[llvm-dev] returning from LowerOperation()

[Jonas Paulsson via llvm-dev]

Hi all,

thanks!

 >  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).

Does this sound good?

diff --git a/include/llvm/Target/TargetLowering.h 
b/include/llvm/Target/TargetLowering.h
index 01c5c51..41edcc6 100644
--- a/include/llvm/Target/TargetLowering.h
+++ b/include/llvm/Target/TargetLowering.h
@@ -2830,7 +2830,8 @@ public:
    /// target, which are registered to use 'custom' lowering, and whose 
defined
    /// values are all legal.  If the target has no operations that 
require custom
    /// lowering, it need not implement this.  The default 
implementation of this
-  /// aborts.
+  /// aborts. Returning SDValue() is ok and signals that the input node
+  /// should be further handled by the caller instead.
    virtual SDValue LowerOperation(SDValue Op, SelectionDAG &DAG) const;

    /// This callback is invoked when a node result type is illegal for the

/ Jonas


On 2017-01-23 22:59, Hal Finkel via llvm-dev wrote:
>
>
> 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
>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
> -- 
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
>
> _______________________________________________
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> llvm-dev at lists.llvm.org
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