A simple loop in ST takes much longer to complete with 9.4.4 than 9.4.3 (NCG)

Consider the following simple program:

st_loop.hs:

module Main where

import Control.Monad.ST
import Data.STRef

programST :: STRef s Integer -> ST s Integer
programST ref = do
  n <- readSTRef ref
  if n <= 0
    then pure n
    else do
      writeSTRef ref $! n - 1
      programST ref

countdownST :: Integer -> (Integer, Integer)
countdownST n = runST $ do
  ref <- newSTRef n
  a <- programST ref
  s <- readSTRef ref
  pure (a, s)

main :: IO ()
main = putStrLn . show $ countdownST 100000000

Now:

unknown@electronics haskell $ /usr/local/haskell/ghc-9.4.3/bin/ghc -fforce-recomp -O --make st_loop.hs
[1 of 2] Compiling Main             ( st_loop.hs, st_loop.o )
[2 of 2] Linking st_loop [Objects changed]
unknown@electronics haskell $ ./st_loop +RTS -s
(0,0)
   1,600,050,840 bytes allocated in the heap
          25,120 bytes copied during GC
          44,328 bytes maximum residency (2 sample(s))
          29,400 bytes maximum slop
               5 MiB total memory in use (0 MB lost due to fragmentation)

                                     Tot time (elapsed)  Avg pause  Max pause
  Gen  0       380 colls,     0 par    0.001s   0.001s     0.0000s    0.0000s
  Gen  1         2 colls,     0 par    0.000s   0.000s     0.0001s    0.0001s

  INIT    time    0.000s  (  0.000s elapsed)
  MUT     time    0.333s  (  0.333s elapsed)
  GC      time    0.001s  (  0.001s elapsed)
  EXIT    time    0.000s  (  0.006s elapsed)
  Total   time    0.334s  (  0.340s elapsed)

  %GC     time       0.0%  (0.0% elapsed)

  Alloc rate    4,804,291,700 bytes per MUT second

  Productivity  99.7% of total user, 97.9% of total elapsed

unknown@electronics haskell $ /usr/local/haskell/ghc-9.4.4/bin/ghc -fforce-recomp -O --make st_loop.hs
[1 of 2] Compiling Main             ( st_loop.hs, st_loop.o )
[2 of 2] Linking st_loop [Objects changed]
unknown@electronics haskell $ ./st_loop +RTS -s
(0,0)
   1,600,050,840 bytes allocated in the heap
          25,120 bytes copied during GC
          44,328 bytes maximum residency (2 sample(s))
          29,400 bytes maximum slop
               5 MiB total memory in use (0 MB lost due to fragmentation)

                                     Tot time (elapsed)  Avg pause  Max pause
  Gen  0       380 colls,     0 par    0.001s   0.001s     0.0000s    0.0000s
  Gen  1         2 colls,     0 par    0.000s   0.000s     0.0001s    0.0001s

  INIT    time    0.000s  (  0.000s elapsed)
  MUT     time    1.843s  (  1.843s elapsed)
  GC      time    0.001s  (  0.001s elapsed)
  EXIT    time    0.000s  (  0.006s elapsed)
  Total   time    1.844s  (  1.850s elapsed)

  %GC     time       0.0%  (0.0% elapsed)

  Alloc rate    868,062,260 bytes per MUT second

  Productivity  99.9% of total user, 99.6% of total elapsed

It takes 6 times longer to complete with 9.4.4 than with 9.4.3.

However, with LLVM there is almost no difference:

unknown@electronics haskell $ /usr/local/haskell/ghc-9.4.3/bin/ghc -fforce-recomp -O -fllvm --make st_loop.hs
[1 of 2] Compiling Main             ( st_loop.hs, st_loop.o )
You are using an unsupported version of LLVM!
Currently only 10 up to 14 (non inclusive) is supported. System LLVM version: 14.0.6
We will try though...
[2 of 2] Linking st_loop [Objects changed]
unknown@electronics haskell $ ./st_loop +RTS -s
(0,0)
   1,600,050,840 bytes allocated in the heap
          25,120 bytes copied during GC
          44,328 bytes maximum residency (2 sample(s))
          29,400 bytes maximum slop
               5 MiB total memory in use (0 MB lost due to fragmentation)

                                     Tot time (elapsed)  Avg pause  Max pause
  Gen  0       380 colls,     0 par    0.001s   0.001s     0.0000s    0.0000s
  Gen  1         2 colls,     0 par    0.000s   0.000s     0.0001s    0.0001s

  INIT    time    0.000s  (  0.000s elapsed)
  MUT     time    0.355s  (  0.355s elapsed)
  GC      time    0.001s  (  0.001s elapsed)
  EXIT    time    0.000s  (  0.004s elapsed)
  Total   time    0.356s  (  0.360s elapsed)

  %GC     time       0.0%  (0.0% elapsed)

  Alloc rate    4,502,712,001 bytes per MUT second

  Productivity  99.7% of total user, 98.7% of total elapsed

unknown@electronics haskell $ /usr/local/haskell/ghc-9.4.4/bin/ghc -fforce-recomp -O -fllvm --make st_loop.hs
[1 of 2] Compiling Main             ( st_loop.hs, st_loop.o )
You are using an unsupported version of LLVM!
Currently only 10 up to 14 (non inclusive) is supported. System LLVM version: 14.0.6
We will try though...
[2 of 2] Linking st_loop [Objects changed]
unknown@electronics haskell $ ./st_loop +RTS -s
(0,0)
   1,600,050,840 bytes allocated in the heap
          25,120 bytes copied during GC
          44,328 bytes maximum residency (2 sample(s))
          29,400 bytes maximum slop
               5 MiB total memory in use (0 MB lost due to fragmentation)

                                     Tot time (elapsed)  Avg pause  Max pause
  Gen  0       380 colls,     0 par    0.001s   0.001s     0.0000s    0.0000s
  Gen  1         2 colls,     0 par    0.000s   0.000s     0.0001s    0.0001s

  INIT    time    0.000s  (  0.000s elapsed)
  MUT     time    0.396s  (  0.396s elapsed)
  GC      time    0.001s  (  0.001s elapsed)
  EXIT    time    0.000s  (  0.003s elapsed)
  Total   time    0.397s  (  0.400s elapsed)

  %GC     time       0.0%  (0.0% elapsed)

  Alloc rate    4,043,468,789 bytes per MUT second

  Productivity  99.7% of total user, 98.9% of total elapsed

Generated Core and STG seem to be the same for both versions, something weird must be going on in the NCG.