[commit: ghc] wip/tdammers/T15019: Fix simplCast perf issues from #14737 (5a157f8)

[git at git.haskell.org]

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

On branch  : wip/tdammers/T15019
Link       : http://ghc.haskell.org/trac/ghc/changeset/5a157f8032633804d9c2f7ba7d558e9d99ad1e78/ghc

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commit 5a157f8032633804d9c2f7ba7d558e9d99ad1e78
Author: Tobias Dammers <tdammers at gmail.com>
Date:   Mon Apr 30 09:33:59 2018 +0200

    Fix simplCast perf issues from #14737
    
    See Trac #15019.
    
    Reflexive coercion performance is a surprisingly delicate matter. On the
    one hand, we want to eliminate reflexive coercions as early as possible,
    to avoid performance bottlenecks later on; on the other hand,
    conclusively doing so involves calling `eqType`, which is expensive.
    
    In #14737, we addressed the `eqType` part by simply removing the
    `eqType` check. This, however, causes regressions, because the check no
    longer catches some of the reflexive coercions that we want to catch.
    
    So we need to find a balance between getting rid of reflexivity as soon
    as it appears (see Trac #11735, #14737, #15019), and avoiding calls to
    `isReflexiveCo` (which is expensive on big types, since it involves
    `eqType`).
    
    In particular, we want to behave well on
    
     *  e |> co1 |> co2
        where the two happent to cancel out entirely. That is quite common;
        e.g. a newtype wrapping and unwrapping cancel
    
     * (f |> co) @t1 @t2 ... @tn x1 .. xm
       Here we wil use pushCoTyArg and pushCoValArg successively, which
       build up NthCo stacks.  Silly to do that if co is reflexive.
    
    A good compromise (determined experimentally) seems to be to call
    isReflexiveCo
     * when composing casts, and
     * at the end


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5a157f8032633804d9c2f7ba7d558e9d99ad1e78
 compiler/coreSyn/CoreOpt.hs         |  6 ++++
 compiler/simplCore/Simplify.hs      | 70 ++++++++++++++++++++++++++-----------
 testsuite/tests/perf/compiler/all.T | 12 ++++---
 3 files changed, 63 insertions(+), 25 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 5a157f8032633804d9c2f7ba7d558e9d99ad1e78