[llvm-dev] returning from LowerOperation()

[Friedman, Eli via llvm-dev]

On 1/23/2017 5:21 AM, Jonas Paulsson wrote:
> Hi Eli,
>
> I would like to clarify generally what the difference is between 
> returning SDValue() and Op (input argument unchanged) from 
> LowerOperation()?
>
> My understanding is that returning SDValue() means that Target gives 
> up, and the common code is supposed to handle it. Returning Op, the 
> unchanged argument, means that the Target is happy with the node as it 
> is, and the common code can move on to something else.

This is right.

> Are there any exceptions to this? Is this commented on anywhere? Are 
> there cases where the target should not return SDValue()?

No exceptions to this.  Not sure if it's described anywhere off the top 
of my head; if there isn't a comment describing this on the declaration 
of LowerOperation or LowerOperationWrapper, it would probably be good to 
add one.  Returning SDValue() should be "fine" in all cases (it might 
lead to a fatal error if we can't actually lower the operation).

-Eli

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