`autoReg @System @(Maybe Int)` can't be used with undefined reset values

[martijnbastiaan]

Minimal example:

import Clash.Prelude

topEntity :: 
  SystemClockResetEnable => 
  Signal System (Maybe Int) ->
  Signal System (Maybe Int)
topEntity = autoReg  (errorX "foo bar")       

Error:

Test.hs:7:1: error:
    
    Clash.Netlist.BlackBox(188): Couldn't instantiate blackbox for Clash.Signal.Internal.register#. Verification procedure reported:
    
    Argument 5 should be literal, as blackbox used ~CONST[5], but was:
    
    Identifier "initial_0" Nothing
    
    The source location of the error is not exact, only indicative, as it is acquired 
    after optimizations. The actual location of the error can be in a function that is 
    inlined. To prevent inlining of those functions, annotate them with a NOINLINE pragma.
  |
7 | topEntity = autoReg  (errorX "foo bar")
  | ^^^^^^^^^

When running with -fclash-debug DebugSilent I get:

Irreducible constant as case subject: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Can be reduced to: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Irreducible constant as case subject: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Can be reduced to: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Irreducible constant as case subject: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Can be reduced to: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Irreducible constant as case subject: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Can be reduced to: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Irreducible constant as case subject: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Can be reduced to: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Irreducible constant as case subject: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Can be reduced to: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Irreducible constant as case subject: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Can be reduced to: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Irreducible constant as case subject: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Can be reduced to: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Irreducible constant as case subject: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Can be reduced to: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Irreducible constant as case subject: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Can be reduced to: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Irreducible constant as case subject: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Can be reduced to: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Irreducible constant as case subject: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Can be reduced to: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Irreducible constant as case subject: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Can be reduced to: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Irreducible constant as case subject: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Can be reduced to: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Irreducible constant as case subject: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Can be reduced to: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Irreducible constant as case subject: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Can be reduced to: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Irreducible constant as case subject: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Can be reduced to: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Irreducible constant as case subject: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Can be reduced to: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Irreducible constant as case subject: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Can be reduced to: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Irreducible constant as case subject: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Can be reduced to: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Irreducible constant as case subject: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Can be reduced to: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Irreducible constant as case subject: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))
Can be reduced to: Clash.XException.errorX
  @(GHC.Maybe.Maybe[3674937295934324792] GHC.Types.Int[3674937295934324766])
  (Clash.Transformations.removedArg @GHC.Stack.Types.CallStack[3674937295934325108])
  (Clash.Transformations.removedArg
     @(GHC.Types.[][3674937295934324788] GHC.Types.Char[3674937295934324752]))

[christiaanb]

It's because errorX is not in this list:

clash-compiler/clash-lib/src/Clash/Normalize/Transformations.hs

Lines 585 to 606 in 7385be8

	| primName pInfo == "Control.Exception.Base.absentError" ->
	let e1 = mkApps (mkTicks (Prim pInfo) ticks)
	[Right ty,msgOrCallStack]
	in changed e1
	#endif
	-- WHNF of subject is _|_, in the form of `absentError`, `patError`,
	-- or `undefined`: that means the entire case-expression is _|_
	(Prim pInfo,repTy:_:msgOrCallStack:_,ticks)
	| primName pInfo `elem` ["Control.Exception.Base.patError"
	#if !MIN_VERSION_ghc(8,2,2)
	,"Control.Exception.Base.absentError"
	#endif
	,"GHC.Err.undefined"] ->
	let e1 = mkApps (mkTicks (Prim pInfo) ticks)
	[repTy,Right ty,msgOrCallStack]
	in changed e1
	-- WHNF of subject is _|_, in the form of our internal _|_-values: that
	-- means the entire case-expression is _|_
	(Prim pInfo,[_],ticks)
	| primName pInfo `elem` [ "Clash.Transformations.undefined"
	, "Clash.GHC.Evaluator.undefined"
	, "EmptyCase"] ->

[leonschoorl]

Because of that it fail to evaluate this:

clash-compiler/clash-prelude/src/Clash/Class/AutoReg/Internal.hs

Line 149 in 7385be8

tagInit = isJust initVal

And can't get the initial value of the tag register