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

[Thomas Huth]

On 30/03/2022 11.34, David Hildenbrand wrote:
> On 30.03.22 11:29, Thomas Huth wrote:
> > On 30/03/2022 10.52, David Hildenbrand wrote:
> > > 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"
> >
> > I think it doesn't really matter whether we compare ar with a1 or 0 here. If
> > an overflow happens, what's the biggest number that we can get? AFAICT it's
> > with a1 = 0x7fffffffffffffff and a2 = 0x7fffffffffffffff. You then get:
> >
> >    0x7fffffffffffffff + 0x7fffffffffffffff = 0xFFFFFFFFFFFFFFFE
> >
> > and that's still < 0 if treated as a signed value. I don't see a way where
> > ar could be in the range between 0 and a1.
> >
> > (OTOH, checking for ar < a1 instead of ar < 0 wouldn't hurt either, I guess).
> >
> > > 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"
> >
> > What about if the uppermost bit gets lost in 64-bit mode:
> >
> >    0x8000000000000000 + 0x8000000000000000 = 0x0000000000000000
> >
> > ar > a1 does not work here anymore, does it?
>
>
> 0 > -9223372036854775808, no?

current coffe level < correct coffee level

... sorry, never mind, you're right of course.

Anyway, 0 is the lowest number we can get for an underflow, so comparing 
with >= 0 should be fine (but comparing with a1 wouldn't hurt either).

 Thomas