Rubbish literals for all representations (#18983)

This patch cleans up the complexity around WW's `mk_absent_let` by
broadening the scope of `LitRubbish`. Rubbish literals now store the
`PrimRep` they represent and are ultimately lowered in Cmm.
This in turn allows absent literals of `VecRep` or `VoidRep`. The latter
allows absent literals for unlifted coercions, as requested in #18983.

I took the liberty to rewrite and clean up `Note [Absent fillers]` and
`Note [Rubbish values]` to account for the new implementation and to
make them more orthogonal in their description.

I didn't add a new regression test, as `T18982` already contains the
test in the ticket and its test output changes as expected.

Fixes #18983.

1 files changed, 21 insertions, 1 deletions

diff --git a/compiler/GHC/StgToCmm/Layout.hs b/compiler/GHC/StgToCmm/Layout.hs
index e45955d119..d10d7f6345 100644
--- a/compiler/GHC/StgToCmm/Layout.hs
+++ b/compiler/GHC/StgToCmm/Layout.hs
@@ -26,7 +26,8 @@ module GHC.StgToCmm.Layout (
         mkVirtConstrSizes,
         getHpRelOffset,
 
-        ArgRep(..), toArgRep, argRepSizeW -- re-exported from GHC.StgToCmm.ArgRep
+        ArgRep(..), toArgRep, argRepSizeW, -- re-exported from GHC.StgToCmm.ArgRep
+        getArgAmode, getNonVoidArgAmodes
   ) where
 
 
@@ -42,6 +43,7 @@ import GHC.StgToCmm.Env
 import GHC.StgToCmm.ArgRep -- notably: ( slowCallPattern )
 import GHC.StgToCmm.Ticky
 import GHC.StgToCmm.Monad
+import GHC.StgToCmm.Lit
 import GHC.StgToCmm.Utils
 
 import GHC.Cmm.Graph
@@ -591,6 +593,24 @@ stdPattern reps
         _ -> Nothing
 
 -------------------------------------------------------------------------
+--        Amodes for arguments
+-------------------------------------------------------------------------
+
+getArgAmode :: NonVoid StgArg -> FCode CmmExpr
+getArgAmode (NonVoid (StgVarArg var)) = idInfoToAmode <$> getCgIdInfo var
+getArgAmode (NonVoid (StgLitArg lit)) = cgLit lit
+
+getNonVoidArgAmodes :: [StgArg] -> FCode [CmmExpr]
+-- NB: Filters out void args,
+--     so the result list may be shorter than the argument list
+getNonVoidArgAmodes [] = return []
+getNonVoidArgAmodes (arg:args)
+  | isVoidRep (argPrimRep arg) = getNonVoidArgAmodes args
+  | otherwise = do { amode  <- getArgAmode (NonVoid arg)
+                   ; amodes <- getNonVoidArgAmodes args
+                   ; return ( amode : amodes ) }
+
+-------------------------------------------------------------------------
 --
 --        Generating the info table and code for a closure
 --