[GHC] #14754: -O1 changes result at runtime

3 Feb 2018

      #14754: -O1 changes result at runtime
-------------------------------------+-------------------------------------
           Reporter:  Bodigrim       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:
          Component:  Compiler       |           Version:  8.4.1-alpha1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect result
  Unknown/Multiple                   |  at runtime
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 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`.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14754
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