Worse performance with -O

See master ticket #1168

Summary

The following program reads a sequence of integers, puts them in an array, and prints them by index.

import Control.Monad
import Data.Array.Unboxed

main :: IO ()
main = do
    n <- readLn
    xs <- map read . words <$> getLine
    let a = listArray (1, n) xs :: UArray Int Int
    q <- readLn
    replicateM_ q $ do
        i <- readLn
        print (a!i)

Surprisingly, this runs a lot worse with -O than without.

From the core it appears that with -O the array is built on every step inside the replicateM_. This turns a simple O(n+q) program into O(nq).

Steps to reproduce

Compile and run the above program with -O.

Expected behavior

The array is constructed once.

Environment

• GHC version used: 9.2.5

Optional:

• Operating System: Ubuntu
• System Architecture: x86_64