The documentation says:

  SUBV Rm, Rn        Rn - Rm -> Rn, underflow -> T

The overflow / underflow can be calculated as:

  T = ((Rn ^ Rm) & (Result ^ Rn)) >> 31

However we were using the incorrect:

  T = ((Rn ^ Rm) & (Result ^ Rm)) >> 31

Fix by using the Rn register instead of Rm.

Cc: qemu-stable@nongnu.org
Fixes: ad8d25a11f ("target-sh4: implement addv and subv using TCG")
Reported-by: Paul Cercueil <paul@crapouillou.net>
Suggested-by: Paul Cercueil <paul@crapouillou.net>
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2318
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
---
 target/sh4/translate.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/target/sh4/translate.c b/target/sh4/translate.c
index 4a1dd0d1f4..3e013b7c7c 100644
--- a/target/sh4/translate.c
+++ b/target/sh4/translate.c
@@ -933,7 +933,7 @@ static void _decode_opc(DisasContext * ctx)
             t0 = tcg_temp_new();
             tcg_gen_sub_i32(t0, REG(B11_8), REG(B7_4));
             t1 = tcg_temp_new();
-            tcg_gen_xor_i32(t1, t0, REG(B7_4));
+            tcg_gen_xor_i32(t1, t0, REG(B11_8));
             t2 = tcg_temp_new();
             tcg_gen_xor_i32(t2, REG(B11_8), REG(B7_4));
             tcg_gen_and_i32(t1, t1, t2);
-- 
2.41.0