codeGen: Optimize continuation argumentswip/T16098

This diff implements code generation for continuation arguments, as
proposed in #16098. For primops like `catch#`/`with#`/`mask`/... which
are defined in the runtime-system there is no mechanism for inlining
them. This often leads to otherwise unnecessary allocations of closures.

This patch introduces the notion of continuation arguments to code
generation, including:

 * A way to control CorePrep to not ANFize certain primops. That is,
   leaving arguments of the form `State# s -> (# State# s, a #)` in
   defined positions.
 * Teaching `CoreToStg` how to translate these to STG by extending STG
   language. Namely the `GenStgArg` type.
 * Inline primops and continuation in code generation.

This patch happily inlines `catch`#:

```
...
I64[Sp - 24] = PicBaseReg + stg_catch_frame_info;
I64[Sp - 16] = %MO_UU_Conv_W32_W64(I32[I64[BaseReg + 872] + 28]);
I64[Sp - 8] = PicBaseReg + (Test.someHandler_closure+2);
...
```

No...

 * ... call to runtime system
 * ... allocation
 * ... copying of free variables

needed

Currently this is implemented only for catch# primop. Once we agree to
merge this I will bring in the rest.

P.S. Also StgCse and Unarise are broken for continuation arguments. I
will fix that in the coming days.

[1] https://gitlab.haskell.org/ghc/ghc/blob/4898df1cc25132dc9e2599d4fa4e1bbc9423cda5/rts/Exception.cmm#L393

1 files changed, 11 insertions, 2 deletions

diff --git a/compiler/GHC/CoreToStg.hs b/compiler/GHC/CoreToStg.hs
index 83799f6e49..c61f378504 100644
--- a/compiler/GHC/CoreToStg.hs
+++ b/compiler/GHC/CoreToStg.hs
@@ -582,12 +582,21 @@ coreToStgArgs (arg : args) = do         -- Non-type argument
     (stg_args, ticks) <- coreToStgArgs args
     arg' <- coreToStgExpr arg
     let
+        arg_ty = exprType arg
         (aticks, arg'') = stripStgTicksTop tickishFloatable arg'
         stg_arg = case arg'' of
                        StgApp v []        -> StgVarArg v
                        StgConApp con [] _ -> StgVarArg (dataConWorkId con)
                        StgLit lit         -> StgLitArg lit
-                       _                  -> pprPanic "coreToStgArgs" (ppr arg)
+                       StgLam bndrs body  ->
+                         let
+                           bndr = case toList bndrs of
+                             [x] -> x
+                             xs  ->
+                               -- TODO: more informative error message
+                               pprPanic "coreToStgArgs" (ppr arg'')
+                         in StgContArg bndr body arg_ty
+                       _  -> pprPanic "coreToStgArgs" (ppr arg'')
 
         -- WARNING: what if we have an argument like (v `cast` co)
         --          where 'co' changes the representation type?
@@ -601,7 +610,7 @@ coreToStgArgs (arg : args) = do         -- Non-type argument
 
     dflags <- getDynFlags
     let
-        arg_rep = typePrimRep (exprType arg)
+        arg_rep = typePrimRep arg_ty
         stg_arg_rep = typePrimRep (stgArgType stg_arg)
         bad_args = not (primRepsCompatible dflags arg_rep stg_arg_rep)