ghc-heap: Introduce closureSize

This function allows the user to compute the (non-transitive) size of a
heap object in words. The "closure" in the name is admittedly confusing
but we are stuck with this nomenclature at this point.

5 files changed, 59 insertions, 0 deletions

diff --git a/libraries/ghc-heap/GHC/Exts/Heap/Closures.hs b/libraries/ghc-heap/GHC/Exts/Heap/Closures.hs
index e624a17b78..2465014e48 100644
--- a/libraries/ghc-heap/GHC/Exts/Heap/Closures.hs
+++ b/libraries/ghc-heap/GHC/Exts/Heap/Closures.hs
@@ -13,6 +13,12 @@ module GHC.Exts.Heap.Closures (
     , GenClosure(..)
     , PrimType(..)
     , allClosures
+#if __GLASGOW_HASKELL__ >= 809
+    -- The closureSize# primop is unsupported on earlier GHC releases but we
+    -- build ghc-heap as a boot library so it must be buildable. Drop this once
+    -- we are guaranteed to bootstsrap with GHC >= 8.9.
+    , closureSize
+#endif
 
     -- * Boxes
     , Box(..)
@@ -321,3 +327,11 @@ allClosures (FunClosure {..}) = ptrArgs
 allClosures (BlockingQueueClosure {..}) = [link, blackHole, owner, queue]
 allClosures (OtherClosure {..}) = hvalues
 allClosures _ = []
+
+#if __GLASGOW_HASKELL__ >= 809
+-- | Get the size of a closure in words.
+--
+-- @since 8.10.1
+closureSize :: Box -> Int
+closureSize (Box x) = I# (closureSize# x)
+#endif
diff --git a/libraries/ghc-heap/tests/all.T b/libraries/ghc-heap/tests/all.T
index a676b4971a..595bd000d4 100644
--- a/libraries/ghc-heap/tests/all.T
+++ b/libraries/ghc-heap/tests/all.T
@@ -6,3 +6,6 @@ test('heap_all',
       omit_ways(['ghci', 'hpc'])
      ],
      compile_and_run, [''])
+test('closure_size',
+     omit_ways(['ghci', 'hpc', 'prof']),
+     compile_and_run, [''])
diff --git a/libraries/ghc-heap/tests/closure_size.hs b/libraries/ghc-heap/tests/closure_size.hs
new file mode 100644
index 0000000000..d77060779e
--- /dev/null
+++ b/libraries/ghc-heap/tests/closure_size.hs
@@ -0,0 +1,34 @@
+{-# LANGUAGE BangPatterns #-}
+{-# LANGUAGE TypeApplications #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+
+import Control.Monad
+import Type.Reflection
+import GHC.Stack
+
+import GHC.Exts.Heap.Closures
+
+assertSize :: forall a. (HasCallStack, Typeable a)
+           => a -> Int -> IO ()
+assertSize !x expected = do
+  let !size = closureSize (asBox x)
+  when (size /= expected) $ do
+    putStrLn $ "closureSize ("++show (typeRep @a)++") == "++show size++", expected "++show expected
+    putStrLn $ prettyCallStack callStack
+{-# NOINLINE assertSize #-}
+
+pap :: Int -> Char -> Int
+pap x _ = x
+{-# NOINLINE pap #-}
+
+main :: IO ()
+main = do
+  assertSize 'a' 2
+  assertSize (Just ()) 2
+  assertSize (Nothing :: Maybe ()) 2
+  assertSize ((1,2) :: (Int,Int)) 3
+  assertSize ((1,2,3) :: (Int,Int,Int)) 4
+  assertSize (id :: Int -> Int) 1
+  assertSize (fst :: (Int,Int) -> Int) 1
+  assertSize (pap 1) 2
+
diff --git a/libraries/ghc-heap/tests/closure_size.stdout b/libraries/ghc-heap/tests/closure_size.stdout
new file mode 100644
index 0000000000..e69de29bb2
--- /dev/null
+++ b/libraries/ghc-heap/tests/closure_size.stdout
diff --git a/libraries/ghc-prim/changelog.md b/libraries/ghc-prim/changelog.md
index ecbc93fea4..2298846cd1 100644
--- a/libraries/ghc-prim/changelog.md
+++ b/libraries/ghc-prim/changelog.md
@@ -1,3 +1,11 @@
+## 0.6.1
+
+- Shipped with GHC 8.10.1
+
+- Added to `GHC.Prim`:
+
+        closureSize# :: a -> Int#
+
 ## 0.6.0
 
 - Shipped with GHC 8.8.1