'GHC.Magic.noinline <var>' should not float out

While working on #15155 (closed), I discovered that the magic function GHC.Magic.noinline tricked the float-out transformation into performing undesirable operations:

Looking at the Core produced:

                  { ghc-prim-0.5.3:GHC.Types.I# x_azMX [Dmd=<S,U>] ->
                  case x_azMX of {
                    __DEFAULT -> jump $j_sMly;
                    3674937295934324920# ->
                      src<compiler/typecheck/TcTyClsDecls.hs:3561:9-35>
                      check_ty_roles_sLb3
                        (src<compiler/typecheck/TcTyClsDecls.hs:3561:24-26> env_aqfK)
                        (src<compiler/typecheck/TcTyClsDecls.hs:3561:28-31> role_aqfL)
                        (ghc-prim-0.5.3:GHC.Magic.noinline @ Kind liftedTypeKind)
                  }
                  }

After simplification we end up with a toplevel variable basically redirecting to an imported variable.

-- RHS size: {terms: 2, types: 1, coercions: 0, joins: 0/0}
lvl330_r12qn :: Kind
[GblId]
lvl330_r12qn
  = ghc-prim-0.5.3:GHC.Magic.noinline @ Kind liftedTypeKind

This would cause IND_STATICs when eventually emitted as assembly.

IIRC GHC.Magic.noinline was introduced with GHC v8.2. This probably regressed the float-out optimisation.

I have a patch to reclassify noinline <some-var> as cheap, so that it won't be floated out.

N.B.: for more details, see: https://github.com/ghc/ghc/pull/224

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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