[llvm-dev] Accessing the associated LLVM IR Instruction for an SDNode used in instruction selection (back end)

[Alex Susu via llvm-dev]

   Hello.
     I come back to this thread to add that moving SDB->clear() at the beginning of the 
method messes up later at instruction selection with an error message like:
       lib/CodeGen/SelectionDAG/DAGCombiner.cpp:1466: llvm::SDValue 
{anonymous}::DAGCombiner::combine(llvm::SDNode*): Assertion `N->getOpcode() != 
ISD::DELETED_NODE && "Node was deleted but visit returned NULL!"' failed.
     So, I added in class SelectionDAGISel another DenseMap crtNodeMap, which I copy in 
SelectionDAGISel::SelectBasicBlock() before the original SDB->clear() is given.

    Doing this no longer gives me any errors.

   Best regards,
     Alex

On 10/24/2016 1:11 PM, Alex Susu wrote:
>   Hello.
>     Quentin, retrieving the LLVM IR instruction from which an SDNode originates is useful
> during the instruction selection phase. For example, I need to recover the LLVM IR
> variable which is used to fill an entire vector with the ISD::BUILD_VECTOR
> target-independent SDNode  .
>     From the recovered LLVM IR variable I can walk on the use-def-chains in order to get
> the most complete definition of this variable (in terms of the input program variables),
> which is something I can't do at the level of SelectionDAGs (because, for example, actual
> function arguments don't appear in the DAG).
>
>   Best regards,
>     Alex
>
>
> On 10/22/2016 12:29 AM, Quentin Colombet wrote:
>> Hi Alex,
>>
>> Out of curiosity, what is your use case for that?
>>
>> Generally speaking I would recommend against doing that. When the SDBuilder is done, I
>> would expect the SDNodes to not query anything outside of the SD layer. We are not here
>> now, though.
>>
>> Cheers, -Quentin
>>> On Oct 21, 2016, at 4:57 AM, Alex Susu via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>>
>>> Hello. I would like to access the LLVM IR Instruction from which an SDNode (from
>>> SelectionDAG) originates. For this I have modified: -
>>> llvm/lib/CodeGen/SelectionDAGISel.cpp, SelectionDAGISel::SelectBasicBlock(), namely I
>>> put SDB->clear() at the beginning of the method in order to avoid clearing NodeMap
>>> after creating the initial SelectionDAG from LLVM IR, since I want to access it after
>>> that; - llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h - added an accessor for
>>> the private NodeMap object relating LLVM IR Instructions to generated SDNodes:
>>> public: // Alex: added a getter for NodeMap DenseMap<const Value*, SDValue>
>>> &getNodeMap() { return NodeMap; }
>>>
>>> A small problem is that it seems that when I access the NodeMap in
>>> [Target]DAGToDAGISel::Select(SDNode *Node) method (part of subclass of
>>> SelectionDAGISel), the NodeMap contains quite a few SDNodes tagged as <<Deleted
>>> Node!>> (for the nodes that were already selected). But the current SDNode, given as
>>> parameter to Select() is valid and we can search for it in NodeMap in order to find
>>> the LLVM IR Instruction (llvm::Value, more exactly) that generated it. As a side
>>> question, is there a better modality to find the key associated to a value in a
>>> DenseMap object than to iterate through all of DenseMap's elements (I could not find
>>> anything better at
>>> http://llvm.org/docs/doxygen/html/classllvm_1_1DenseMapBase.html)?
>>>
>>> Could you please tell me if there is a better modality to retrieve the LLVM IR
>>> Instruction from which an SDNode originates?
>>>
>>> Thank you, Alex _______________________________________________ LLVM Developers
>>> mailing list llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>