llvm dev - Oct 2016 - Accessing the associated LLVM IR Instruction for an SDNode used in instruction selection (back end)

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

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

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
>
>