[GHC] #11285: Static linking is really slow sometimes

[GHC]

#11285: Static linking is really slow sometimes
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           Reporter:  ezyang         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I'm testing on Cabal `Setup.hs`, which links against Cabal.

 On a beefy machine with many cores and lots of RAM, I see a x2 regression
 in linking time from GHC 7.10.2 to GHC 8.0 (a recent HEAD) using GNU ld
 (not gold):

 {{{
 [ezyang at hs01 ezyang]$ rm A; time ghc --make A.hs -fforce-recomp
 [1 of 1] Compiling Main             ( A.hs, A.o )
 Linking A ...

 real    0m1.273s
 user    0m0.990s
 sys     0m0.210s

 [ezyang at hs01 ezyang]$ rm A; time ghc-8.0/usr/bin/ghc --make A.hs -fforce-
 recomp
 [1 of 1] Compiling Main             ( A.hs, A.o )
 Linking A ...

 real    0m3.270s
 user    0m2.727s
 sys     0m0.523s
 }}}

 On a puny eight year-old laptop, I see a x2 regression from 7.6 to 7.10
 (with not much change with 8.0)

 {{{
 ezyang at sabre:~$ rm A; time ghc --make -O0 A.hs -fforce-recomp
 rm: cannot remove ‘A’: No such file or directory
 [1 of 1] Compiling Main             ( A.hs, A.o )
 Linking A ...

 real    0m3.058s
 user    0m1.860s
 sys     0m1.164s
 ezyang at sabre:~$ rm A; time ghc-7.10 --make -O0 A.hs -fforce-recomp
 [1 of 1] Compiling Main             ( A.hs, A.o )
 Linking A ...

 real    0m7.139s
 user    0m4.616s
 sys     0m2.488s

 }}}

 There must be something which is causing the linker to run slowly in one
 case, and quickly in the other. It would be really good to figure out what
 this is. Slow linking is NOT NICE.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11285>
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