Use a more efficient representation than [DynFlag]

[igloo]

In compiler/typecheck/TcRnMonad.lhs, if I replace the existing definitions with

traceOptIf _ _ = return ()
traceOptTcRn _ _ = return ()
dumpTcRn _ = return ()

then we see an improvement in nofib's "Compile Times" sections:

-1 s.d.		-----	-8.5%
+1 s.d.		-----	+1.8%
Average		-----	-3.5%

By using some sort of bitmap representation, we ought to see a modest compile time improvement from all the checks that the trace functions make, as well as various other checks that are made (do we need to warn about missing sigs, are overlapping instances allowed, etc).

The tricky bit is designing it so that it is hard to shoot yourself in the foot. Lexer.x already has a bitmap (genericsBit, ffiBit etc) but the mapping of feature to bit number is done by hand, which isn't ideal. Perhaps this is even important enough to deserve a language extension?

[igloo]

With plain HEAD, and HEAD with bitmap.dpatch, running

time ghc -v0 --make -O Setup.hs

in a Cabal tree (which thus also builds most of Cabal) I get:

Run 1
    HEAD:           111.38 user 1.39 system 1:52.88 elapsed
    Bitmap:         109.01 user 1.38 system 1:50.41 elapsed
Run 2
    HEAD:           112.18 user 1.32 system 1:53.55 elapsed
    Bitmap:         110.58 user 1.38 system 1:51.98 elapsed
Run 3
    HEAD:           97.51 user 1.23 system 1:38.75 elapsed
    Bitmap:         110.95 user 1.26 system 1:52.24 elapsed
Run 4
    HEAD:           111.60 user 1.33 system 1:52.92 elapsed
    Bitmap:         109.93 user 1.30 system 1:51.31 elapsed
Run 5
    HEAD:           111.63 user 1.66 system 1:53.31 elapsed
    Bitmap:         88.90 user 1.19 system 1:30.41 elapsed
Run 6
    HEAD:           95.12 user 1.26 system 1:36.47 elapsed
    Bitmap:         111.27 user 1.48 system 1:52.88 elapsed
Run 7
    HEAD:           112.18 user 1.35 system 1:53.66 elapsed
    Bitmap:         109.51 user 1.27 system 1:50.85 elapsed
Run 8
    HEAD:           112.51 user 1.16 system 1:53.74 elapsed
    Bitmap:         110.33 user 1.26 system 1:51.71 elapsed
Run 9
    HEAD:           111.69 user 1.36 system 1:53.37 elapsed
    Bitmap:         109.26 user 1.40 system 1:50.65 elapsed
Run 10
    HEAD:           114.77 user 1.38 system 1:56.24 elapsed
    Bitmap:         110.08 user 1.47 system 1:51.54 elapsed

so with the patch it takes 109-110 rather than 111-112s (although occasionally a build is much quicker).

So not a huge performance increase, but it removes one of the slivers, and it would be nice to get the dopt noise out of profiles. I think some sort of extension (or generic program?) that makes bitmaps from enumerations would be generally useful.

[igloo]

I don't think we should apply the patch, incidentally. It would be a maintenance nightmare.

[simonmar]

Replying to igloo:

I don't think we should apply the patch, incidentally. It would be a maintenance nightmare.

That's what I thought too when I saw it. Have you tried just using an IntMap?