RFR: 8269878: Handle redundant reg-2-reg moves in X86 backend

[Vladimir Kozlov]

On Tue, 6 Jul 2021 17:04:28 GMT, Jatin Bhateja <jbhateja at openjdk.org> wrote:

> Chain patterns which connects various legacy, VL and non-legacy USE - DEF operands during selection need not emit a reg-2-reg move instruction. C2 register allocator can handle these cases by back propagating the register mask from USE to DEF e.g.
> 
> USE (regD) <- DEF( legRegD) : Not an issue since regD can accommodate ranges allocated to legRegD.
> 
> USE (legRegD) <- USE (regD) : Not an issue since Allocator will back propagate legRegD mask from USE to its DEF node, which will constrain the allocation set to lower register bank.
> 
> Same logic applies to vlRegD , rRegD, legRegD combinations.
> 
> These machine nodes are redundant post selection and can be removed during generic operand resolution phase.

Testing tier1-3 passed.

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

Marked as reviewed by kvn (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/4693