Fix discarding of unreachable code in the register allocator (#9155)

A previous fix to this was wrong: f5879acd018494b84233f26fba828ce376d0f81d
and left some unreachable code behind.  So rather than try to be clever and
do this at the same time as the strongly-connected-component analysis, I'm
doing a separate reachability pass first.

2 files changed, 31 insertions, 0 deletions

diff --git a/testsuite/tests/codeGen/should_compile/T9155.hs b/testsuite/tests/codeGen/should_compile/T9155.hs
new file mode 100644
index 0000000000..6fac0bcee6
--- /dev/null
+++ b/testsuite/tests/codeGen/should_compile/T9155.hs
@@ -0,0 +1,30 @@
+{-# LANGUAGE ScopedTypeVariables #-}
+module M () where
+
+import Data.Bits ((.&.))
+
+bitsSet :: Int -> Int -> Bool
+bitsSet mask i
+  = (i .&. mask == mask)
+
+class Eq b => BitMask b where
+  assocBitMask :: [(b,Int)]
+
+  fromBitMask  :: Int -> b
+  fromBitMask i
+    = walk assocBitMask
+    where
+      walk []         = error "Graphics.UI.WX.Types.fromBitMask: empty list"
+      walk [(x,0)]    = x
+      walk ((x,m):xs) | bitsSet m i = x
+                      | otherwise   = walk xs
+
+data Align   = AlignLeft
+             | AlignCentre
+             deriving Eq
+
+instance BitMask Align where
+  assocBitMask
+    = [(AlignCentre,512)
+      ,(AlignLeft,  256)
+      ]
diff --git a/testsuite/tests/codeGen/should_compile/all.T b/testsuite/tests/codeGen/should_compile/all.T
index 487b6b653c..ae8d0dd24a 100644
--- a/testsuite/tests/codeGen/should_compile/all.T
+++ b/testsuite/tests/codeGen/should_compile/all.T
@@ -22,3 +22,4 @@ test('massive_array',
 test('T7237', normal, compile, [''])
 test('T7574', [cmm_src, omit_ways(['llvm', 'optllvm'])], compile, [''])
 test('T8205', normal, compile, ['-O0'])
+test('T9155', normal, compile, ['-O2'])