[llvm-dev] multiprecision add/sub

[Nemanja Ivanovic via llvm-dev]

Sorry for the delay in responding.
The use case is basically for being able to emit instructions that
correspond to these nodes. For example on Power, we use these instructions
that set and use the Carry bit for comparisons without using compare
instructions.
One example where we would be able to make use of these would be in
https://reviews.llvm.org/D28637.

On Tue, Mar 7, 2017 at 7:56 AM, Mehdi Amini <mehdi.amini at apple.com> wrote:

>
> On Feb 21, 2017, at 9:54 PM, Nemanja Ivanovic via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> I believe that providing additional intrinsics that would directly produce
> the ISD::ADDC/ISD::SUBC nodes would provide the additional advantage of
> being able to directly produce these nodes for code that doesn't have
> anything to do with multiprecision addition/subtraction. I am not aware of
> any way currently available to produce IR that will generate these nodes
> directly.
>
>
> But what is the use case? What IR pattern it would replace?
>
> —
> Mehdi
>
>
>
>
>
> On Fri, Feb 17, 2017 at 8:52 PM, Bagel via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> On 02/16/2017 12:08 PM, Stephen Canon wrote:
>> >> On Feb 16, 2017, at 9:12 AM, Bagel <bagel99 at gmail.com
>> >> <mailto:bagel99 at gmail.com>> wrote:
>> >>
>> >> I figured that the optimization of this would bedifficult (else it
>> would
>> >> have already been done :-))
>> >
>> > Don’t make this assumption. There’s lots of opportunities for
>> optimization
>> > scattered around. Some of them are left because they’re genuinely
>> difficult,
>> > but most either simply haven’t been noticed by anyone, or are known but
>> simply
>> > haven’t been done because no one has pushed for them (I’m fairly sure
>> that this
>> > particular optimization falls into that category—folks have known the
>> codegen
>> > wasn't great since these intrinsics were added, but no one has really
>> > complained about it, so people have worked on higher-impact stuff).
>>
>> But I am still curious as to whether the optimization is architecture
>> independent or must be done again for each backend?
>>
>> bagel
>>
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>
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