llvm new pass manager: how to get -analyze -loops output

Question

I used to be able to do:

$ opt -analyze -loops app2/app2.ll
Printing analysis 'Natural Loop Information' for function 'rotf':
Loop at depth 1 containing: %4<header><exiting>,%7,%10,%27,%31<latch>,%14,%24
    Loop at depth 2 containing: %10<header><exiting>,%14,%24<latch>
Printing analysis 'Natural Loop Information' for function 'rotf2':
Loop at depth 1 containing: %4<header><exiting>,%7,%10,%27,%31<latch>,%14,%24
    Loop at depth 2 containing: %10<header><exiting>,%14,%24<latch>
Printing analysis 'Natural Loop Information' for function 'main':
Loop at depth 1 containing: %4<header><exiting>,%7,%15,%11,%19,%24,%28,%29<latch>

How do I get this information with the new pass manager? The docs are failing me.

$ opt  -p print-loops app2/app2.ll
WARNING: You're attempting to print out a bitcode file.
This is inadvisable as it may cause display problems. If
you REALLY want to taste LLVM bitcode first-hand, you
can force output with the `-f' option.

opt: unknown pass name 'print-loops'
$  opt  -p print-analyze-loops app2/app2.ll
WARNING: You're attempting to print out a bitcode file.
This is inadvisable as it may cause display problems. If
you REALLY want to taste LLVM bitcode first-hand, you
can force output with the `-f' option.

opt: unknown pass name 'print-analyze-loops'

Answer 1

It turns out, the opt program has a bug, as pointed out here: https://discourse.llvm.org/t/need-usage-help-w-new-pass-manager-for-opt-analysis-natural-loop-information/75874.

The LoopPrinterPass fails to define isRequired() so the output is dropped.

A work-around is to generate the LLVM IR code (via clang) with the -O1 flag, which has other side effects; but then allows opt to generate the Natural Loop Information with the following syntax:

clang -S -emit-llvm app2.c -O1 -o app2.ll
opt app2.ll -passes="print<loops>" -disable-output