[llvm-dev] returning from LowerOperation()

[Hal Finkel via llvm-dev]

On 01/23/2017 02:41 PM, Matt Arsenault via llvm-dev wrote:
>
>> On Jan 23, 2017, at 12:36, Friedman, Eli via llvm-dev 
>> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>
>> 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.
>
> This sounds backwards. Returning SDValue() means the node should be 
> treated as legal. Returning the original operation should hit the 
> expand path.

No, I think that was correct. The code in 
lib/CodeGen/SelectionDAG/LegalizeDAG.cpp reads:

     switch (Action) {
     case TargetLowering::Legal:
       return;
     case TargetLowering::Custom: {
       // FIXME: The handling for custom lowering with multiple results is
       // a complete mess.
       if (SDValue Res = TLI.LowerOperation(SDValue(Node, 0), DAG)) {
         if (!(Res.getNode() != Node || Res.getResNo() != 0))
           return;

         ...

         ReplaceNode(Node, ResultVals.data());
         return;
       }
       LLVM_FALLTHROUGH;
     }
     case TargetLowering::Expand:
       if (ExpandNode(Node))
         return;

So, if you return an SDValue() then it will Expand. If you return the 
original node, that is equivalent to Legal. Otherwise, you're requesting 
a replacement. The logic for loads/stores and vectors is handled 
separately (but is similar).

  -Hal

>
> -Matt
>
>
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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