Fix HO blackboxes not propagating usage metadata

changelog/2026-02-10T18_38_12+01_00_fix_iteratei_verilog_reg

@@ -0,0 +1 @@
+FIXED: HO blackboxes not propagating usage metadata for generated results [#3147](https://github.com/clash-lang/clash-compiler/issues/3141)

clash-lib/src/Clash/Primitives/DSL.hs

@@ -1,6 +1,6 @@
 {-|
   Copyright   :  (C) 2019,      Myrtle Software Ltd.
-                     2020-2024, QBayLogic B.V.
+                     2020-2026, QBayLogic B.V.
                      2021,      Myrtle.ai
                      2022-2023, Google Inc
   License     :  BSD2 (see the file LICENSE)
@@ -102,7 +102,7 @@ import           Data.IntMap                     (IntMap)
 import qualified Data.IntMap                     as IntMap
 import           Data.List                       (intersperse)
 import           Data.List.Extra                 (zipEqual)
-import           Data.Maybe                      (fromMaybe)
+import           Data.Maybe                      (fromMaybe, listToMaybe)
 import           Data.Monoid                     (Ap(getAp))
 import           Data.Semigroup                  hiding (Product)
 import           Data.String
@@ -835,6 +835,10 @@ instHO bbCtx fPos (resTy, bbResTy) argsWithTypes = do
 
   resName <- declare' (ctxName <> "_" <> "ho" <> showt fPos <> "_"
                                <> showt fSubPos <> "_res") resTy
+  -- Pick the fSubPos-th HO instantiation (if present) and reuse its usage.
+  case IntMap.lookup fPos (bbFunctions bbCtx) >>= listToMaybe . drop fSubPos of
+    Just (_, usage, _, _, _, _) -> declareUseOnce usage resName
+    Nothing -> pure ()
   let res = ([Text (Id.toLazyText resName)], bbResTy)
 
   -- Render HO argument to plain text

tests/Main.hs

@@ -669,6 +669,7 @@ runClashTest = defaultMain
         , let _opts = def { hdlTargets = [VHDL], hdlLoad = [], hdlSim = []}
            in runTest "T3084" _opts
         , runTest "T3141" def{hdlSim=[], hdlLoad=[], clashFlags=["-itests/shouldwork/Issues/T3141", "-itests/shouldwork/Issues/T3141"]}
+        , outputTest "T3147" def{hdlTargets=[Verilog], hdlSim=[]}
         ] <>
         if compiledWith == Cabal then
           -- This tests fails without environment files present, which are only

tests/shouldwork/Issues/T3147.hs

@@ -0,0 +1,36 @@
+module T3147 where
+
+import Clash.Prelude
+import Data.List (isInfixOf)
+import System.Environment (getArgs)
+import System.FilePath ((</>))
+import qualified Prelude as P
+
+delayedCounter ::
+  forall dom d n.
+  (HiddenClockResetEnable dom, KnownNat d, KnownNat n) =>
+  SNat d ->
+  Signal dom (Unsigned n)
+delayedCounter d = last $ iterate (succSNat d) dflipflop cnt
+ where
+  cnt = register 0 (cnt + 1)
+
+topEntity :: Clock System -> Reset System -> Enable System -> Signal System (Unsigned 8)
+topEntity = exposeClockResetEnable $ delayedCounter d3
+
+assertIn :: String -> String -> IO ()
+assertIn needle haystack
+  | needle `isInfixOf` haystack = return ()
+  | otherwise =
+      P.error $ P.concat ["Expected:\n\n  ", needle, "\n\nIn:\n\n", haystack]
+
+mainVerilog :: IO ()
+mainVerilog = do
+  [topDir] <- getArgs
+  content <- readFile (topDir </> show 'topEntity </> "topEntity.v")
+  assertIn "reg [7:0] c$bb_res_res" content
+  assertIn "reg [7:0] c$bb_res_res_0" content
+  assertIn "reg [7:0] c$bb_res_res_1" content
+  assertIn "assign iterateI_ho1_0_res = c$bb_res_res3;" content
+  assertIn "assign iterateI_ho1_1_res = c$bb_res_res_0;" content
+  assertIn "assign iterateI_ho1_2_res = c$bb_res_res_1;" content