From: David Hildenbrand <david@redhat.com>
To: Ilya Leoshkevich <iii@linux.ibm.com>,
Richard Henderson <richard.henderson@linaro.org>,
Cornelia Huck <cohuck@redhat.com>, Thomas Huth <thuth@redhat.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>,
qemu-s390x@nongnu.org, qemu-devel@nongnu.org
Subject: Re: [PATCH v2 3/5] target/s390x: Fix cc_calc_sla_64() missing overflows
Date: Wed, 12 Jan 2022 16:45:11 +0100 [thread overview]
Message-ID: <c9544f5c-545a-8edf-dac6-a960aa00d4dd@redhat.com> (raw)
In-Reply-To: <add9d2da455bf9b07cf77c942e95183f5bf6f090.camel@linux.ibm.com>
>> 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
next prev parent reply other threads:[~2022-01-12 16:17 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-01-12 4:39 [PATCH v2 0/5] target/s390x: Fix shift instructions Ilya Leoshkevich
2022-01-12 4:39 ` [PATCH v2 1/5] target/s390x: Fix SLDA sign bit index Ilya Leoshkevich
2022-01-12 8:33 ` David Hildenbrand
2022-01-12 4:39 ` [PATCH v2 2/5] target/s390x: Fix SRDA CC calculation Ilya Leoshkevich
2022-01-12 8:46 ` David Hildenbrand
2022-01-12 4:39 ` [PATCH v2 3/5] target/s390x: Fix cc_calc_sla_64() missing overflows Ilya Leoshkevich
2022-01-12 8:59 ` David Hildenbrand
2022-01-12 12:51 ` Ilya Leoshkevich
2022-01-12 15:45 ` David Hildenbrand [this message]
2022-01-12 16:38 ` Ilya Leoshkevich
2022-01-12 4:39 ` [PATCH v2 4/5] target/s390x: Fix shifting 32-bit values for more than 31 bits Ilya Leoshkevich
2022-01-12 8:43 ` David Hildenbrand
2022-01-12 4:39 ` [PATCH v2 5/5] tests/tcg/s390x: Test shift instructions Ilya Leoshkevich
2022-01-12 9:07 ` David Hildenbrand
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