Re: Global Value Numbering and dependence on SSA in GCC

[Andrey Belevantsev <abel at ispras dot ru>]

Hi Kartik,

On Sun, 10 Feb 2013 15:41:17 +0530, Kartik Singhal 
<kartiksinghal@gmail.com> wrote:
> Thanks Richard for pointing out tree-ssa-sccvn.c
>
> On Wed, Feb 6, 2013 at 8:14 PM, Richard Biener
> <richard.guenther@gmail.com> wrote:
> > Well, to ignore SSA form simply treat each SSA name as separate 
> > variable.
> > You of course need to handle PHI nodes as copies on CFG edges then.
>
> I am not sure if I understood this correctly.
>
> Consider the following example:
>
>      if (...)
>        a_1 = 5;
>      else if (...)
>        a_2 = 2;
>      else
>        a_3 = 13;
>
>      # a_4 = PHI <a_1, a_2, a_3>
>      return a_4;
>
> Do you mean that I treat a_1, a_2 and a_3 as 3 different variables? 
> In
> this approach, I lose the information that they are actually the same
> variables.
>
> Or should I write a mini lexer function to convert the SSA names into
> original variable names by removing _1, _2, etc. as suffix from each?
>
> Regarding PHI nodes, I think you mean, I should treat them as 
> identity
> functions, but I am not clear exactly how in the first approach 
> above.
> In second, I can just treat it as a=a.

Richard means that you can treat the above code as

      if (...) {
         a_1 = 5;
         a_4 = a_1;
      } else if (...) {
         a_2 = 2;
         a_4 = a_2;
      } else {
         a_3 = 13;
         a_4 = a_3;
      }
      return a_4;

The extra code above is "copies on CFG edges".

Andrey

> --
> Kartik
> http://k4rtik.wordpress.com/