Global Value Numbering and dependence on SSA in GCC

[Andrey Belevantsev]

 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/