Re: [PATCH 1/2] target/s390x: Fix determination of overflow condition code after addition

[David Hildenbrand]

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 <bruno@clisp.org>
> Signed-off-by: Thomas Huth <thuth@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