[llvm-dev] Question about ISD::SUBCARRY

[Friedman, Eli via llvm-dev]

I would tend to lean towards making SUBCARRY return the same value on 
all targets, assuming it doesn't complicate the target-specific code 
much.  We want to make the target-independent code more straightforward 
if we can, and it probably makes the code easier to understand if 
SUBCARRY and USUBO are consistent.

If it somehow ends up being too tricky to lower SUBCARRY on ARM, it 
probably makes more sense to add new opcodes, rather than make the 
semantics of the existing opcodes depend on a target flag. (The end 
result is essentially equivalent, but it's harder to accidentally 
mistake which kind of node you're dealing with.)

-Eli

On 6/28/2017 9:58 AM, Roger Ferrer Ibanez via llvm-dev wrote:
>
> Hi James,
>
> thanks for your answer.
>
> Your suggestion looks sensible to me, then I understand that a 
> combiner like the following one (DAGCombiner::visitSUBCARRY)
>
> http://www.llvm.org/doxygen/DAGCombiner_8cpp_source.html#l02498
>
> that assumes that “subcarry x, y, 0” can be simplified as “usubo x, y” 
> should be parameterized using TargetLowering, as this assumes that the 
> third input of subcarry is a borrow (e.g. x86) rather than a “carry” 
> (e.g. ARM).
>
> Does this make sense? Maybe I’m mixing things here.
>
> ( For context: I’m trying to establish if I can avoid adding ARM 
> specific combiners in https://reviews.llvm.org/D34515 and use the 
> generic ones instead. )
>
> Kind regards,
>
> Roger
>
> *From:*James Y Knight [mailto:jyknight at google.com]
> *Sent:* 27 June 2017 17:00
> *To:* Roger Ferrer Ibanez
> *Cc:* llvm-dev; nd
> *Subject:* Re: [llvm-dev] Question about ISD::SUBCARRY
>
> That's an excellent question. :)
>
> I'd say it should _probably_ be defined as having the same boolean 
> value as the hardware's carry-flag, and there should be a 
> TargetLowering function which describes the semantics upon subtract.
>
> On Tue, Jun 27, 2017 at 4:53 AM, Roger Ferrer Ibanez via llvm-dev 
> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>
> Dear all,
>
> a couple of new generic DAG nodes ISD::ADCARRY and ISD::SUBCARRY were 
> recently introduced in https://reviews.llvm.org/D29872
>
> These nodes have three inputs and two outputs, the second output being 
> the "carry". I understand that carry is well defined for ADDCARRY but 
> my question is about SUBCARRY.
>
> Some architectures set the "carry" of a "x - y" subtraction is set 
> when x < y (e.g x86, "borrow") and some others set it when x >= y 
> (e.g. ARM). Does the ISD::SUBCARRY picks one interpretation (and uses 
> it for combiners on top of this node) or leaves the interpretation to 
> the target.
>
> Maybe my whole question does not make sense and even if only one 
> interpretation is chosen this does not impact the target?
>
> Thank you very much,
> Roger
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