[GHC] #14364: Reduce repetition in derived Read instances

[GHC]

#14364: Reduce repetition in derived Read instances
-------------------------------------+-------------------------------------
        Reporter:  bgamari           |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  deriving
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #10980 #7258      |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by tdammers):

 http://git.haskell.org/ghc.git/commitdiff/a6dd03e751d17467be10eea3ff1b1773d8d35893
 factors out the field reader into `readField` and `readSymField` (the
 latter being used for symbol-named fields).

 Performance improvement is significant; before and after profiling output
 for a 500-field record example:

 Before:

 {{{

         Wed Oct 18 09:02 2017 Time and Allocation Profiling Report
 (Final)

            ghc-stage2 +RTS -p -h -RTS -B/home/tobias/well-
 typed/devel/ghc/inplace/lib -B/home/tobias/well-
 typed/devel/ghc/inplace/lib -fforce-recomp -c /home/tobias/Downloads/W3.hs

         total time  =       25.50 secs   (25505 ticks @ 1000 us, 1
 processor)
         total alloc = 24,693,071,936 bytes  (excludes profiling overheads)

 COST CENTRE        MODULE      SRC
 %time %alloc

 RegAlloc-linear    AsmCodeGen
 compiler/nativeGen/AsmCodeGen.hs:(658,27)-(660,55)   29.6   25.6
 pprNativeCode      AsmCodeGen
 compiler/nativeGen/AsmCodeGen.hs:(529,37)-(530,65)   16.9   22.2
 regLiveness        AsmCodeGen
 compiler/nativeGen/AsmCodeGen.hs:(591,17)-(593,52)    8.8    6.5
 StgCmm             HscMain
 compiler/main/HscMain.hs:(1426,13)-(1427,62)          8.7    8.3
 NativeCodeGen      CodeOutput  compiler/main/CodeOutput.hs:171:18-78
 7.3    7.9
 genMachCode        AsmCodeGen
 compiler/nativeGen/AsmCodeGen.hs:(580,17)-(582,62)    6.0    6.3
 SimplTopBinds      SimplCore   compiler/simplCore/SimplCore.hs:761:39-74
 2.8    3.9
 CoreTidy           HscMain     compiler/main/HscMain.hs:1253:27-67
 2.6    3.2
 layoutStack        CmmPipeline compiler/cmm/CmmPipeline.hs:(97,13)-(99,40)
 2.1    2.6
 deSugar            HscMain     compiler/main/HscMain.hs:511:7-44
 2.0    2.7
 CorePrep           HscMain
 compiler/main/HscMain.hs:(1313,24)-(1314,57)          1.4    2.0
 generateJumpTables AsmCodeGen  compiler/nativeGen/AsmCodeGen.hs:689:17-50
 1.3    0.7
 fixStgRegisters    AsmCodeGen  compiler/nativeGen/AsmCodeGen.hs:566:17-42
 1.1    0.7
 cmmToCmm           AsmCodeGen  compiler/nativeGen/AsmCodeGen.hs:571:17-50
 1.1    1.1
 OccAnal            SimplCore
 compiler/simplCore/SimplCore.hs:(739,22)-(740,67)     1.1    1.1
 }}}

 After:

 {{{

         Wed Oct 18 15:41 2017 Time and Allocation Profiling Report
 (Final)

            ghc-stage2 +RTS -p -h -RTS -B/home/tobias/well-
 typed/devel/ghc/inplace/lib -B/home/tobias/well-
 typed/devel/ghc/inplace/lib -fforce-recomp -c /home/tobias/Downloads/W3.hs

         total time  =       14.78 secs   (14784 ticks @ 1000 us, 1
 processor)
         total alloc = 14,528,601,400 bytes  (excludes profiling overheads)

 COST CENTRE        MODULE      SRC
 %time %alloc

 RegAlloc-linear    AsmCodeGen
 compiler/nativeGen/AsmCodeGen.hs:(658,27)-(660,55)   26.0   22.0
 pprNativeCode      AsmCodeGen
 compiler/nativeGen/AsmCodeGen.hs:(529,37)-(530,65)   14.7   19.2
 StgCmm             HscMain
 compiler/main/HscMain.hs:(1426,13)-(1427,62)          8.8    7.8
 regLiveness        AsmCodeGen
 compiler/nativeGen/AsmCodeGen.hs:(591,17)-(593,52)    7.8    5.7
 NativeCodeGen      CodeOutput  compiler/main/CodeOutput.hs:171:18-78
 6.3    6.8
 genMachCode        AsmCodeGen
 compiler/nativeGen/AsmCodeGen.hs:(580,17)-(582,62)    5.3    5.5
 SimplTopBinds      SimplCore   compiler/simplCore/SimplCore.hs:761:39-74
 4.5    6.5
 CoreTidy           HscMain     compiler/main/HscMain.hs:1253:27-67
 4.4    5.5
 deSugar            HscMain     compiler/main/HscMain.hs:511:7-44
 3.4    4.5
 CorePrep           HscMain
 compiler/main/HscMain.hs:(1313,24)-(1314,57)          2.5    3.2
 OccAnal            SimplCore
 compiler/simplCore/SimplCore.hs:(739,22)-(740,67)     2.1    1.9
 layoutStack        CmmPipeline compiler/cmm/CmmPipeline.hs:(97,13)-(99,40)
 1.9    2.3
 Stg2Stg            HscMain     compiler/main/HscMain.hs:1489:12-44
 1.4    1.0
 generateJumpTables AsmCodeGen  compiler/nativeGen/AsmCodeGen.hs:689:17-50
 1.2    0.6
 fixStgRegisters    AsmCodeGen  compiler/nativeGen/AsmCodeGen.hs:566:17-42
 1.1    0.6
 tc_rn_src_decls    TcRnDriver
 compiler/typecheck/TcRnDriver.hs:(493,4)-(555,7)      1.1    0.8
 cmmToCmm           AsmCodeGen  compiler/nativeGen/AsmCodeGen.hs:571:17-50
 1.0    1.0
 occAnalBind.assoc  OccurAnal   compiler/simplCore/OccurAnal.hs:853:13-60
 0.9    1.0
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14364#comment:6>
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