`getModificationTime` is about 10x slower on windows compared to linux

[mpickering]

#!/usr/bin/env -S cabal run
{- cabal:
build-depends: base >=4.17 && <5
             , directory >=1.3.8
             , filepath  >=1.4.100
             , time
default-language: GHC2021
ghc-options: -O2
-}

import System.OsPath
import System.Directory.OsPath        (getModificationTime)
import Control.Monad

main :: IO ()
main = do
  let rel_path = unsafeEncodeUtf "Main.hs"

  replicateM_ 1000000 (getModificationTime rel_path)

When I run this benchmark on linux, I get

 447,660,936 bytes allocated in the heap
          14,656 bytes copied during GC
          35,984 bytes maximum residency (1 sample(s))
          29,552 bytes maximum slop
               7 MiB total memory in use (0 MiB lost due to fragmentation)

                                     Tot time (elapsed)  Avg pause  Max pause
  Gen  0       107 colls,     0 par    0.001s   0.001s     0.0000s    0.0000s
  Gen  1         1 colls,     0 par    0.000s   0.000s     0.0001s    0.0001s

  INIT    time    0.000s  (  0.000s elapsed)
  MUT     time    0.584s  (  0.585s elapsed)
  GC      time    0.001s  (  0.001s elapsed)
  EXIT    time    0.000s  (  0.000s elapsed)
  Total   time    0.585s  (  0.586s elapsed)

  %GC     time       0.0%  (0.0% elapsed)

  Alloc rate    766,951,617 bytes per MUT second

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

on windows, my colleague ran the program, and it allocates about 10x more.

  44,760,076,432 bytes allocated in the heap
       7,301,816 bytes copied during GC
          34,928 bytes maximum residency (2 sample(s))
          26,928 bytes maximum slop
               6 MiB total memory in use (0 MiB lost due to fragmentation)

                                     Tot time (elapsed)  Avg pause  Max pause
  Gen  0     10644 colls,     0 par    0.016s   0.056s     0.0000s    0.0001s
  Gen  1         2 colls,     0 par    0.000s   0.001s     0.0005s    0.0008s

  INIT    time    0.000s  (  0.000s elapsed)
  MUT     time    4.281s  ( 18.487s elapsed)
  GC      time    0.016s  (  0.057s elapsed)
  EXIT    time    0.000s  (  0.000s elapsed)
  Total   time    4.297s  ( 18.544s elapsed)

  %GC     time       0.0%  (0.0% elapsed)

  Alloc rate    10,454,908,363 bytes per MUT second

  Productivity  99.6% of total user, 99.7% of total elapsed

It seems that in the windows code paths there is much more file path manipulation happening which converts from OsPath to FilePath and back again.

[mpickering]