Re: [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:  highest           |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  8.4.1-alpha1
      Resolution:                    |             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:                    |
-------------------------------------+-------------------------------------
Changes (by bgamari):

 * priority:  high => highest
 * milestone:   => 8.4.1

Old description:
...
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`.
New description:

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

 {{{#!hs
 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`.

--

Comment:

 Yikes! Thank you for catching this.

 I guess this means my plans for another alpha release today are on hold.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14754#comment:3
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