(synopsis is provisional, change as you see fit)

$ CONF=linux-x86-server-fastdebug make images run-test TEST=compiler/c1/TestTraceLinearScanLevel.java

walk_to(4) **************************************************************
active to handled 5 fixed [ecx|I] 5 -1 [0, 4[ "no definition"
active to inactive 3 fixed [eax|I] 3 171 [0, 1[ [78, 80[ "no spill store"
+++++ activating interval -1 object [ANY] 161 5 [4, 38[ 4 M 38 M "no spill store"
      split_parent: 161, insert_move_when_activated: 0
      normal allocation of register
trying to find free register for -1 object [ANY] 161 5 [4, 38[ 4 M 38 M "no spill store"
      state of registers:
      reg 0 ([esi|I]): use_pos: 2147483647
      reg 1 ([edi|I]): use_pos: 2147483647
      reg 2 ([ebx|I]): use_pos: 2147483647
      reg 3 ([eax|I]): use_pos: 78
      reg 4 ([edx|I]): use_pos: 2147483647
      reg 5 ([ecx|I]): use_pos: 2147483647
      hint registers 5 ([ecx|I]), 6 (# To suppress the following error report, specify this argument
# after -XX: or in .hotspotrc: SuppressErrorAt=/c1_LinearScan.cpp:3234
#
# A fatal error has been detected by the Java Runtime Environment:
#
# Internal Error (/home/shade/trunks/jdk/src/hotspot/share/c1/c1_LinearScan.cpp:3234), pid=2618467, tid=2618483
# assert(false) failed: unknown register
#
# JRE version: OpenJDK Runtime Environment (16.0) (fastdebug build 16-internal+0-adhoc.shade.jdk)
# Java VM: OpenJDK Server VM (fastdebug 16-internal+0-adhoc.shade.jdk, mixed mode, tiered, g1 gc, linux-x86)
# Problematic frame:
# V [libjvm.so+0x631910] LinearScan::print_reg_num(outputStream*, int)+0xc0
#

I suspect the refactoring in JDK-8251093 introduced this.