[GHC] #9961: compile-time performance regression compiling genprimcode

[GHC]

#9961: compile-time performance regression compiling genprimcode
-------------------------------------+-------------------------------------
              Reporter:  hvr         |             Owner:  simonpj
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:  7.10.1
             Component:  Compiler    |           Version:  7.10.1-rc1
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  Compile-time
  Unknown/Multiple                   |  performance bug
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
-------------------------------------+-------------------------------------
 compiling `utils/genprimopcode` (where compiling the Alex-generated Lexer
 takes up the most amount of time) with GHC 7.8.4:

 ```
 $ time /opt/ghc/7.8.4/bin/ghc  -Rghc-timing --make -O Main.hs
 [1 of 5] Compiling ParserM          ( ParserM.hs, ParserM.o )
 [2 of 5] Compiling Lexer            ( Lexer.hs, Lexer.o )
 [3 of 5] Compiling Syntax           ( Syntax.hs, Syntax.o )
 [4 of 5] Compiling Parser           ( Parser.hs, Parser.o )
 [5 of 5] Compiling Main             ( Main.hs, Main.o )
 Linking Main ...
 <<ghc: 20742311240 bytes, 2092 GCs, 60174814/140258264 avg/max bytes
 residency (33 samples), 332M in use, 0.00 INIT (0.00 elapsed), 7.84 MUT
 (8.35 elapsed), 6.93 GC (6.93 elapsed) :ghc>>

 real    0m15.331s
 user    0m15.173s
 sys     0m0.172s
 ```

 (btw, is it ok that GC takes up so much?)

 the same with a GHC 7.10 snapshot takes *alot* longer:

 ```
 $ time /opt/ghc/7.10.1/bin/ghc  -Rghc-timing --make -O Main.hs
 [1 of 5] Compiling ParserM          ( ParserM.hs, ParserM.o )
 [2 of 5] Compiling Lexer            ( Lexer.hs, Lexer.o )
 [3 of 5] Compiling Syntax           ( Syntax.hs, Syntax.o )
 [4 of 5] Compiling Parser           ( Parser.hs, Parser.o )
 [5 of 5] Compiling Main             ( Main.hs, Main.o )
 Linking Main ...
 <<ghc: 147681534568 bytes, 37824 GCs, 203338614/375314176 avg/max bytes
 residency (538 samples), 876M in use, 0.001 INIT (0.001 elapsed), 81.374
 MUT (81.888 elapsed), 235.240 GC (235.136 elapsed) :ghc>>

 real    5m17.061s
 user    5m16.702s
 sys     0m0.499s
 ```

 ...and here the GC/MUT ratio is even more off.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9961>
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