[GHC] #14118: stg2stg passes appear to produce invalid STG

#14118: stg2stg passes appear to produce invalid STG -------------------------------------+------------------------------------- Reporter: bgamari | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 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: -------------------------------------+------------------------------------- While building with GHC with `-dstg-lint -g3 -O0` (after fixing #14116 and #14117) 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, {{{#!hs svoid [InlPrag=INLINE (sat-args=1)] :: forall s. (State# s -> State# s) -> S s () [GblId, Arity=2, Caf=NoCafRefs, Str=, Unf=OtherCon []] = \r [m0_scBy s1_scBz] src case m0_scBy s1_scBz of s'_scBA { __DEFAULT -> src (#,#) [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, {{{#!hs -- 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=, 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 case m0_scBy s1_scBz of s'_scBA { __DEFAULT -> src (# s'_scBA, GHC.Tuple.() #) } }}} The reason the linter fails here is due to the following logic in `lintStgExpr`, {{{#!hs 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. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14118 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler