I can reproduce it like this:
JDK23:
./java -XX:CompileCommand=compileonly,Test*::* -XX:CompileCommand=TraceAutoVectorization,Test*::*,SW_REJECTIONS,POINTERS,ALIGN_VECTOR -XX:+TraceNewVectors -Xbatch -XX:+TraceLoopOpts TestMemorySegmentMainLoopAlignment.java
JDK21:
/oracle-work/jdk-21.0.3/fastdebug/bin/java --enable-preview --source 21 -XX:CompileCommand=compileonly,Test*::* -XX:+TraceNewVectors -Xbatch -XX:+TraceLoopOpts TestMemorySegmentMainLoopAlignment.java
JDK20: reproducer does not compile, because MemorySegment API was different.
It is possible that with VarHandle or Unsafe this might fail with even older JDK's.
-------------------------------------
VLoopVPointers::print:
VPointer[mem: 1633 LoadI, base: 1, adr: 1669, base[ 1] + offset( 4) + invar( 0) + scale( 4) * iv]
VPointer[mem: 1639 LoadI, base: 65, adr: 65, base[ 65] + offset( 20) + invar[1149] + scale( 4) * iv]
VPointer[mem: 1645 LoadI, base: 65, adr: 65, base[ 65] + offset( 16) + invar[1149] + scale( 4) * iv]
VPointer[mem: 1470 LoadI, base: 65, adr: 65, base[ 65] + offset( 24) + invar[1149] + scale( 4) * iv]
VPointer[mem: 873 LoadI, base: 65, adr: 65, base[ 65] + offset( 28) + invar[1149] + scale( 4) * iv]
VPointer[mem: 1648 LoadI, base: 1, adr: 1669, base[ 1] + offset( 0) + invar( 0) + scale( 4) * iv]
VPointer[mem: 1465 LoadI, base: 1, adr: 1669, base[ 1] + offset( 8) + invar( 0) + scale( 4) * iv]
VPointer[mem: 781 LoadI, base: 1, adr: 1669, base[ 1] + offset( 12) + invar( 0) + scale( 4) * iv]
SuperWord::transform_loop:
Loop: N1657/N886 counted [int,int),+4 (17 iters) main has_sfpt strip_mined
1657 CountedLoop === 1657 1181 886 [[ 1633 1637 1648 1657 1658 1465 781 1177 ]] inner stride: 4 main of N1657 strip mined !orig=[1476],[1182],[1030],[921],[912],[117] !jvms: TestMemorySegmentMainLoopAlignment::test @ bci:23 (line 27)
SuperWord::output Loop: N1657/N886 counted [int,int),+4 (17 iters) main has_sfpt strip_mined
adjust_pre_loop_limit_to_align_main_loop_vectors:
align_to_ref: 1648 LoadI === 1657 63 1711 [[ 1644 ]] @rawptr:BotPTR, idx=Raw; unaligned unsafe #int (does not depend only on test, unknown control) !orig=1465,781 !jvms: Unsafe::getIntUnaligned @ bci:5 (line 3576) ScopedMemoryAccess::getIntUnalignedInternal @ bci:15 (line 1893) ScopedMemoryAccess::getIntUnaligned @ bci:6 (line 1881) VarHandleSegmentAsInts::get @ bci:48 (line 108) VarHandleGuards::guard_LJ_I @ bci:49 (line 999) AbstractMemorySegmentImpl::get @ bci:8 (line 772) TestMemorySegmentMainLoopAlignment::memorySegmentGet @ bci:12 (line 19) 0x0000000039228418::apply @ bci:4 TestMemorySegmentMainLoopAlignment::test @ bci:30 (line 27)
aw: 16
stride: 4
scale: 4
offset: 0
base: 1669 CastX2P === _ 1590 [[ 1715 1707 1709 1711 ]] !orig=[1671]
invar: null
old_limit: 32 ConI === 0 [[ 1705 1270 1149 1265 1405 1430 1712 1686 1551 1289 1325 1354 1642 1381 1653 1679 ]] #int:2
orig_limit: 1015 SubI === _ 25 1012 [[ 1430 1162 1276 1306 ]] !orig=[1017]
AW = aw(16) / abs(scale(4)) = 4
xboi: 31 ConI === 0 [[ 48 1012 1028 1179 80 984 1274 1272 80 80 ]] #int:0
xbase: 1729 CastP2X === _ 1669 [[ ]]
xbase: 1730 ConvL2I === _ 1729 [[ ]] #int
xboi: 1731 SubI === _ 31 1730 [[ ]]
log2_abs_scale: 32 ConI === 0 [[ 1705 1270 1149 1265 1405 1430 1712 1686 1551 1289 1325 1354 1642 1381 1653 1679 1732 ]] #int:2
XBOI: 1732 URShiftI === _ 1731 32 [[ ]]
XBOI_OP_old_limit: 1733 SubI === _ 1732 32 [[ ]]
mask_AW: 1718 ConI === 0 [[ 1474 1734 ]] #int:3
adjust_pre_iter: 1734 AndI === _ 1733 1718 [[ ]]
new_limit: 1735 AddI === _ 32 1734 [[ ]]
constrained_limit: 1736 MinI === _ 1735 1015 [[ ]]
TraceNewVectors [SuperWord]: 1737 LoadVector === 1001 63 1646 [[ 1644 1632 1464 874 ]] @int[int:16] (java/lang/Cloneable,java/io/Serializable):NotNull:exact+any *, idx=6; mismatched #vectorx[4]:{int} !orig=[1645],[1470],[873] !jvms: TestMemorySegmentMainLoopAlignment::test @ bci:39 (line 27)
TraceNewVectors [SuperWord]: 1738 LoadVector === 1657 63 1711 [[ 1644 1632 1464 874 ]] @rawptr:BotPTR, idx=Raw; mismatched #vectorx[4]:{int} (does not depend only on test, unknown control) !orig=[1648],[1465],[781] !jvms: Unsafe::getIntUnaligned @ bci:5 (line 3576) ScopedMemoryAccess::getIntUnalignedInternal @ bci:15 (line 1893) ScopedMemoryAccess::getIntUnaligned @ bci:6 (line 1881) VarHandleSegmentAsInts::get @ bci:48 (line 108) VarHandleGuards::guard_LJ_I @ bci:49 (line 999) AbstractMemorySegmentImpl::get @ bci:8 (line 772) TestMemorySegmentMainLoopAlignment::memorySegmentGet @ bci:12 (line 19) 0x0000000039228418::apply @ bci:4 TestMemorySegmentMainLoopAlignment::test @ bci:30 (line 27)
TraceNewVectors [SuperWord]: 1739 AddVI === _ 1737 1738 [[ 1643 1631 1463 875 ]] #vectorx[4]:{int} !orig=[1644],[1464],[874] !jvms: TestMemorySegmentMainLoopAlignment::test @ bci:40 (line 27)
TraceNewVectors [SuperWord]: 1740 AddReductionVI === _ 1658 1739 [[ 1658 1183 1338 ]] !orig=[1643],[1463],[875],1485 !jvms: TestMemorySegmentMainLoopAlignment::test @ bci:41 (line 27)
SuperWord::transform_loop: success
TraceNewVectors [UnorderedReduction]: 1741 Replicate === _ 31 [[ ]] #vectorx[4]:{int}
TraceNewVectors [UnorderedReduction]: 1742 AddVI === _ 1658 1739 [[ 1338 1183 1658 ]] #vectorx[4]:{int} !orig=[1740],[1643],[1463],[875],1485 !jvms: TestMemorySegmentMainLoopAlignment::test @ bci:41 (line 27)
TraceNewVectors [UnorderedReduction]: 1743 AddReductionVI === _ 1313 1742 [[ 1338 1183 ]]
Bad graph detected in compute_lca_of_uses
n: 1311 Bool === _ 1312 [[ 1329 ]] [lt] !orig=1176,[1027]
early(n): 1348 IfTrue === 1347 [[ 1513 1492 1363 ]] #1
n->in(1): 1312 CmpI === _ 1324 1430 [[ 1311 ]] !orig=1175,[1026]
early(n->in(1)): 1348 IfTrue === 1347 [[ 1513 1492 1363 ]] #1
n->in(1)->in(1): 1324 AddI === _ 1328 44 [[ 1312 1328 1340 1345 ]] !orig=1174,[1025]
early(n->in(1)->in(1)): 1326 CountedLoop === 1326 1206 1330 [[ 1315 1326 1328 1329 1331 ]] inner stride: 1 pre of N1182 !orig=[1182],[1030],[921],[912],[117] !jvms: TestMemorySegmentMainLoopAlignment::test @ bci:23 (line 27)
n->in(1)->in(2): 1430 Opaque1 === _ 1736 1015 [[ 1312 ]]
early(n->in(1)->in(2)): 1348 IfTrue === 1347 [[ 1513 1492 1363 ]] #1
LCA(n): 1326 CountedLoop === 1326 1206 1330 [[ 1315 1326 1328 1329 1331 ]] inner stride: 1 pre of N1182 !orig=[1182],[1030],[921],[912],[117] !jvms: TestMemorySegmentMainLoopAlignment::test @ bci:23 (line 27)
n->out(0): 1329 CountedLoopEnd === 1326 1311 [[ 1330 1343 ]] [lt] P=0.500000, C=18415.000000 !orig=1177,[1031],[1007],[918]
n->out(0)->out(0): 1330 IfTrue === 1329 [[ 1326 ]] #1 !orig=886 !jvms: TestMemorySegmentMainLoopAlignment::test @ bci:20 (line 26)
n->out(0)->out(1): 1343 IfFalse === 1329 [[ 1347 ]] #0
idoms of early "1348 IfTrue":
idom[3]: 1326 CountedLoop
idom[2]: 1329 CountedLoopEnd
idom[1]: 1343 IfFalse
idom[0]: 1347 If
n: 1348 IfTrue
idoms of (wrong) LCA "1326 CountedLoop":
n: 1326 CountedLoop
Real LCA of early "1348 IfTrue" (idom[3]) and wrong LCA "1326 CountedLoop":
1326 CountedLoop === 1326 1206 1330 [[ 1315 1326 1328 1329 1331 ]] inner stride: 1 pre of N1182 !orig=[1182],[1030],[921],[912],[117] !jvms: TestMemorySegmentMainLoopAlignment::test @ bci:23 (line 27)
*** Use 1326 isn't dominated by def 1311 ***
#
# A fatal error has been detected by the Java Runtime Environment:
#
# Internal Error (/oracle-work/jdk-fork2/open/src/hotspot/share/opto/loopnode.cpp:5901), pid=1708090, tid=1708104
# assert(!had_error) failed: bad dominance
#
# JRE version: Java(TM) SE Runtime Environment (23.0) (fastdebug build 23-internal-2024-04-05-0704446.emanuel...)
# Java VM: Java HotSpot(TM) 64-Bit Server VM (fastdebug 23-internal-2024-04-05-0704446.emanuel..., mixed mode, tiered, compressed oops, compressed class ptrs, g1 gc, linux-amd64)
# Problematic frame:
# V [libjvm.so+0x129be37] PhaseIdealLoop::compute_lca_of_uses(Node*, Node*, bool)+0x987
#
# Core dump will be written. Default location: Core dumps may be processed with "/usr/share/apport/apport -p%p -s%s -c%c -d%d -P%P -u%u -g%g -- %E" (or dumping to /oracle-work/jdk-fork2/build/linux-x64-debug/jdk/bin/core.1708090)
#
# An error report file with more information is saved as:
# /oracle-work/jdk-fork2/build/linux-x64-debug/jdk/bin/hs_err_pid1708090.log
#
# Compiler replay data is saved as:
# /oracle-work/jdk-fork2/build/linux-x64-debug/jdk/bin/replay_pid1708090.log
#
# If you would like to submit a bug report, please visit:
# https://bugreport.java.com/bugreport/crash.jsp
#
Aborted (core dumped)