[commit: ghc] master: Improve performance for PM check on literals (Fixes #11160 and #11161) (ae4398d)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/ae4398d64655b43606386dddb01637bbfcf0171e/ghc

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

commit ae4398d64655b43606386dddb01637bbfcf0171e
Author: George Karachalias <george.karachalias at gmail.com>
Date:   Fri Dec 4 07:11:55 2015 +0100

    Improve performance for PM check on literals (Fixes #11160 and #11161)
    
    Two changes:
    
    1. Instead of generating constraints of the form (x ~ e) (as we do in
    the paper), generate constraints of the form (e ~ e). The term oracle
    (`tmOracle` in deSugar/TmOracle.hs) is not really efficient and in the
    presence of many (x ~ e) constraints behaves quadratically. For
    literals, constraints of the form (False ~ (x ~ lit)) are pretty common,
    so if we start with { y ~ False, y ~ (x ~ lit) } we end up givng to the
    solver (a) twice as many constraints as we need and (b) half of them
    trigger the solver's weakness. This fixes #11160.
    
    2. Treat two overloaded literals that look different as different. This
    is not entirely correct but it is what both the previous and the current
    check did. I had the ambitious plan to do the *right thing* (equality
    between overloaded literals is undecidable in the general case) and just
    use this assumption when issuing the warnings. It seems to generate much
    more constraints than I expected (breaks #11161) so I just do it
    immediately now instead of generating everything and filtering
    afterwards.
    
    Even if it is not (strictly speaking) correct, we have the following:
      * Gives the "expected" warnings (the ones Ocaml or the previous
        algorithm would give) and,
      * Most importantly, it is safe. Unless a catch-all clause exists, a
        match against literals is always non-exhaustive. So, effectively
        this affects only what is shown to the user (and, evidently,
        performance!).


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ae4398d64655b43606386dddb01637bbfcf0171e
 compiler/deSugar/Check.hs    | 128 ++++++++--------------
 compiler/deSugar/PmExpr.hs   |  24 ++---
 compiler/deSugar/TmOracle.hs | 247 +++----------------------------------------
 3 files changed, 70 insertions(+), 329 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 ae4398d64655b43606386dddb01637bbfcf0171e