[GHC] #2224: -fhpc inteferes/prevents rewrite rules from firing

[GHC]

#2224: -fhpc inteferes/prevents rewrite rules from firing
-------------------------------------+-------------------------------------
              Reporter:  dons        |            Owner:  andy@…
                  Type:  bug         |           Status:  new
              Priority:  lowest      |        Milestone:  7.10.1
             Component:  Code        |          Version:  6.8.2
  Coverage                           |         Keywords:  rules, hpc
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------
Changes (by thomie):

 * failure:   => None/Unknown


Old description:

> Use case:
>
> I'm writing tests for rewrite rules, and using HPC to determine if rules
> were fired (and their code exercised). HPC is quite cool here, since it
> lets us see which rules fired, without needing to explicitly export
> functions to test.
>
> However, -fhpc seems to prevent many rules from firing (likely due to
> ticks getting in the way?)
>
> For example:
>
> {{{
> import qualified  Data.ByteString.Char8 as C
>
> main = print (C.pack "literal")
> }}}
>
> When compiled normally, triggers a nice rewrite rule:
>
> {{{
> $ ghc -O2 A.hs -ddump-simpl-stats A.hs -c
>
>     1 ByteString pack/packAddress
> }}}
>
> Now with -fhpc:
>
> {{{
> 2 RuleFired
>     1 unpack
>     1 unpack-list
> }}}
>
> What's the best way to ensure the same code is exercised with and without
> -fhpc here? (I'd quite like to get this working, since rewrite rules
> benefit from testing.)

New description:

 Use case:

 I'm writing tests for rewrite rules, and using HPC to determine if rules
 were fired (and their code exercised). HPC is quite cool here, since it
 lets us see which rules fired, without needing to explicitly export
 functions to test.

 However, -fhpc seems to prevent many rules from firing (likely due to
 ticks getting in the way?)

 For example:

 {{{
 import qualified  Data.ByteString.Char8 as C

 main = print (C.pack "literal")
 }}}

 When compiled normally, triggers a nice rewrite rule:

 {{{
 $ ghc -O2 A.hs -ddump-rule-firings A.hs -c
 Rule fired: unpack
 Rule fired: Class op show
 Rule fired: unpack-list
 Rule fired: ByteString packChars/packAddress
 Rule fired: unpack
 Rule fired: Class op show
 Rule fired: unpack-list
 Rule fired: ByteString packChars/packAddres
 }}}

 Now with -fhpc:

 {{{
 $ ghc -O2 A.hs -ddump-rule-firings A.hs -c -fhpc
 Rule fired: unpack
 Rule fired: Class op show
 Rule fired: unpack-list
 Rule fired: unpack
 Rule fired: Class op show
 Rule fired: unpack-list
 }}}

 What's the best way to ensure the same code is exercised with and without
 -fhpc here? (I'd quite like to get this working, since rewrite rules
 benefit from testing.)

--

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/2224#comment:16>
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