[commit: ghc] master: make iToBase62's inner loop stricter in one of its arguments (ed78951)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/ed789516e201e4fad771e5588da47a62e53b42b8/ghc

>---------------------------------------------------------------

commit ed789516e201e4fad771e5588da47a62e53b42b8
Author: Alp Mestanogullari <alp at well-typed.com>
Date:   Sun Sep 2 22:06:55 2018 +0200

    make iToBase62's inner loop stricter in one of its arguments
    
    Summary:
    hadrian's support for dynamic ways is currently broken (see hadrian#641 [1]).
    The stage 1 GHCs that hadrian produces end up producing bad code for
    the `iToBase62` function after a few optimisation passes.
    
    In the case where `quotRem` returns (overflowError, 0),
    GHC isn't careful enough to realise q is _|_ and happily inlines,
    distributes and floats code around until we end up trying to access
    index `minBound :: Int` of an array of 62 chars, as a result of inlining
    the definition of `quotRem` for Ints, in particular the minBound branch [2].
    
    I will separately look into reproducing the bad transformation on a small
    self-contained example and filling a ticket.
    
    [1]: https://github.com/snowleopard/hadrian/issues/641
    [2]: https://git.haskell.org/ghc.git/blob/HEAD:/libraries/base/GHC/Real.hs#l366
    
    Test Plan: fixes hadrian#641
    
    Reviewers: bgamari, tdammers
    
    Reviewed By: tdammers
    
    Subscribers: tdammers, rwbarton, carter
    
    Differential Revision: https://phabricator.haskell.org/D5106


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ed789516e201e4fad771e5588da47a62e53b42b8
 compiler/basicTypes/Unique.hs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/compiler/basicTypes/Unique.hs b/compiler/basicTypes/Unique.hs
index 4a709d2..b5c0fce 100644
--- a/compiler/basicTypes/Unique.hs
+++ b/compiler/basicTypes/Unique.hs
@@ -325,7 +325,7 @@ iToBase62 n_
     go n cs | n < 62
             = let !c = chooseChar62 n in c : cs
             | otherwise
-            = go q (c : cs) where (q, r) = quotRem n 62
+            = go q (c : cs) where (!q, r) = quotRem n 62
                                   !c = chooseChar62 r
 
     chooseChar62 :: Int -> Char