RFR: 8253734: C2: Optimize Move nodes

[Tobias Hartmann]

On Fri, 23 Oct 2020 08:27:50 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

> Introduce the following transformations for Move nodes: 
> 1. `MoveI2F (MoveF2I x) => x`
> 
> 1. `MoveI2F (LoadI mem) => LoadF mem` 
> 
> 1. `StoreI mem (MoveF2I x) => StoreF mem x` 
> 
> (The same applies to MoveL2D/MoveD2L.)
> 
> №1 eliminates redundant operations and №2/№3 avoid reg-to-reg moves in generated code:
> 0x000000010d09964c:   vmovss 0x20(%rsi),%xmm1
> 0x000000010d099651:   vmovd  %xmm1,%eax                   ;*invokestatic floatToRawIntBits
> vs
> 0x0000000110c5a6cc:   mov    0x20(%rsi),%eax              ;*invokestatic floatToRawIntBits
>  
> 
> (№2 and №3 are performed late (after loop opts are over) to avoid high-level optimizations passes to handle newly introduced mismatched accesses.)
> 
> Testing: tier1-5.

Maybe add some comments with examples to the reinterpret methods.

src/hotspot/share/opto/movenode.hpp line 126:

> 124:   public:
> 125:   MoveL2DNode(Node* value) : MoveNode(value) {
> 126:     init_class_id(Class_Move);

Shouldn't `init_class_id` be moved to the `MoveNode` constructor?

src/hotspot/share/opto/movenode.hpp line 108:

> 106:   virtual Node* Ideal(PhaseGVN* phase, bool can_reshape);
> 107:   virtual Node* Identity(PhaseGVN* phase);
> 108: //  virtual const Type* Value(PhaseGVN* phase) const;

Can be removed.

-------------

Changes requested by thartmann (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/826