are phi instead of if)

On Fri, Oct 10, 2014 at 6:18 PM, Richard Biener
<richard.guenther@gmail.com> wrote:
> On Fri, Oct 10, 2014 at 3:44 PM, Evgeniya Maenkova
> <evgeniya.maenkova@gmail.com> wrote:
>> Hi,
>> could anyone clarify about predicated code motion in lim?
>>
>> After reading a TODO in /tree-ssa-loop-im.c (see [1]) I tried several
>> examples, say [2]. However, in all of them the code was moved out of
>> the loop successfully (either by pre or by lim, as in [2]).
>>
>> So my question is: what the author of this code did mean by
>> "predicated code motion"? (what is the TODO)
>
> It means transforming
>
>> Thanks,
>>
>> Evgeniya
>>
>> [1]
>> TODO:  Support for predicated code motion.  I.e.
>>
>>    while (1)
>>      {
>>        if (cond)
>>          {
>>            a = inv;
>>            something;
>>          }
>>      }
>
> this to
>
>     if (cond)
>      {
>        a= inv;
>        something;
>      }
>    while (1)
>      ;
>
> which is currently supported in a very limited way by emitting
> this as
>
>    a = cond ? inv : a;
>
> for at most two statements.  As it executes stmts unconditionally
> that way it is limited to non-trapping operations.
>
> Richard.
>
>
>> [2]
>>
>> void foo(int cond, int inv)
>> {
>>
>>     int a;
>>     int i = 0;
>>     int j = 0;
>>     while (j++ < 100) {
>>         while (i++ < 2000)
>>          {
>>            if (j % 2)
>>             {
>>                 a = 528*j;
>>                 printf("Hey1%d %d", a, i);//something;
>>              }
>>         }
>>
>>      }
>> }
>>
>> lim:
>>
>> ;; Function foo (foo, funcdef_no=0, decl_uid=1394, cgraph_uid=0, symbol_order=0)
>>
>> foo (int cond, int inv)
>> {
>>   int j;
>>   int i;
>>   int a;
>>   unsigned int j.0_12;
>>   unsigned int _13;
>>   unsigned int _18;
>>   unsigned int _21;
>>
>>   <bb 2>:
>>   goto <bb 8>;
>>
>>   <bb 3>:
>>   if (_13 != 0)
>>     goto <bb 4>;
>>   else
>>     goto <bb 10>;
>>
>>   <bb 10>:
>>   goto <bb 5>;
>>
>>   <bb 4>:
>>   printf ("Hey1%d %d", a_14, i_11);
>>
>>   <bb 5>:
>>
>>   <bb 6>:
>>   # i_1 = PHI <i_22(8), i_11(5)>
>>   i_11 = i_1 + 1;
>>   if (i_1 <= 1999)
>>     goto <bb 3>;
>>   else
>>     goto <bb 7>;
>>
>>   <bb 7>:
>>   # i_20 = PHI <i_11(6)>
>>   j_9 = j_23 + 1;
>>   if (j_23 != 100)
>>     goto <bb 11>;
>>   else
>>     goto <bb 9>;
>>
>>   <bb 11>:
>>
>>   <bb 8>:
>>   # i_22 = PHI <i_20(11), 0(2)>
>>   # j_23 = PHI <j_9(11), 1(2)>
>>   j.0_12 = (unsigned int) j_23;
>>   _13 = j.0_12 & 1;
>>   _21 = (unsigned int) j_23;
>>   _18 = _21 * 528;
>>   a_14 = (int) _18;
>>   goto <bb 6>;
>>
>>   <bb 9>:
>>   return;
>>
>> }
>>
>>
>> However, in loopinit (the optimization before lim) there was no motion
>> (So this was done by lim:
>>   <bb 4>:
>>   a_14 = j_23 * 528;
>>   printf ("Hey1%d %d", a_14, i_11);



-- 
Thanks,

Evgeniya

perfstories.wordpress.com


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