On Sun, Feb 10, 2013 at 11:45 AM, Andrey Belevantsev <a...@ispras.ru> wrote:
> Hi Kartik,
>
>
> On Sun, 10 Feb 2013 15:41:17 +0530, Kartik Singhal <kartiksing...@gmail.com>
> wrote:
>>
>> Thanks Richard for pointing out tree-ssa-sccvn.c
>>
>> On Wed, Feb 6, 2013 at 8:14 PM, Richard Biener
>> <richard.guent...@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".
Yes. And of course the various a_* are not "the same variable" - they may have
overlapping lifetimes, so an assign to a_2 does not kill a_1.
Richard.
> Andrey
>
>>
>>
>>
>> --
>> Kartik
>> http://k4rtik.wordpress.com/
