Always do the worker/wrapper split for NOINLINEs

Trac #10069 revealed that small NOINLINE functions didn't get split
into worker and wrapper. This was due to `certainlyWillInline`
saying that any unfoldings with a guidance of `UnfWhen` inline
unconditionally. That isn't the case for NOINLINE functions, so we
catch this case earlier now.

Nofib results:

--------------------------------------------------------------------------------
        Program         Allocs    Instrs
--------------------------------------------------------------------------------
 fannkuch-redux          -0.3%      0.0%
             gg          +0.0%     +0.1%
       maillist          -0.2%     -0.2%
        minimax           0.0%     -0.8%
--------------------------------------------------------------------------------
            Min          -0.3%     -0.8%
            Max          +0.0%     +0.1%
 Geometric Mean          -0.0%     -0.0%

Fixes #10069.

-------------------------
Metric Increase:
    T9233
-------------------------

2 files changed, 37 insertions, 7 deletions

diff --git a/compiler/coreSyn/CoreUnfold.hs b/compiler/coreSyn/CoreUnfold.hs
index 3ac35c9848..e55e12487b 100644
--- a/compiler/coreSyn/CoreUnfold.hs
+++ b/compiler/coreSyn/CoreUnfold.hs
@@ -1118,13 +1118,14 @@ smallEnoughToInline _ _
 ----------------
 
 certainlyWillInline :: DynFlags -> IdInfo -> Maybe Unfolding
--- Sees if the unfolding is pretty certain to inline
--- If so, return a *stable* unfolding for it, that will always inline
+-- ^ Sees if the unfolding is pretty certain to inline.
+-- If so, return a *stable* unfolding for it, that will always inline.
 certainlyWillInline dflags fn_info
   = case unfoldingInfo fn_info of
       CoreUnfolding { uf_tmpl = e, uf_guidance = g }
-        | loop_breaker -> Nothing       -- Won't inline, so try w/w
-        | otherwise    -> do_cunf e g   -- Depends on size, so look at that
+        | loop_breaker -> Nothing      -- Won't inline, so try w/w
+        | noinline     -> Nothing      -- See Note [Worker-wrapper for NOINLINE functions]
+        | otherwise    -> do_cunf e g  -- Depends on size, so look at that
 
       DFunUnfolding {} -> Just fn_unf  -- Don't w/w DFuns; it never makes sense
                                        -- to do so, and even if it is currently a
@@ -1134,6 +1135,7 @@ certainlyWillInline dflags fn_info
 
   where
     loop_breaker = isStrongLoopBreaker (occInfo fn_info)
+    noinline     = inlinePragmaSpec (inlinePragInfo fn_info) == NoInline
     fn_unf       = unfoldingInfo fn_info
 
     do_cunf :: CoreExpr -> UnfoldingGuidance -> Maybe Unfolding
@@ -1148,9 +1150,6 @@ certainlyWillInline dflags fn_info
         --    See Note [certainlyWillInline: INLINABLE]
     do_cunf expr (UnfIfGoodArgs { ug_size = size, ug_args = args })
       | not (null args)  -- See Note [certainlyWillInline: be careful of thunks]
-      , case inlinePragmaSpec (inlinePragInfo fn_info) of
-          NoInline -> False -- NOINLINE; do not say certainlyWillInline!
-          _        -> True  -- INLINE, INLINABLE, or nothing
       , not (isBottomingSig (strictnessInfo fn_info))
               -- Do not unconditionally inline a bottoming functions even if
               -- it seems smallish. We've carefully lifted it out to top level,
diff --git a/compiler/stranal/WorkWrap.hs b/compiler/stranal/WorkWrap.hs
index 34cfd64ecd..8f34b3b2ec 100644
--- a/compiler/stranal/WorkWrap.hs
+++ b/compiler/stranal/WorkWrap.hs
@@ -242,6 +242,37 @@ NOINLINE pragma to the worker.
 
 (See Trac #13143 for a real-world example.)
 
+It is crucial that we do this for *all* NOINLINE functions. Trac #10069
+demonstrates what happens when we promise to w/w a (NOINLINE) leaf function, but
+fail to deliver:
+
+  data C = C Int# Int#
+
+  {-# NOINLINE c1 #-}
+  c1 :: C -> Int#
+  c1 (C _ n) = n
+
+  {-# NOINLINE fc #-}
+  fc :: C -> Int#
+  fc c = 2 *# c1 c
+
+Failing to w/w `c1`, but still w/wing `fc` leads to the following code:
+
+  c1 :: C -> Int#
+  c1 (C _ n) = n
+
+  $wfc :: Int# -> Int#
+  $wfc n = let c = C 0# n in 2 #* c1 c
+
+  fc :: C -> Int#
+  fc (C _ n) = $wfc n
+
+Yikes! The reboxed `C` in `$wfc` can't cancel out, so we are in a bad place.
+This generalises to any function that derives its strictness signature from
+its callees, so we have to make sure that when a function announces particular
+strictness properties, we have to w/w them accordingly, even if it means
+splitting a NOINLINE function.
+
 Note [Worker activation]
 ~~~~~~~~~~~~~~~~~~~~~~~~
 Follows on from Note [Worker-wrapper for INLINABLE functions]