[GHC] #16122: `round :: Double -> Int64` much slower than `fromIntegral @Int @Int64 . round`

[GHC]

#16122: `round :: Double -> Int64` much slower than `fromIntegral @Int @Int64 .
round`
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        Reporter:  Fuuzetsu          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by akio):

 Replying to [comment:4 simonpj]:
 > * Rules are applied to the RHS of other rules, if the phases allow; this
 seems reasonable. After all, that's what will happen whenever the rule is
 applied.

 Hmm, this doesn't appear to be true in general. It looks like user-defined
 rules are never applied to the RHS of other rules. Only built-in rules
 are. I checked this by compiling the following module:

 {{{#!hs
 module Foo where

 import qualified GHC.List as List

 foo, bar:: Int
 foo = 0
 bar = 0

 baz :: [Int]
 baz = []

 {-# RULES "foo" foo = 1 #-}

 {-# RULES "bar/foo" bar = foo #-} -- the "foo" rule doesn't fire
 {-# RULES "baz" baz = List.concat [] #-} -- the "concat" rule from
 GHC.List doesn't fire
 }}}

 Here is what I think is the relevant code
 https://gitlab.haskell.org/ghc/ghc/blob/9ea8dcea3e5ba96808ef91028e0efde9d31f7272/compiler/simplCore/Simplify.hs#L3558.
 The rule RHS is simplified under `rule_env`, which has the `sm_rules`
 field set to `False`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16122#comment:6>
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