Fix erroneous float in CoreOpt

The simple optimiser was making an invalid transformation
to join points -- yikes.  The fix is easy.

I also added some documentation about the fact that GHC uses
a slightly more restrictive version of join points than does
the paper.

Fix #16918

3 files changed, 34 insertions, 0 deletions

diff --git a/testsuite/tests/simplCore/should_compile/T16918.hs b/testsuite/tests/simplCore/should_compile/T16918.hs
new file mode 100644
index 0000000000..87113b4d96
--- /dev/null
+++ b/testsuite/tests/simplCore/should_compile/T16918.hs
@@ -0,0 +1,7 @@
+module Bug where
+
+pokeArray :: () -> ()
+pokeArray = pokeArray
+
+pokeSockAddr :: String -> () -> ()
+pokeSockAddr path p = (case path of ('\0':_) -> pokeArray) p
diff --git a/testsuite/tests/simplCore/should_compile/T16918a.hs b/testsuite/tests/simplCore/should_compile/T16918a.hs
new file mode 100644
index 0000000000..8b676f83c4
--- /dev/null
+++ b/testsuite/tests/simplCore/should_compile/T16918a.hs
@@ -0,0 +1,25 @@
+{-# LANGUAGE ForeignFunctionInterface #-}
+module Bug where
+
+import Data.Word
+import Foreign
+import Foreign.C.String
+import Foreign.C.Types
+
+type CSaFamily = (Word16)
+data SockAddr = SockAddrUnix String
+
+pokeSockAddr :: Ptr a -> SockAddr -> IO ()
+pokeSockAddr p (SockAddrUnix path) = do
+    case path of
+      ('\0':_) -> zeroMemory p (110)
+      _        -> return ()
+    ((\hsc_ptr -> pokeByteOff hsc_ptr 0)) p ((1) :: CSaFamily)
+    let pathC = map castCharToCChar path
+        poker = case path of ('\0':_) -> pokeArray; _ -> pokeArray0 0
+    poker (((\hsc_ptr -> hsc_ptr `plusPtr` 2)) p) pathC
+
+foreign import ccall unsafe "string.h" memset :: Ptr a -> CInt -> CSize -> IO ()
+
+zeroMemory :: Ptr a -> CSize -> IO ()
+zeroMemory dest nbytes = memset dest 0 (fromIntegral nbytes)
diff --git a/testsuite/tests/simplCore/should_compile/all.T b/testsuite/tests/simplCore/should_compile/all.T
index 2fbe84a49e..768012d451 100644
--- a/testsuite/tests/simplCore/should_compile/all.T
+++ b/testsuite/tests/simplCore/should_compile/all.T
@@ -303,3 +303,5 @@ test('T15631',
 test('T15673', normal, compile, ['-O'])
 test('T16288', normal, multimod_compile, ['T16288B', '-O -dcore-lint -v0'])
 test('T16348', normal, compile, ['-O'])
+test('T16918', normal, compile, ['-O'])
+test('T16918a', normal, compile, ['-O'])