## Summary I found this test failure while working on nixpkgs on aarch64. This is *extremely alarming*, before I reduced it to a simpler program using only base, at which point I am out-of-this-world alarmed. https://github.com/haskell-foundation/foundation/issues/571 ## Steps to reproduce Stick this in `Main.hs`: ```haskell {-# LANGUAGE NoImplicitPrelude, OverloadedStrings, RankNTypes, ScopedTypeVariables, NoRebindableSyntax #-} module Main where import Prelude import System.Environment import Data.Word testF :: Word8 -> Word8 -> Bool testF a b = if b == 0 then True else a == (a `div` b) * b + (a `mod` b) main :: IO () main = do args <- getArgs let [a :: Word8, b] = read <$> args putStrLn . show $ testF a b ``` Then: ``` co/ghc942bug » ghc -O1 app/Main.hs [1 of 2] Compiling Main ( app/Main.hs, app/Main.o ) [2 of 2] Linking app/Main co/ghc942bug » app/Main 217 161 False co/ghc942bug » ghc -O0 app/Main.hs [1 of 2] Compiling Main ( app/Main.hs, app/Main.o ) [Optimisation flags changed] [2 of 2] Linking app/Main [Objects changed] co/ghc942bug » app/Main 217 161 True ``` Note that it's fine on 9.2.4: ``` co/ghc942bug » ghcup set 9.2.4 [ Warn ] This is an old-style command for setting GHC. Use 'ghcup set ghc' instead. [ Info ] GHC 9.2.4 successfully set as default version co/ghc942bug » ghc -O0 app/Main.hs [1 of 1] Compiling Main ( app/Main.hs, app/Main.o ) Linking app/Main ... co/ghc942bug » app/Main 217 161 True co/ghc942bug » ghc -O1 app/Main.hs [1 of 1] Compiling Main ( app/Main.hs, app/Main.o ) [Optimisation flags changed] Linking app/Main ... co/ghc942bug » app/Main 217 161 True ``` On Linux, GHC 9.4.2 from nixpkgs (qemu on Apple Silicon): ``` ~ » ghc -O1 Repro.hs [1 of 2] Compiling Main ( Repro.hs, Repro.o ) [2 of 2] Linking Repro ~ » ./Repro 217 161 False ~ » ghc -O0 Repro.hs [1 of 2] Compiling Main ( Repro.hs, Repro.o ) [Optimisation flags changed] [2 of 2] Linking Repro [Objects changed] ~ » ./Repro 217 161 True ~ » uname -a Linux thinnix 5.15.47 #1-NixOS SMP Tue Jun 14 16:36:28 UTC 2022 aarch64 GNU/Linux ``` ## Expected behavior It should return True in both optimization configurations. ## Environment * GHC version used: 9.4.2 from ghcup although it also happens on ghc from nix. It's fine in ghc 9.2.4. Optional: * Operating System: macOS, although it also happens on aarch64-linux * System Architecture: aarch64