On 23.03.22 17:26, Thomas Huth wrote:
> 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_add_overflow (x, y, &sum);
> }
>
> int overflow_64 (long long x, long long y)
> {
> long sum;
> return ! __builtin_add_overflow (x, y, &sum);
> }
>
> int a1 = -2147483648;
> int b1 = -2147483648;
> long long a2 = -9223372036854775808L;
> 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));
> }
> }
>
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/616
> Suggested-by: Bruno Haible <br...@clisp.org>
> Signed-off-by: Thomas Huth <th...@redhat.com>
> ---
> target/s390x/tcg/cc_helper.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/target/s390x/tcg/cc_helper.c b/target/s390x/tcg/cc_helper.c
> index 8d04097f78..e11cdb745d 100644
> --- a/target/s390x/tcg/cc_helper.c
> +++ b/target/s390x/tcg/cc_helper.c
> @@ -136,7 +136,7 @@ static uint32_t cc_calc_subu(uint64_t borrow_out,
> uint64_t result)
>
> static uint32_t cc_calc_add_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)) {
Intuitively, I'd have checked for any overflow/underflow by comparing
with one of the input variables:
a) Both numbers are positive
Adding to positive numbers has to result in something that's bigger than
the input parameters.
"a1 > 0 && a2 > 0 && ar < a1"
b) Both numbers are negative
Adding to negative numbers has to result in something that's smaller
than the input parameters.
"a1 < 0 && a2 < 0 && ar > a1"
--
Thanks,
David / dhildenb
