[aarch64-port-dev ] RFR: aarch64: elide DecodeN when followed by CmpP 0

[Edward Nevill]

Hi,

Please review the following webrev

http://cr.openjdk.java.net/~enevill/8136615/webrev/

JIRA issue here: https://bugs.openjdk.java.net/browse/JDK-8136615

C2 currently generates code like

  0x000003ff9d3e8920: ldr w11, [x1,#16]
  0x000003ff9d3e8924: lsl x10, x11, #3
  0x000003ff9d3e8928: cbnz x10, 0x000003ff9d3e8950

when doing a pointer comparison against 0

The lsl instruction is generated by a DecodeN node which can be elided in this case.

The following is a simple test case

public class decode {
    public static void main(String[] args) {
        if (args[0] != null) {
          System.out.println("not null");
        }
    }
}

java -XX:-TieredCompilation -Xcomp -XX:CompileOnly=decode::main -XX:+UnlockDiagnosticVMOptions -XX:+PrintAssembly -XX:+PrintOptoAssembly -XX:+PrintIdeal decode

The above is a trivial case, but I have seen many of these redundant lsl instructions generated in various list chasing code.

Tested with jtreg hotspot and langtools with no regressions.

Thanks for your review,
Ed.