RFC : Approach to handle Allocation Merges in C2 Scalar Replacement

[Vladimir Kozlov]

We can't do that unless we know that p0 does not escape in call() (we do such analysis in EA code already).
Otherwise there could be concurrent p0's fields updates.

Thanks,
Vladimir K

On 2/11/22 1:38 AM, Nils Eliasson wrote:
> This is also useful if only one of the paths are a local allocation. We must just be sure that the original phi actually 
> can be removed.
> 
> public int ex1(boolean cond, int first, int second) {
> 
>      p0 = call(...)
> 
>      if (cond) {
>          p1 = Allocate(...);
>          ...
>          p1.x = second;
>          p1.y = first;
>      }
> 
>      p = phi(p0, p1) // unused and will be removed
> 
> 
>      return phi(p0.x,p1.x) - phi(p0.y, p1.y);
> }
> 
> Regards,
> Nils
> 
> On 2022-02-11 05:07, Vladimir Kozlov wrote:
>> For this simple case we can teach C2's IGVN to split fields loads through Phi so that phi(p0, p1) is not used and 
>> allocations as well. We can do that because we know that allocations and phi do not escape.
>>
>> public int ex1(boolean cond, int first, int second) {
>>     p0 = Allocate(...);
>>     ...
>>     p0.x = first;
>>     p0.y = second;
>>
>>     if (cond) {
>>         p1 = Allocate(...);
>>         ...
>>         p1.x = second;
>>         p1.y = first;
>>     }
>>
>>     p = phi(p0, p1) // unused and will be removed
>>
>>
>>     return phi(p0.x,p1.x) - phi(p0.y, p1.y);
>> }
>>
>> Thanks,
>> Vladimir K 
>