[GHC] #12076: "lazy" leads to undefined reference to `stg_ap_0_upd_info'

[GHC]

#12076: "lazy" leads to undefined reference to `stg_ap_0_upd_info'
-------------------------------------+-------------------------------------
        Reporter:  ezyang            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1-rc2
  (CodeGen)                          |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Description changed by ezyang:

@@ -41,1 +41,22 @@
- afterwards that eliminates the trivial thunk assignment.
+ afterwards that eliminates the trivial thunk assignment. Here's the STG:
+
+ {{{
+ z_r2aC :: ()
+ [GblId, Caf=NoCafRefs, Str=DmdType, Unf=OtherCon []] =
+     NO_CCS ()! [];
+
+ g_r2aD :: () -> () -> ()
+ [GblId, Arity=2, Caf=NoCafRefs, Str=DmdType, Unf=OtherCon []] =
+     sat-only \r srt:SRT:[] [ds_s2rW ds1_s2rX] () [];
+
+ M.f :: () -> ()
+ [GblId, Arity=1, Caf=NoCafRefs, Str=DmdType, Unf=OtherCon []] =
+     \r srt:SRT:[] [ds_s2rY]
+         let {
+           x_s2rZ :: ()
+           [LclId, Str=DmdType, Unf=OtherCon []] =
+               \u srt:SRT:[] [] z_r2aC;
+         } in  g_r2aD x_s2rZ x_s2rZ;
+ }}}
+
+ so we need, instead, something like `x_s2rZ = z_r2aC`.

New description:

 This is basically the same thing as #11155 but triggered differently.

 {{{
 -- M.hs
 {-# OPTIONS_GHC -O0 #-}
 module M(f) where

 import GHC.Exts

 {-# NOINLINE z #-}
 z = ()

 f :: () -> ()
 f _ = let x = lazy z
       in g x x

 {-# NOINLINE g #-}
 g :: () -> () -> ()
 g _ _ = ()

 -- MM.hs
 import M
 main = f `seq` return ()
 }}}

 On GHC 8.0, I get:

 {{{
 ezyang at sabre:~$ ghc-8.0 --make MM.hs -fforce-recomp
 [1 of 2] Compiling M                ( M.hs, M.o )
 [2 of 2] Compiling Main             ( MM.hs, MM.o )
 Linking MM ...
 ./M.o: In function `r2aD_info':
 (.text+0x4a): undefined reference to `stg_ap_0_upd_info'
 collect2: error: ld returned 1 exit status
 `gcc' failed in phase `Linker'. (Exit code: 1)
 }}}

 Error goes away when you turn on optimization. My understanding is that
 removing `lazy` in `CorePrep` is too late, because there's no pass
 afterwards that eliminates the trivial thunk assignment. Here's the STG:

 {{{
 z_r2aC :: ()
 [GblId, Caf=NoCafRefs, Str=DmdType, Unf=OtherCon []] =
     NO_CCS ()! [];

 g_r2aD :: () -> () -> ()
 [GblId, Arity=2, Caf=NoCafRefs, Str=DmdType, Unf=OtherCon []] =
     sat-only \r srt:SRT:[] [ds_s2rW ds1_s2rX] () [];

 M.f :: () -> ()
 [GblId, Arity=1, Caf=NoCafRefs, Str=DmdType, Unf=OtherCon []] =
     \r srt:SRT:[] [ds_s2rY]
         let {
           x_s2rZ :: ()
           [LclId, Str=DmdType, Unf=OtherCon []] =
               \u srt:SRT:[] [] z_r2aC;
         } in  g_r2aD x_s2rZ x_s2rZ;
 }}}

 so we need, instead, something like `x_s2rZ = z_r2aC`.

 This is blocking Phab:D2211, where the insertion of a `noinline` (which is
 optimized out similarly to `lazy`) triggers a stage 2 linker failure when
 you don't compile with optimization (unfortunately, this is NOT caught
 validate since we build GHC with optimization... but with this test it
 will be!)

 Perhaps there should be an ASSERT in the codegen so it doesn't create this
 symbol?

--

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12076#comment:1>
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