Re: [PATCH v2 3/5] target/s390x: Fix cc_calc_sla_64() missing overflows

[David Hildenbrand]

>> If the sign is false, the shifted bits (mask) have to be 0.
>> If the sign bit is true, the shifted bits (mask) have to be set.
> 
> IIUC this logic handles sign bit + "shift - 1" bits. So if the last
> shifted bit is different, the overflow is not detected.

Ah, right, because of the - 1ULL ...

[...]

>> This looks like some black magic :)
> 
> Yeah, I felt this way too, but didn't come up with anything better and
> just left a comment warning not to simplify.
> 

I wonder if all we want is

const uint64_t sign = 1ULL << 63;
uint64_t mask = (-1ULL << (63 - shift)) & ~sign;

For shift =
*  0: 0000000...0b
*  1: 0100000...0b
*  2: 0110000...0b
* 63: 0111111...1b

Seems to survive your tests.

-- 
Thanks,

David / dhildenb