Fix #19931

The issue was the renderer for x86 addressing modes assumes native size
registers, but we were passing in a possibly-smaller index in
conjunction with a native-sized base pointer.

The easist thing to do is just extend the register first.

I also changed the other NGC backends implementing jump tables
accordingly. On one hand, I think PowerPC and Sparc don't have the small
sub-registers anyways so there is less to worry about. On the other
hand, to the extent that's true the zero extension can become a no-op.

I should give credit where it's due: @hsyl20 really did all the work for
me in
https://gitlab.haskell.org/ghc/ghc/-/merge_requests/4717#note_355874,
but I was daft and missed the "Oops" and so ended up spending a silly
amount of time putting it all back together myself.

The unregisterised backend change is a bit different, because here we
are translating the actual case not a jump table, and the fix is to
handle right-sized literals not addressing modes. But it makes sense to
include here too because it's the same change in the subsequent commit
that exposes both bugs.

3 files changed, 37 insertions, 0 deletions

diff --git a/testsuite/tests/numeric/should_run/T19931.hs b/testsuite/tests/numeric/should_run/T19931.hs
new file mode 100644
index 0000000000..1ab9002d2e
--- /dev/null
+++ b/testsuite/tests/numeric/should_run/T19931.hs
@@ -0,0 +1,35 @@
+{-# LANGUAGE MagicHash #-}
+{-# LANGUAGE UnboxedTuples #-}
+
+module Main where
+
+import GHC.Exts
+import GHC.Types
+import GHC.Num
+
+data T = T Word8#
+
+instance Num T where
+  fromInteger i = T (wordToWord8# (integerToWord# i))
+
+instance Eq T where
+  (==) (T a) (T b) = isTrue# (a `eqWord8#` b)
+  (/=) (T a) (T b) = isTrue# (a `neWord8#` b)
+
+main :: IO ()
+main = do
+  let !addr = "\0\1\2\3\4\5\6\7\8"#
+
+  w8 <- IO (\s -> case readWord8OffAddr# (plusAddr# addr 5#) 0# s of
+                      (# s', w8 #) -> (# s', T w8 #))
+  -- w8 must be small enough for one of the branch to be taken.
+  -- we need several alternatives for a jump table to be used
+  print $ case w8 of
+      0 -> 1000
+      1 -> 1001
+      2 -> 1002
+      3 -> 1003
+      4 -> 1004
+      5 -> 1005
+      6 -> 1006
+      _ -> 1010
diff --git a/testsuite/tests/numeric/should_run/T19931.stdout b/testsuite/tests/numeric/should_run/T19931.stdout
new file mode 100644
index 0000000000..49bc2728c7
--- /dev/null
+++ b/testsuite/tests/numeric/should_run/T19931.stdout
@@ -0,0 +1 @@
+1005
diff --git a/testsuite/tests/numeric/should_run/all.T b/testsuite/tests/numeric/should_run/all.T
index 028677e52d..1123984bdd 100644
--- a/testsuite/tests/numeric/should_run/all.T
+++ b/testsuite/tests/numeric/should_run/all.T
@@ -74,5 +74,6 @@ test('T18499', normal, compile_and_run, [''])
 test('T18509', normal, compile_and_run, [''])
 test('T18515', normal, compile_and_run, [''])
 test('T18604', normal, compile_and_run, [''])
+test('T19931', normal, compile_and_run, ['-O2'])
 
 test('IntegerToFloat', normal, compile_and_run, [''])