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Opened by Ben Gamari@bgamari🐢Maintainer

Strangeness regarding STG alternative types and linter

While building with GHC with -dstg-lint -g3 -O0 (after fixing #14116 (closed) and #14117 (closed)) I encountered a rather peculiar error,

"inplace/bin/ghc-stage1" -hisuf hi -osuf  o -hcsuf hc -static  -O -H64m -Wall       -this-unit-id integer-gmp-1.0.1.0 -hide-all-packages -i -ilibraries/integer-gmp/src/ -ilibraries/integer-gmp/dist-install/build
 -Ilibraries/integer-gmp/dist-install/build -ilibraries/integer-gmp/dist-install/build/./autogen -Ilibraries/integer-gmp/dist-install/build/./autogen -Ilibraries/integer-gmp/include    -optP-include -optPlibrari
es/integer-gmp/dist-install/build/./autogen/cabal_macros.h -package-id ghc-prim-0.5.1.0 -this-unit-id integer-gmp -Wall -XHaskell2010 -O -dcore-lint -g3 -ddump-to-file -ddump-stg -dcore-lint -dstg-lint -dcmm-lin
t  -no-user-package-db -rtsopts  -Wno-deprecated-flags     -Wnoncanonical-monad-instances  -odir libraries/integer-gmp/dist-install/build -hidir libraries/integer-gmp/dist-install/build -stubdir libraries/intege
r-gmp/dist-install/build   -dynamic-too -c libraries/integer-gmp/src//GHC/Integer/Type.hs -o libraries/integer-gmp/dist-install/build/GHC/Integer/Type.o -dyno libraries/integer-gmp/dist-install/build/GHC/Integer
/Type.dyn_o
ghc-stage1: panic! (the 'impossible' happened)
  (GHC version 8.3.20170815 for x86_64-unknown-linux):
          *** Stg Lint ErrMsgs: in Stg2Stg ***
  <no location info>: warning:
       [in body of lambda with binders m0_scBy :: State# s_a2Em
                                                  -> State# s_a2Em,
                                       s1_scBz :: State# s_a2Em]
      s'_scBA is out of scope
  <no location info>: warning:
       [in body of lambda with binders wild1_sdUv :: Int#]
      qr_sdUp is out of scope

Looking at the STG it appears that these warnings are absolutely correct,

  svoid [InlPrag=INLINE (sat-args=1)]
    :: forall s. (State# s -> State# s) -> S s ()
  [GblId,
   Arity=2,
   Caf=NoCafRefs,
   Str=<C(S),1*C1(U)><S,U>,
   Unf=OtherCon []] =
      \r [m0_scBy s1_scBz]
          src<libraries/integer-gmp/src/GHC/Integer/Type.hs:1957:1-48>
          case m0_scBy s1_scBz of s'_scBA {
            __DEFAULT ->
                src<libraries/integer-gmp/src/GHC/Integer/Type.hs:1957:37-48>
                (#,#) [s'_scBA ()];
          };

This is quite strange given that s'_scBA is clearly in scope, being bound as the case binder.

We had the following from Core Prep,

-- RHS size: {terms: 10, types: 17, coercions: 0, joins: 0/0}                               
GHC.Integer.Type.svoid [InlPrag=INLINE (sat-args=1)]                                        
  :: forall s.                                                                              
     (GHC.Prim.State# s -> GHC.Prim.State# s) -> GHC.Integer.Type.S s ()                    
[GblId,                                                                                     
 Arity=2,                                                                                   
 Caf=NoCafRefs,                                                                             
 Str=<C(S),1*C1(U)><S,U>,                                                                   
 Unf=OtherCon []]                                                                           
GHC.Integer.Type.svoid                                                                      
  = \ (@ s_a2Em)                                                                            
      (m0_scBy [Occ=Once!]                                                                  
         :: GHC.Prim.State# s_a2Em -> GHC.Prim.State# s_a2Em)                               
      (s1_scBz [Occ=Once] :: GHC.Prim.State# s_a2Em) ->                                     
      src<libraries/integer-gmp/src/GHC/Integer/Type.hs:1957:1-48>                          
      case m0_scBy s1_scBz of s'_scBA { __DEFAULT ->                                        
      src<libraries/integer-gmp/src/GHC/Integer/Type.hs:1957:37-48>                         
      (# s'_scBA, GHC.Tuple.() #)                                                           
      }

The reason the linter fails here is due to the following logic in lintStgExpr,

    in_scope <- MaybeT $ liftM Just $
     case alts_type of
        AlgAlt tc     -> check_bndr (tyConPrimRep tc) >> return True
        PrimAlt rep   -> check_bndr [rep]             >> return True
        MultiValAlt _ -> return False -- Binder is always dead in this case
        PolyAlt       -> return True

    MaybeT $ addInScopeVars [bndr | in_scope] $
             lintStgAlts alts scrut_ty

In the svoid case above we hit MultiValAlt path, which causes us to ignore the case binder. The fact that we hit MultiValAlt at all is a bit surprising given that the result is not an unboxed sum or tuple.

Trac metadata
Trac field Value
Version 8.2.1
Type Bug
TypeOfFailure OtherFailure
Priority normal
Resolution Unresolved
Component Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture
Edited by Ben Gamari