Question on higher degree polynomial in scalar evolution

[Richard Biener]

On Wed, Feb 6, 2013 at 7:27 AM, Bin.Cheng <amker.cheng@gmail.com> wrote:
> Hi,
> Considering below program,
>
> extern int d;
>
> int func(void)
> {
>     int a, b, c;
>     a = 2;
>     c = 5;
>     do
>     {
>     b = a + 1;
>     d = a + c;
>     a = b;
>     c = d;
>     } while (a < 100);
>
>     return 0;
> }
>
> The gimple dump before pass_scev_cprop is like:
>
>   <bb 2>:
>
>   <bb 3>:
>   # a_1 = PHI <2(2), a_5(4)>
>   # c_2 = PHI <5(2), c_6(4)>
>   a_5 = a_1 + 1;
>   c_6 = a_1 + c_2;
>   if (a_5 != 100)
>     goto <bb 4>;
>   else
>     goto <bb 5>;
>
>   <bb 4>:
>   goto <bb 3>;
>
>   <bb 5>:
>   # d_lsm.4_12 = PHI <c_6(3)>
>   d = d_lsm.4_12;
>   return 0;
>
> When analyzing c_2=PHI<5,c_6> in scev_const_prop, resolver_mixers
> returns chrec_dont_know, rather than {5, +, 2, +, 1}.
> So two questions about this:
> 1. Is it expected to handle higher degree polynomial lie this?

Yes.

> If yes,
> 2. does it mean we don't use higher degree in misc. loop optimizations in GCC?

Yes.

> I just started reading this part of code, so please correct if I
> missed something.
>
> Thanks in advance.
>
> --
> Best Regards.