[GHC] #15113: Do not make CAFs from literal strings

[GHC]

#15113: Do not make CAFs from literal strings
-------------------------------------+-------------------------------------
        Reporter:  simonpj           |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.2.2
      Resolution:                    |             Keywords:  CAFs
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4717
       Wiki Page:                    |
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Comment (by osa1):

 I started looking into this, but I realized that nofib can't be run with
 GHC 8.6 (and probably with HEAD) so I'll need to fix nofib first.

 What I've done so far is I implemented two counters:

 - A runtime counter for SRT traversals done by the GC
 - A compiler counter to count how many SRTs are generated (I'm also
 recording sum of sizes of the SRTs although this is probably not too
 useful)

 The runtime counter is printed with `+RTS -t`:

 {{{
 <<ghc: 98529880 bytes, 63 GCs, 3481032/7949096 avg/max bytes residency (6
 samples), 20M in use, 0.000 INIT (0.000 elapsed), 0.038 MUT (0.198
 elapsed), 0.059 GC (0.059 elapsed), 226752 SRT scavs :ghc>>
 }}}

 The "226752 SRT scavs" part is new, and it means that we scavenged 226752
 SRTs.

 The compiler counter prints once per binding group and it looks like this:

 {{{
 SRTs: 4 SRT(s), total size: 17
 }}}

 This means that 4 SRTs are generated, and total size of all those SRTs are
 17 words.

 The patch is here:
 https://github.com/osa1/ghc/commit/c46fe24a02591edd3ce7b6aa70246493826d218d

 I'll fix nofib first and then try to collect these numbers from nofib
 output.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15113#comment:6>
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