target/sh4: Fix SUBV opcode

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.

Add tests provided by Paul Cercueil.

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
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Yoshinori Sato <ysato@users.sourceforge.jp>
Message-Id: <20240430163125.77430-3-philmd@linaro.org>

2 files changed, 33 insertions, 0 deletions

diff --git a/tests/tcg/sh4/Makefile.target b/tests/tcg/sh4/Makefile.target
index 521b8b0..7852fa6 100644
--- a/tests/tcg/sh4/Makefile.target
+++ b/tests/tcg/sh4/Makefile.target
@@ -20,3 +20,6 @@ TESTS += test-macw
 
 test-addv: CFLAGS += -O -g
 TESTS += test-addv
+
+test-subv: CFLAGS += -O -g
+TESTS += test-subv
diff --git a/tests/tcg/sh4/test-subv.c b/tests/tcg/sh4/test-subv.c
new file mode 100644
index 0000000..a3c2db9
--- /dev/null
+++ b/tests/tcg/sh4/test-subv.c
@@ -0,0 +1,30 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+
+#include <limits.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+static void subv(const int a, const int b, const int res, const int carry)
+{
+    int o = a, c;
+
+    asm volatile("subv %2,%0\n"
+                 "movt %1\n"
+                 : "+r"(o), "=r"(c) : "r"(b) : );
+
+    if (c != carry || o != res) {
+        printf("SUBV %d, %d = %d/%d [T = %d/%d]\n", a, b, o, res, c, carry);
+        abort();
+    }
+}
+
+int main(void)
+{
+    subv(INT_MIN, 1, INT_MAX, 1);
+    subv(INT_MAX, -1, INT_MIN, 1);
+    subv(INT_MAX, 1, INT_MAX - 1, 0);
+    subv(0, 1, -1, 0);
+    subv(-1, -1, 0, 0);
+
+    return 0;
+}