[lworld] RFR: 8264340: [lworld] [AArch64] TestLWorld.java assertion failure in OopFlow::build_oop_map

[Nick Gasson]

This happens reliably in TestLWorld::test9() scenario 0 on AArch64:


  # A fatal error has been detected by the Java Runtime Environment:
  #
  # Internal Error (/mnt/nicgas01-pc/valhalla/src/hotspot/share/opto/buildOopMap.cpp:360), pid=8866, tid=8882
  # assert(false) failed: there should be a oop in OopMap instead of a live raw oop at safepoint
  #


The crash can also be reproduced on x86 by running with -XX:+OptoScheduling (this is the default on AArch64).

The problem seems to be caused by a CheckCastPP node whose input is a raw pointer being scheduled after a SafePoint node such that the raw pointer is live in a register over the safepoint.

Before scheduling we have a basic block like:


  R0      73  Phi  ===  15  74  30  [[ 72  71  70  69  68  67  84 ]]  #rawptr:BotPTR !jvms: SchedCrash$MyValue1::setX @ bci:-1 (line 27) SchedCrash::test9 @ bci:25 (line 39)
  ...
  R0      84  checkCastPP  ===  11  73  [[ 2 ]] SchedCrash$MyValue1:NotNull:exact *  Oop:SchedCrash$MyValue1:NotNull:exact * !jvms: SchedCrash$MyValue1::<init> @ bci:65 (line 24) SchedCrash$MyValue1::setX @ bci:21 (line 27) SchedCrash::test9 @ bci:25 (line 39)
  ...
          6  safePoint  ===  9  0  33  0  0  7  0  78  40  0  140  135  136  139  138  141  [[ 8  4 ]]  !jvms: SchedCrash::test9 @ bci:33 (line 37)


But after scheduling this is transformed into:


  R0      73  Phi  ===  15  74  30  [[ 72  71  70  69  68  67  84 ]]  #rawptr:BotPTR !jvms: SchedCrash$MyValue1::setX @ bci:-1 (line 27) SchedCrash::test9 @ bci:25 (line 39)
  ...
          6  safePoint  ===  9  0  33  0  0  7  0  78  40  0  140  135  136  139  138  141  | 164  [[ 8  4 ]]  !jvms: SchedCrash::test9 @ bci:33 (line 37)
  ...
  R0      84  checkCastPP  ===  11  73  | 67  68  69  70  71  72  [[ 2 ]] SchedCrash$MyValue1:NotNull:exact *  Oop:SchedCrash$MyValue1:NotNull:exact * !jvms: SchedCrash$MyValue1::<init> @ bci:65 (line 24) SchedCrash$MyValue1::setX @ bci:21 (line 27) SchedCrash::test9 @ bci:25 (line 39)


Where R0 is holding the live raw pointer over the safepoint, which triggers the assertion failure.

The fix here is to add a precedence edge from any CheckCastPP with a raw pointer input to the following safepoint, which prevents them being rearranged. I'm not very familiar with this code so I can't be sure this is the correct solution, but the same logic exists in GCM's PhaseCFG::schedule_late().

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

Commit messages:
 - 8264340: [lworld] [AArch64] TestLWorld.java assertion failure in OopFlow::build_oop_map

Changes: https://git.openjdk.java.net/valhalla/pull/479/files
 Webrev: https://webrevs.openjdk.java.net/?repo=valhalla&pr=479&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264340
  Stats: 9 lines in 1 file changed: 9 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/479.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/479/head:pull/479

PR: https://git.openjdk.java.net/valhalla/pull/479