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  1. JDK
  2. JDK-8148328

aarch64: redundant lsr instructions in stub code

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    Details

    • Type: Enhancement
    • Status: Resolved
    • Priority: P4
    • Resolution: Fixed
    • Affects Version/s: None
    • Fix Version/s: 9
    • Component/s: hotspot
    • Labels:
      None
    • Subcomponent:
    • Introduced In Version:
      9
    • Resolved In Build:
      b106
    • CPU:
      aarch64
    • OS:
      linux

      Backports

        Description

        For JDK8 & JDK9, there two redundant lsr instructions in jbyte_arraycopy and jbyte_disjoint_arraycopy stub as shown in the following code snippet:
        StubRoutines::jbyte_disjoint_arraycopy [0x0000007f7c0bef80, 0x0000007f7c0bf020[ (160 bytes)
          0x0000007f7c0bef80: stp x29, x30, [sp,#-16]!
          0x0000007f7c0bef84: mov x29, sp
          0x0000007f7c0bef88: cmp x2, #0x10
          0x0000007f7c0bef8c: b.cc Stub::jbyte_disjoint_arraycopy+100 0x0x7f7c0befe4
          0x0000007f7c0bef90: neg x9, x0
          0x0000007f7c0bef94: and x9, x9, #0xf
          0x0000007f7c0bef98: cbz x9, Stub::jbyte_disjoint_arraycopy+92 0x0x7f7c0befdc
          0x0000007f7c0bef9c: lsr x9, x9, #0
          0x0000007f7c0befa0: sub x2, x2, x9
          0x0000007f7c0befa4: tbz w9, #3, Stub::jbyte_disjoint_arraycopy+48 0x0x7f7c0befb0
          0x0000007f7c0befa8: ldr x8, [x0],#8
          0x0000007f7c0befac: str x8, [x1],#8
          0x0000007f7c0befb0: tbz w9, #2, Stub::jbyte_disjoint_arraycopy+60 0x0x7f7c0befbc
          0x0000007f7c0befb4: ldr w8, [x0],#4
          0x0000007f7c0befb8: str w8, [x1],#4
          0x0000007f7c0befbc: tbz w9, #1, Stub::jbyte_disjoint_arraycopy+72 0x0x7f7c0befc8
          0x0000007f7c0befc0: ldrh w8, [x0],#2
          0x0000007f7c0befc4: strh w8, [x1],#2
          0x0000007f7c0befc8: tbz w9, #0, Stub::jbyte_disjoint_arraycopy+84 0x0x7f7c0befd4
          0x0000007f7c0befcc: ldrb w8, [x0],#1
          0x0000007f7c0befd0: strb w8, [x1],#1
          ........

        A proposed patch to fix the issue:
        diff -r 119702fc4dea src/cpu/aarch64/vm/stubGenerator_aarch64.cpp
        --- a/src/cpu/aarch64/vm/stubGenerator_aarch64.cpp Tue Jan 26 17:13:18 2016 +0100
        +++ b/src/cpu/aarch64/vm/stubGenerator_aarch64.cpp Wed Jan 27 12:11:24 2016 +0800
        @@ -954,7 +954,7 @@
                            Register count, Register tmp, int step) {
             copy_direction direction = step < 0 ? copy_backwards : copy_forwards;
             bool is_backwards = step < 0;
        - int granularity = uabs(step);
        + int shift, granularity = uabs(step);
             const Register t0 = r3, t1 = r4;

             if (is_backwards) {
        @@ -962,7 +962,7 @@
               __ lea(d, Address(d, count, Address::lsl(exact_log2(-step))));
             }

        - Label done, tail;
        + Label tail;

             __ cmp(count, 16/granularity);
             __ br(Assembler::LO, tail);
        @@ -985,9 +985,12 @@
                 __ neg(rscratch2, s);
                 __ andr(rscratch2, rscratch2, 2 * wordSize - 1);
               }
        + shift = exact_log2(granularity);
               // rscratch2 is the byte adjustment needed to align s.
               __ cbz(rscratch2, aligned);
        - __ lsr(rscratch2, rscratch2, exact_log2(granularity));
        + if (shift > 0) {
        + __ lsr(rscratch2, rscratch2, shift);
        + }
               __ sub(count, count, rscratch2);

         #if 0

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                Assignee:
                fyang Fei Yang
                Reporter:
                fyang Fei Yang
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                  Created:
                  Updated:
                  Resolved: