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  1. JDK
  2. JDK-8247838

PostLoopMultiversioning is broken and generates incorrect result

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    Details

    • Type: Bug
    • Status: Closed
    • Priority: P4
    • Resolution: Duplicate
    • Affects Version/s: 14, 15
    • Fix Version/s: 15
    • Component/s: hotspot
    • Labels:
    • Subcomponent:
    • CPU:
      x86
    • OS:
      generic

      Description

      C2 optimization PostLoopMultiversioning is broken and generates incorrect result with latest jdk (14/15). This can be reproduced by below program on x86 with UseAVX=3.

      public class Foo {

        private static final int SIZE = 65536;

        private static void bar(int[] a, int[] b, int[] c, int start, int limit) {
          for (int i = start; i < limit; i += 1) {
            c[i] = a[i] + b[i];
          }
        }

        public static void main(String[] args) {
          int[] a = new int[SIZE];
          int[] b = new int[SIZE];
          int[] c = new int[SIZE];

          for (int i = 0; i < SIZE; i++) {
            a[i] = i;
            b[i] = i;
            c[i] = 0;
          }

          for (int i = 0; i < 20000; i++) {
            bar(a, b, c, 16384, 32768);
          }

          int sum = 0;
          for (int i = 32760; i < 32780; i++) {
            sum += c[i];
          }
          System.out.println(sum);
        }
      }

      $java -XX:+UnlockExperimentalVMOptions -XX:+PostLoopMultiversioning Foo
      #
      # A fatal error has been detected by the Java Runtime Environment:
      #
      # SIGSEGV (0xb) at pc=0x00007f04f181cb15, pid=20589, tid=20611
      #
      # JRE version: OpenJDK Runtime Environment (16.0) (slowdebug build 16-internal+0-adhoc..jdksrc)
      # Java VM: OpenJDK 64-Bit Server VM (slowdebug 16-internal+0-adhoc..jdksrc, mixed mode, tiered, compressed oops, g1 gc, linux-amd64)
      # Problematic frame:
      # V [libjvm.so+0x10dbb15] SuperWord::transform_loop(IdealLoopTree*, bool)+0x513
      #
      # Core dump will be written. Default location: /home/ent-user/case/core
      #
      # An error report file with more information is saved as:
      # /home/ent-user/case/hs_err_pid20589.log
      #
      # Compiler replay data is saved as:
      # /home/ent-user/case/replay_pid20589.log
      #
      # If you would like to submit a bug report, please visit:
      # https://bugreport.java.com/bugreport/crash.jsp
      #
      Aborted (core dumped)

      The SIGSEGV occurs at C2 code superword.cpp here (http://hg.openjdk.java.net/jdk/jdk/file/cc7b6598df7e/src/hotspot/share/opto/superword.cpp#l173).

      Cause is that lpt_next could be null after loop strip mining. The code block checks which post loop (the normal vector post or the multi-versioned post) resides after the main loop. But if the main loop is strip-mined, the _next loop would be null. So to fix this crash we can check if it's strip-mined and search the _parent->_next loop if it is.

      Patch:
      diff --git a/src/hotspot/share/opto/superword.cpp b/src/hotspot/share/opto/superword.cpp
      index 0f4da5e8cfa..caf59461164 100644
      --- a/src/hotspot/share/opto/superword.cpp
      +++ b/src/hotspot/share/opto/superword.cpp
      @@ -169,7 +169,7 @@ void SuperWord::transform_loop(IdealLoopTree* lpt, bool do_optimization) {
           SLP_extract();
           if (PostLoopMultiversioning && Matcher::has_predicated_vectors()) {
             if (cl->is_vectorized_loop() && cl->is_main_loop() && !cl->is_reduction_loop()) {
      - IdealLoopTree *lpt_next = lpt->_next;
      + IdealLoopTree *lpt_next = cl->is_strip_mined() ? lpt->_parent->_next : lpt->_next;
               CountedLoopNode *cl_next = lpt_next->_head->as_CountedLoop();
               _phase->has_range_checks(lpt_next);
               if (cl_next->is_post_loop() && !cl_next->range_checks_present()) {

      After this fix, there's no crash but C2 still generates incorrect result.

      $java Foo
      524216

      $java -XX:+UnlockExperimentalVMOptions -XX:+PostLoopMultiversioning Foo
      917462



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              Assignee:
              thartmann Tobias Hartmann
              Reporter:
              pli Pengfei Li
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                Updated:
                Resolved: