RFR 8227494 [lworld][c1] Optimize withfield bytecode into putfield

[Tobias Hartmann]

Hi Ioi,

this looks good to me!

Why is the change in c1_GraphBuilder.cpp:1879 necessary?

Best regards,
Tobias

On 24.07.19 07:30, Ioi Lam wrote:
> https://bugs.openjdk.java.net/browse/JDK-8227494
> http://cr.openjdk.java.net/~iklam/valhalla/8227494-withfield-optimization.v01/
> 
> 
> This optimization significantly reduces the amount of code generated by C1 for
> withfield bytecodes.
> 
> In the <init> methods of value classes, it's quite common to have a sequence like
> this, where we load a value object from a local slot, and overwrite the same local
> slot with a modified copy of the value object.
> 
>      defaultvalue #1 // class compiler/valhalla/valuetypes/MyValue1
>      astore 9
>      ...
>      iload_0
>      aload 9
>      swap
>      withfield #7 // Field x:I
>      astore 9
> 
> If this object was created by defaultvalue, and has not escaped, and is not stored
> in any other local slots, we can effectively treat the withfield/astore
> sequence as a single putfield bytecode.
> 
> I implemented a very rudimentary escape analysis in c1_GraphBuilder.cpp. It seems like
> there's no need to follow back branches like this:
> 
>         static FooValue test4() {
>             FooValue v = FooValue.default;
>             for (int i=1; i<=2; i++) {
>                 v = __WithField(v.x, i);
>                 v = __WithField(v.y, i);
>                 set_foo_static_if_null(v);
>             }
> 
>             return v;
>         }
> 
> The local slot for "v" in the loop body is a Phi node (not a NewValueTypeInstance node),
> so it will fail the "obj->is_optimizable_for_withfield()" check (which will only return true
> from NewValueTypeInstance) in GraphBuilder::withfield().
> 
> Thanks
> - Ioi
>