-O1 changes result at runtime

I found a program, which works as expected in GHC 8.4.1-alpha1 with -O0, but freezes with -O1.

module Main where

import Debug.Trace

main :: IO ()
main = print (alg 3 1)

alg :: Word -> Word -> Word
alg a b
  | traceShow (a, b) False = undefined
  | c < b = alg b c
  | c > b = alg c b
  | otherwise = c
  where
    c = a - b

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.4.0.20180118
$ ghc -O0 alg.hs
[1 of 1] Compiling Main             ( alg.hs, alg.o )
Linking alg ...
$ ./alg
(3,1)
(2,1)
1
$ ghc -O1 alg.hs
[1 of 1] Compiling Main             ( alg.hs, alg.o ) [Optimisation flags changed]
Linking alg ...
$ ./alg 2>&1 | head
(3,1)
(1,2)
(2,18446744073709551615)
(18446744073709551615,3)
^C

For some reason an optimised program chooses a wrong case at the very first invocation of alg.