[GHC] #7782: flag to run the demand analysis a second time

[GHC]

#7782: flag to run the demand analysis a second time
----------------------------+----------------------------------------------
        Reporter:  nfrisby  |            Owner:  nfrisby
            Type:  task     |           Status:  new
        Priority:  normal   |        Milestone:  7.8.1
       Component:           |          Version:  7.7
  Compiler                  |         Keywords:
      Resolution:           |     Architecture:  Unknown/Multiple
Operating System:           |       Difficulty:  Unknown
  Unknown/Multiple          |       Blocked By:
 Type of failure:           |  Related Tickets:  #4941, #5302, #6087, #4962
  None/Unknown              |
       Test Case:           |
        Blocking:           |
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Description changed by nfrisby:

Old description:

> There are some tickets documenting runtime bugs that can be cleaned up by
> running the demand analyzer (followed by a simplifier run) a second time
> at the end of the pipeline: #4941, #5302, #6087. #6070 ? Others?
>
> Here's the effects on nofib. Run time didn't seem to change as
> drastically.  The "X/Y" column headers mean "library-flags/test-flags"
> given to GHC when compiling the respective bit.
>
> {{{
> Allocations
>
> -------------------------------------------------------------------------------
>         Program                O2/O2     late-dmd+O2/O2    late-dmd+O2
> /late-dmd+O2
> -------------------------------------------------------------------------------
>    cryptarithm2             25078168           +0.0%           +8.0%
>        nucleic2             98331744           +0.0%           +3.2%
>
>        cichelli             80310632           +0.0%          -22.9%
>           fasta            401159024           -9.1%           -9.1%
>          fulsom            321427240           +0.0%           -2.6%
>    k-nucleotide           4125102928           -0.0%           -4.8%
>         knights              2037984           +0.0%           -3.7%
>         mandel2              1041840           +0.0%          -21.4%
>         parstof              3103208           +0.0%           -1.4%
> reverse-complem            155188304          -12.8%          -12.8%
>          simple            226412800           -0.0%           -1.0%
> }}}
> All other changes less than 1% allocation.
> Note that it improves a couple tests significantly just via changes in
> the base libraries.
>
> For cryptarithm2, (cf remarks in #4941)
>  * 4% increase allocation is due to reboxing
>  * 4% is due to dead closures, because the fix in #4962 isn't working for
> some reason.
>
> For nucleic2, in var_most_distant_atom, an let-bound function is inlined
> after w/w, and hence grows numerous closures by a significant amount. I'm
> not sure where to lay the blame for this. Note however, that just making
> nucleic2's data types use strict !Float fields changes its allocation
> -72.4%, so maybe this "bad practice" corner case is a small issue.
>
> The main remaining task beyond the attached patch is to repair the
> treatment of wrapper's strictness signatures in .hi files. Because the
> second demand analysis might change a these signatures, the current
> compaction mechanism that .hi uses results in ill-formed Core. For the
> attached patch, I've just disabled that .hi compaction mechanism, but
> this just needlessly bloats .hi files.

New description:

 There are some tickets documenting runtime bugs that can be cleaned up by
 running the demand analyzer (followed by a simplifier run) a second time
 at the end of the pipeline: #4941, #5302, #6087. #6070 ? Others?

 The -flate-dmd-anal flag has been committed to HEAD (cf comment:10 below).

 The remaining task is to determine if `-O2` should imply `-flate-dmd-
 anal`. It currently does not: late demand analysis is ''off'' by default.

 See LateDmd for more info.

--

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7782#comment:11>
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