compile-time performance regression (probably due to Generics)
This is a continuation of #9583 (closed)
Compiling latest Cabal from Git's Language.Haskell.Extension module shows a significant performance regression with respect to GHC 7.8.3:
$ uname -a
Linux duo 3.17.0-031700rc6-generic #201409211935 SMP Sun Sep 21 23:37:11 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux
$ git describe
cabal-install_merged-1912-g7161e4c
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$ /opt/ghc/7.8.3/bin/ghc --version
The Glorious Glasgow Haskell Compilation System, version 7.8.3
$ /opt/ghc/7.8.3/bin/ghc -Rghc-timing -O -c Language/Haskell/Extension.hs
<<ghc: 5228751760 bytes, 543 GCs,
34135677/114940864 avg/max bytes residency (14 samples),
273M in use, 0.00 INIT (0.03 elapsed),
4.96 MUT (5.28 elapsed), 3.26 GC (3.47 elapsed) :ghc>>
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$ /opt/ghc/head/bin/ghc --version
The Glorious Glasgow Haskell Compilation System, version 7.9.20140923
$ /opt/ghc/head/bin/ghc -Rghc-timing -O -c Language/Haskell/Extension.hs
<<ghc: 125917854120 bytes, 3181 GCs,
349136895/833496072 avg/max bytes residency (30 samples),
2096M in use, 0.001 INIT (0.001 elapsed),
133.003 MUT (142.730 elapsed), 73.241 GC (78.234 elapsed) :ghc>>Or in other words, this represents a ~7.7x "memory in use" and ~26x elapsed time regression