target/s390x: Fix determination of overflow condition code after subtraction

Reported by Paul Eggert in
https://lists.gnu.org/archive/html/bug-gnulib/2021-09/msg00050.html

This program currently prints different results when run with TCG instead
of running on real s390x hardware:

 #include <stdio.h>

 int overflow_32 (int x, int y)
 {
   int sum;
   return __builtin_sub_overflow (x, y, &sum);
 }

 int overflow_64 (long long x, long long y)
 {
   long sum;
   return __builtin_sub_overflow (x, y, &sum);
 }

 int a1 = 0;
 int b1 = -2147483648;
 long long a2 = 0L;
 long long b2 = -9223372036854775808L;

 int main ()
 {
   {
     int a = a1;
     int b = b1;
     printf ("a = 0x%x, b = 0x%x\n", a, b);
     printf ("no_overflow = %d\n", ! overflow_32 (a, b));
   }
   {
     long long a = a2;
     long long b = b2;
     printf ("a = 0x%llx, b = 0x%llx\n", a, b);
     printf ("no_overflow = %d\n", ! overflow_64 (a, b));
   }
 }

Signed-off-by: Bruno Haible <bruno@clisp.org>
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/618
Message-Id: <20220323162621.139313-3-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

diff --git a/target/s390x/tcg/cc_helper.c b/target/s390x/tcg/cc_helper.c
index e11cdb745dcf3242add1aeebf46020d673d02b3e..b2e8d3d9f5973611f1728a91b978791c393ab687 100644 (file)
--- a/target/s390x/tcg/cc_helper.c
+++ b/target/s390x/tcg/cc_helper.c
@@ -151,7 +151,7 @@ static uint32_t cc_calc_add_64(int64_t a1, int64_t a2, int64_t ar)
 
 static uint32_t cc_calc_sub_64(int64_t a1, int64_t a2, int64_t ar)
 {
-    if ((a1 > 0 && a2 < 0 && ar < 0) || (a1 < 0 && a2 > 0 && ar > 0)) {
+    if ((a1 >= 0 && a2 < 0 && ar < 0) || (a1 < 0 && a2 > 0 && ar > 0)) {
         return 3; /* overflow */
     } else {
         if (ar < 0) {
@@ -211,7 +211,7 @@ static uint32_t cc_calc_add_32(int32_t a1, int32_t a2, int32_t ar)
 
 static uint32_t cc_calc_sub_32(int32_t a1, int32_t a2, int32_t ar)
 {
-    if ((a1 > 0 && a2 < 0 && ar < 0) || (a1 < 0 && a2 > 0 && ar > 0)) {
+    if ((a1 >= 0 && a2 < 0 && ar < 0) || (a1 < 0 && a2 > 0 && ar > 0)) {
         return 3; /* overflow */
     } else {
         if (ar < 0) {