LLVM code generator produces mal-formed LLVM blocks
It appears that the recent addition (673efccb) of some instances for types in GHC.Generics
has uncovered a bug in the LLVM code generator (at least on ARM). libraries/base/GHC/Generics.hs
fails to compile with,
Entry block to function must not have predecessors!
label %cjGw
/home/ben/ghc/roots/llvm-3.7/bin/opt: libraries/base/GHC/Generics.ll: error: input module is broken!
The problematic block in question appears to be,
define internal ghccc void @rhSw_info$def(i32* noalias nocapture %Base_Arg, i32* noalias nocapture %Sp_Arg, i32* noal
ias nocapture %Hp_Arg, i32 %R1_Arg, i32 %R2_Arg, i32 %R3_Arg, i32 %R4_Arg, i32 %SpLim_Arg) align 4 nounwind prefix <{
i32, i32, i32}><{i32 65541, i32 0, i32 15}>
{
clpH:
br label %clpH
}
Which arose from this Cmm,
==================== Post control-flow optimisations ====================
2016-02-26 10:39:28.656744 UTC
[section ""data" . lvl_rhSw_closure" {
lvl_rhSw_closure:
const lvl_rhSw_info;
},
lvl_rhSw_entry() // []
{ info_tbl: [(clpH,
label: lvl_rhSw_info
rep:HeapRep static { Fun {arity: 1 fun_type: ArgSpec 5} })]
stack_info: arg_space: 0 updfr_space: Nothing
}
{offset
clpH:
goto clpH; // CmmBranch
}
}]
Which arose from this input Cmm,
==================== Cmm produced by new codegen ====================
2016-02-26 10:39:28.611641 UTC
[section ""data" . lvl_rhSw_closure" {
lvl_rhSw_closure:
const lvl_rhSw_info;
},
lvl_rhSw_entry() // [R2]
{ info_tbl: [(clpD,
label: lvl_rhSw_info
rep:HeapRep static { Fun {arity: 1 fun_type: ArgSpec 5} })]
stack_info: arg_space: 4 updfr_space: Just 4
}
{offset
clpD:
_shWa::P32 = R2; // CmmAssign
goto clpz; // CmmBranch
clpz:
if ((old + 0) - <highSp> < SpLim) goto clpE; else goto clpF; // CmmCondBranch
clpE:
R2 = _shWa::P32; // CmmAssign
R1 = lvl_rhSw_closure; // CmmAssign
call (stg_gc_fun)(R2, R1) args: 4, res: 0, upd: 4; // CmmCall
clpF:
goto clpy; // CmmBranch
clpy:
goto shWb; // CmmBranch
shWb:
goto clpH; // CmmBranch
clpH:
goto shWb; // CmmBranch
}
}]
Which arose from this STG,
lvl_rhSw
:: forall a_a99i.
GHC.Generics.U1 a_a99i -> GHC.Generics.U1 [a_a99i]
[GblId,
Arity=1,
Caf=NoCafRefs,
Str=DmdType <L,U>x,
Unf=OtherCon []] =
sat-only \r [eta_shWa]
let-no-escape {
some_v_shWb [Occ=LoopBreaker] :: GHC.Generics.U1 [a12_a99i]
[LclId, Str=DmdType b] =
NO_CCS[] \u [] some_v_shWb;
} in some_v_shWb;
Which arose from this Core (from -ddump-simpl
),
lvl_rhSw :: forall a_a99i. U1 a_a99i -> U1 [a_a99i]
[GblId, Arity=1, Caf=NoCafRefs, Str=DmdType <L,U>x]
lvl_rhSw =
\ (@ a12_a99i) _ [Occ=Dead] ->
letrec {
some_v_a99l [Occ=LoopBreaker] :: U1 [a12_a99i]
[LclId, Str=DmdType b]
some_v_a99l = some_v_a99l; } in
some_v_a99l
Which is apparently the body of the some
implementation in the Alternative
instance for GHC.Generics.U1
.