RFR 8249893: AARCH64: optimize the construction of the value from the bits of the other two

[Andrew Haley]

Hi,

On 23/08/2020 19:20, Boris Ulasevich wrote:
  >
  > Please review the updated change to C2 and AArch64 which introduces
  > a new BitfieldInsert node to replace Or+Shift+And sequence when possible.
  > Single BFI instruction is emitted for the new node.
  >
  > With the current change all the transformation logic is moved out of
  > aarch64.ad file into the common C2 code.
  >
  > http://bugs.openjdk.java.net/browse/JDK-8249893
  > http://cr.openjdk.java.net/~bulasevich/8249893/webrev.01
  >
  > The change in compiler.cpp was done to implicitly ask IGVN to run
  > the idealization once again after the loop optimization phase.
  > This extra step is necessary to make the BFI transform happen
  > only after loop optimization.

So here's a strange thing. When I run a simple JMH test

      @State(Scope.Benchmark)
      public static class Result {
          public int a, b;
          public long x;
      }

      @Benchmark
      public static int bfm(Result r) {
          return (r.a & 0xFF) | ((r.b & 0xFF) << 8);
      }

I get

    0x0000ffff84644df0:   ubfiz	w12, w11, #8, #8
    0x0000ffff84644df4:   and	w10, w10, #0xff
    0x0000ffff84644df8:   orr	w2, w10, w12                ;*ior {reexecute=0 rethrow=0 return_oop=0}
                                                              ; - org.openjdk.Rotates::bfm at 19 (line 22)
                                                              ; - org.openjdk.generated.Rotates_bfm_jmhTest::bfm_avgt_jmhStub at 20 (line 199)

instead of

    0x0000ffff808554b4: and	w10, w10, #0xff
    0x0000ffff808554b8: and	w12, w12, #0xff
    0x0000ffff808554bc: orr	w2, w12, w10, lsl #8  ;*ior
                                                  ; - org.openjdk.Rotates::bfm at 19 (line 22)
                                                  ; - org.openjdk.generated.Rotates_bfm_jmhTest::bfm_avgt_jmhStub at 20 (line 199)

Do you have any ideas why this might be? Thanks.

-- 
Andrew Haley  (he/him)
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
https://keybase.io/andrewhaley
EAC8 43EB D3EF DB98 CC77 2FAD A5CD 6035 332F A671