rts/CNF: Fix fixup comparison function

Previously we would implicitly convert the difference between two words
to an int, resulting in an integer overflow on 64-bit machines.

Fixes #16992

1 files changed, 22 insertions, 0 deletions

diff --git a/libraries/ghc-compact/tests/T16992.hs b/libraries/ghc-compact/tests/T16992.hs
new file mode 100644
index 0000000000..6505aa7b00
--- /dev/null
+++ b/libraries/ghc-compact/tests/T16992.hs
@@ -0,0 +1,22 @@
+import Data.Bifunctor
+import Foreign.Ptr
+import qualified Data.ByteString as BS
+import qualified Data.ByteString.Unsafe as BS
+import qualified GHC.Compact as Compact
+import qualified GHC.Compact.Serialized as CompactSerialize
+
+-- | Minimal test case for reproducing compactFixupPointers# bug for large compact regions.
+-- See Issue #16992.
+main :: IO ()
+main = do
+  let
+    large = 1024 * 1024 * 128
+    largeString = replicate large 'A'
+
+  region <- Compact.compact largeString
+
+  Just deserialized <- CompactSerialize.withSerializedCompact region $ \s -> do
+    blks <- mapM (BS.unsafePackCStringLen . bimap castPtr fromIntegral) (CompactSerialize.serializedCompactBlockList s)
+    CompactSerialize.importCompactByteStrings s blks
+
+  print (Compact.getCompact deserialized == largeString)