[commit: ghc] master: Restrict exprOkForSpeculation/case to unlifted types (a0b7b10)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/a0b7b100c649f01325bc6807e418b7563885120e/ghc

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commit a0b7b100c649f01325bc6807e418b7563885120e
Author: Simon Peyton Jones <simonpj at microsoft.com>
Date:   Tue Aug 22 13:34:31 2017 +0100

    Restrict exprOkForSpeculation/case to unlifted types
    
    Consider
      case x of y
        DEFAULT -> let v::Int# = case y of
                                   True  -> e1
                                   False -> e2
                    in ...
    
    Previously this would have been ok-for-speculation because
    y is evaluated.  But the binder-swap done
    by SetLevels would transform the inner alternative to
         DEFAULT -> let v::Int# = case x of { ... }
                    in ...)
    which does /not/ satisfy the let/app invariant, because x is
    not evaluated.
    
    I don't know why this has never bitten us before, but it began
    to bite when I did upcoming refactoring of the Simplifier.
    
    So this patch narrows exprOkForSpeculation to only work for
    /unlifted/ cases.
    
    To make this work I had to make exprOkForSpeculation non-polymorphic
    in the binder type, which has a little knock-on for is use in
    SetLevels.
    
    (It's annoying that we need to handle cases at all, but see
     Note [exprOkForSpeculation: case expressions])


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a0b7b100c649f01325bc6807e418b7563885120e
 compiler/coreSyn/CoreUtils.hs   | 130 ++++++++++++++++++++++++++--------------
 compiler/simplCore/SetLevels.hs |   5 +-
 2 files changed, 88 insertions(+), 47 deletions(-)

Diff suppressed because of size. To see it, use:

    git diff-tree --root --patch-with-stat --no-color --find-copies-harder --ignore-space-at-eol --cc a0b7b100c649f01325bc6807e418b7563885120e