[llvm-dev] Question about ISD::SUBCARRY

[Roger Ferrer Ibanez via llvm-dev]

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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