Make sure clashSimulation is False in HDL even with -O2

The SpecConstr pass in GHC is able to poke through the OPAQUE of
clashSimulation.
Adding a noinline seems to block it from seeing the True inside.

Fixes #2736

clash-prelude/src/Clash/Magic.hs

@@ -44,6 +44,7 @@ module Clash.Magic
   ) where
 
 import Data.String.Interpolate     (__i)
+import GHC.Magic                   (noinline)
 import GHC.Stack                   (HasCallStack, withFrozenCallStack)
 import Clash.NamedTypes            ((:::))
 import GHC.TypeLits                (Nat,Symbol)
@@ -259,7 +260,8 @@ noDeDup = id
 
 -- | 'True' in Haskell/Clash simulation. Replaced by 'False' when generating HDL.
 clashSimulation :: Bool
-clashSimulation = True
+clashSimulation = noinline True
+-- The 'noinline' is here to prevent SpecConstr from poking through the OPAQUE, see #2736
 -- See: https://github.com/clash-lang/clash-compiler/pull/2511
 {-# CLASH_OPAQUE clashSimulation #-}
 

tests/Main.hs

@@ -387,7 +387,7 @@ runClashTest = defaultMain $ clashTestRoot
         , runTest "NameInstance" def{hdlSim=[]}
         , outputTest "NameInstance" def
         , outputTest "SetName" def{hdlTargets=[VHDL]}
-        , outputTest "SimulationMagic" def{hdlTargets=[VHDL]}
+        , outputTest "SimulationMagic2736" def{hdlTargets=[VHDL]}
         , runTest "PatError" def{hdlSim=[]}
         , runTest "ByteSwap32" def
         , runTest "CharTest" def

tests/clash-testsuite.cabal

@@ -104,6 +104,7 @@ library
     -- an external interface file. See
     -- https://github.com/clash-lang/clash-compiler/issues/1796 for an example.
     shouldwork/LoadModules/precompiled
+    shouldwork/Basic/precompiled
 
   exposed-modules:
     Test.Tasty.Common
@@ -122,6 +123,9 @@ library
     -- From tests/shouldwork/LoadModules/precompiled
     T1796a
 
+    -- From tests/shouldwork/Basic/precompiled
+    SimulationMagic2736a
+
   other-modules:
     Paths_clash_testsuite
 

tests/shouldwork/Basic/SimulationMagic.hs → tests/shouldwork/Basic/SimulationMagic2736.hs

@@ -1,16 +1,17 @@
-module SimulationMagic where
+{-# LANGUAGE TemplateHaskellQuotes #-}
+{-# OPTIONS_GHC -O2 -fspec-constr #-}
+module SimulationMagic2736 where
 
 import Prelude as P
 import Data.List (isInfixOf)
 import System.Environment (getArgs)
 import System.FilePath ((</>))
 
 import Clash.Prelude
-import Clash.Magic (clashSimulation)
+import SimulationMagic2736a
 
 f :: Int
-f | clashSimulation = 123
-  | otherwise       = 456
+f = f0
 {-# ANN f (defSyn "f") #-}
 
 assertNotIn :: String -> String -> IO ()

tests/shouldwork/Basic/precompiled/SimulationMagic2736a.hs

@@ -0,0 +1,8 @@
+{-# OPTIONS_GHC -O2 -fspec-constr #-}
+module SimulationMagic2736a where
+
+import Clash.Prelude
+
+f0 :: Int
+f0 | clashSimulation = 123
+   | otherwise       = 456