Consider the following simple program: st_loop.hs: ```haskell 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.