RFC : Approach to handle Allocation Merges in C2 Scalar Replacement

[Nils Eliasson]

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