[llvm-dev] x.with.overflow semantics question

[Sanjoy Das via llvm-dev]

John Regehr wrote:
> Ok, thanks folks. Sounds like it's clear that x.with.overflow performs
> two's complement operations. Should I patch the LangRef to be more
> explicit about this?
>

SGTM


> John
>
>
> On 5/9/16 8:14 AM, Sanjoy Das wrote:
>> CGP also relies on the add being a simple two's complement add, since it
>> will transform
>>
>> define void @test1(i64 %a, i64 %b, i64* %res_i64, i1* %res_i1) {
>> entry:
>> %add = add i64 %b, %a
>> %cmp = icmp ult i64 %add, %a
>> store i1 %cmp, i1* %res_i1
>> store i64 %add, i64* %res_i64
>> ret void
>> }
>>
>> to
>>
>> define void @test1(i64 %a, i64 %b, i64* %res_i64, i1* %res_i1) {
>> entry:
>> %uadd.overflow = call { i64, i1 } @llvm.uadd.with.overflow.i64(i64 %b,
>> i64 %a)
>> %uadd = extractvalue { i64, i1 } %uadd.overflow, 0
>> %overflow = extractvalue { i64, i1 } %uadd.overflow, 1
>> store i1 %overflow, i1* %res_i1
>> store i64 %uadd, i64* %res_i64
>> ret void
>> }
>>
>>
>> Now if we _know_ that the arithmetic result is used only if it does not
>> overflow, then we can "pretend" that the arithmetic was nsw/nuw. This
>> is what I tried to do in http://reviews.llvm.org/rL265912, but I had to
>> back out the change due to an unrelated issue with SCEV.
>>
>> -- Sanjoy
>>
>> Nuno Lopes via llvm-dev wrote:
>>>>> Or do you mean that the result of an add may not even be defined? In
>>>>> that case would reading it be considered UB in
>>>>> the case where the overflow bit was set?
>>>>
>>>> Yeah, this is the case I'm worried about: that for example
>>>> sadd.with.overflow(INT_MAX, 1) might be designed to return
>>>> { poison, true } instead of giving a useful result in the first
>>>> element of the struct.
>>>
>>> Any argument against that? I guess that would be the most natural
>>> definition given the motivation to have these
>>> intrinsics (e.g., check if the 'nmemb * size' operation of calloc
>>> overflows; if so return null).
>>>
>>> InstCombine will remove these instrinsics if the overflow bit is
>>> unused, for example. AFAICT, InstCombine never adds
>>> nsw/nuw to the replacements. I didn't check GVN. But I think they
>>> should unless there's some reason not to.
>>>
>>> Nuno
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>>