When debug the VM in gdb and perform "call help()", the output is
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(gdb) call help()

"Executing help"
basic
  pp(void* p) - try to make sense of p
  pv(intptr_t p)- ((PrintableResourceObj*) p)->print()
  ps() - print current thread stack
  pss() - print all thread stacks
  pm(int pc) - print Method* given compiled PC
  findm(intptr_t pc) - finds Method*
  find(intptr_t x) - finds & prints nmethod/stub/bytecode/oop based on pointer into it
  pns(void* sp, void* fp, void* pc) - print native (i.e. mixed) stack trace. E.g.
                   pns($sp, $rbp, $pc) on Linux/amd64 and Solaris/amd64 or
                   pns($sp, $ebp, $pc) on Linux/x86 or
                   pns($sp, 0, $pc) on Linux/ppc64 or
                   pns($sp + 0x7ff, 0, $pc) on Solaris/SPARC
                 - in gdb do 'set overload-resolution off' before calling pns()
                 - in dbx do 'frame 1' before calling pns()
misc.
  flush() - flushes the log file
  events() - dump events from ring buffers
compiler debugging
  debug() - to set things up for compiler debugging
  ndebug() - undo debug
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The help-info for pns(...) on Linux/mips is lost.
However, it exists in jdk8u.

To support debugging on as many platforms as possible, it would be better to keep it.
This can be fixed by the following patch:
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diff -r df3d253aaf81 src/hotspot/share/utilities/debug.cpp
--- a/src/hotspot/share/utilities/debug.cpp Wed Feb 27 02:49:52 2019 +0000
+++ b/src/hotspot/share/utilities/debug.cpp Wed Feb 27 15:33:31 2019 +0800
@@ -643,6 +643,7 @@
   tty->print_cr(" pns($sp, $rbp, $pc) on Linux/amd64 and Solaris/amd64 or");
   tty->print_cr(" pns($sp, $ebp, $pc) on Linux/x86 or");
   tty->print_cr(" pns($sp, 0, $pc) on Linux/ppc64 or");
+ tty->print_cr(" pns($sp, $s8, $pc) on Linux/mips or");
   tty->print_cr(" pns($sp + 0x7ff, 0, $pc) on Solaris/SPARC");
   tty->print_cr(" - in gdb do 'set overload-resolution off' before calling pns()");
   tty->print_cr(" - in dbx do 'frame 1' before calling pns()");
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