[GHC] #9326: Minor change to list comprehension structure leads to poor performance

[GHC]

#9326: Minor change to list comprehension structure leads to poor performance
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              Reporter:  dfeuer      |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.8.3
            Resolution:              |          Keywords:
Differential Revisions:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  Runtime
  Unknown/Multiple                   |  performance bug
            Difficulty:  Unknown     |         Test Case:
            Blocked By:              |          Blocking:
       Related Tickets:              |
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Description changed by dfeuer:

Old description:

> According to [http://stackoverflow.com/questions/24690406/haskell-list-
> comprehension-speed-inconsistencies] ''and my own testing'', the code in
> compspeed.hs reliably runs several times faster than the code in
> compspeedslow.hs with ghc 7.8.3, although this apparently does not happen
> with 7.6.3. The basic distinction is between a list comprehension of the
> form
>
> {{{
> [(a,h) | ..., let h = expr]
> }}}
>
> and
>
> {{{
> [(a,expr) | ...]
> }}}
>
> The core produced by -ddump-simpl is completely different in each case,
> so I can't figure out what's going on.

New description:

 According to [http://stackoverflow.com/questions/24690406/haskell-list-
 comprehension-speed-inconsistencies] ''and my own testing'', the code in
 compspeed.hs reliably runs several times faster than the code in
 compspeedslow.hs with ghc 7.8.3. It seems 7.8.3 is compiling the "fast"
 code better than 7.6.3 did, but compiling the "slow" code about the same.
 The basic distinction is between a list comprehension of the form

 {{{
 [(a,h) | ..., let h = expr]
 }}}

 and

 {{{
 [(a,expr) | ...]
 }}}

 The core produced by -ddump-simpl is completely different in each case, so
 I can't figure out what's going on.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9326#comment:1>
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