codeGen: Fix two buglets in -fbounds-check logic

@Bodigrim noticed that the `compareByteArray#` bounds-checking logic had
flipped arguments and an off-by-one. For the sake of clarity I also
refactored occurrences of `cmmOffset` to rather use `cmmOffsetB`. I
suspect the former should be retired.

2 files changed, 20 insertions, 0 deletions

diff --git a/testsuite/tests/codeGen/should_fail/CheckBoundsCompareByteArray.hs b/testsuite/tests/codeGen/should_fail/CheckBoundsCompareByteArray.hs
new file mode 100644
index 0000000000..e20dcc1647
--- /dev/null
+++ b/testsuite/tests/codeGen/should_fail/CheckBoundsCompareByteArray.hs
@@ -0,0 +1,19 @@
+{-# LANGUAGE NondecreasingIndentation  #-}
+{-# LANGUAGE UnboxedTuples #-}
+{-# LANGUAGE MagicHash #-}
+
+module Main where
+
+import GHC.Exts
+import GHC.IO
+
+main :: IO ()
+main = do
+    IO $ \s0 ->
+      case newByteArray# 4# s0 of
+        (# s1, a_marr #) -> case newByteArray# 4# s1 of
+          (# s2, b_marr #) -> case unsafeFreezeByteArray# a_marr s2 of
+            (# s3, a_arr #) -> case unsafeFreezeByteArray# b_marr s2 of
+              (# s4, b_arr #) -> case compareByteArrays# a_arr 0# b_arr 1# 4# of
+                0# -> (# s4, () #)
+
diff --git a/testsuite/tests/codeGen/should_fail/all.T b/testsuite/tests/codeGen/should_fail/all.T
index 01a4802d09..e23fa03f82 100644
--- a/testsuite/tests/codeGen/should_fail/all.T
+++ b/testsuite/tests/codeGen/should_fail/all.T
@@ -15,4 +15,5 @@ check_bounds_test('CheckBoundsIndexArray')
 check_bounds_test('CheckBoundsReadInt8Array')
 check_bounds_test('CheckBoundsReadWord8ArrayAsInt32')
 check_bounds_test('CheckBoundsCopyByteArray')
+check_bounds_test('CheckBoundsCompareByteArray')