[llvm-dev] Doubts

Nemanja Ivanovic via llvm-dev llvm-dev at lists.llvm.org
Mon Jun 6 08:32:31 PDT 2016

It is not a keyword. It is a node defined in
include/llvm/Target/TargetSelectionDAG.td. You can likely find most of the
definitions you're wondering about there.
In terms of its purpose, perhaps someone can elaborate on that a bit more,
but there is no corresponding ISD node for this and the way I look at it is
that its purpose is to help Tbl-gen infer the result type of the node (in
this case the "add" node). For example, you may have an instruction that
adds two i32's and produces an i64 and then another one that produces an
i32 (I know this is kind of contrived in the case of addition, but you get
the point). The DAGs would look identical except that the set node would
have a different type.
However, I may be wrong about this.

Of course, "store" is quite different. That is a full fledged node and has
fairly straightforward semantics - it's a store node. So for example a DAG
that looks like:
(store i64:$src, addr:$dst)
actually says store the 64-bit integer $src at address $dst.


On Mon, Jun 6, 2016 at 5:16 PM, Pedro Lopes <pedro.fraiao at gmail.com> wrote:

> Thanks, indeed it was on the LegalizeDAG.cpp and the information proved
> very useful.
> I also realized that the customization, promotion or expansion will occur
> whenever any operand, with the same type as the type specified on the
> second argument (MVT) of setOperationAction function, appears. (Correct me
> if I'm wrong).
> The second doubt I have regards instruction matching.
> When I define a pattern such as:
> [(set i32:$dst, (add (mul i32:$src1, i32:$src2), i32:$src3))]>
> What is the meaning of the word "set" or "store" which appear quite often?
> From what I can understand this pattern is only interest in an add node
> with a mul node as one operator and an immediate as the other.
> Does the "set" (keyword?) translates to an output of the add node, or just
> another node?
> Thanks,
> Patosga
> 2016-06-06 6:22 GMT+01:00 Nemanja Ivanovic <nemanja.i.ibm at gmail.com>:
>> Hi Patosga,
>> you can control what type the operands are promoted to as follows:
>> AddPromotedToType (ISD::ADD, MVT::i1, MVT::i64);
>> For the "Expand" case, I believe you'll find what you're looking for in
>> the file:
>> lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
>> and function SelectionDAGLegalize::ExpandNode(), but if I'm wrong here
>> perhaps others can correct me.
>> Finally, MVT::f32 just states that the operand is a 32-bit single
>> precision floating point type.
>> Nemanja
>> On Sun, Jun 5, 2016 at 11:05 PM, Pedro Lopes via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>> Sorry, glad I'm in the right place.
>>> Before I start, I want to state that I'm a beginer and I'm trying to
>>> develop a backend by adapting an existent target to my platform.
>>> My first doubt is about the SelectionDAG and the TargetLowering class.
>>> When I use, for example:
>>> setOperationAction(ISD::ADD, MVT::i1, Promote);
>>> Is it correct to say that I'm promoting any operand used by the ISD::ADD
>>> node to a larger type? If so, what would that value type be?
>>> If I use the same function with the expand as the third argument like
>>> below:
>>> setOperationAction(ISD::FSIN, MVT::f32, Expand);
>>> What will the expansion look like, since I don´t provide any custom
>>> implementation of the node? Also, what is the meaning of the MVT::f32
>>> in this case?
>>> Thanks,
>>> Patosga
>>> 2016-06-05 19:16 GMT+01:00 Pierre Gagelin <Pierre.Gagelin at myport.ac.uk>:
>>>> Hi Patosga,
>>>> You are on the right place to ask your questions and get help!
>>>> Try to be more specific with your problem. We need some details to help
>>>> you =)
>>>> Pierre
>>>> On 5 June 2016 at 18:12, via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>>>> Hello sir,
>>>>> I'm having some problems understading how llvm works. I'm following a
>>>>> concrete example and visualizing the DAG and the .s output and some stuff
>>>>> doesn´t seem to match.
>>>>> I sorry to bother you but I don´t know where I can expose my doubts
>>>>> and I wonder if you could redirect me to some discussion place where
>>>>> someone might help me.
>>>>> Thanks,
>>>>> Patosga
>>>>> _____________________________________
>>>>> Sent from http://llvm.1065342.n5.nabble.com
>>>>> _______________________________________________
>>>>> LLVM Developers mailing list
>>>>> llvm-dev at lists.llvm.org
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> --
> Cumprimentos,
> José Pedro Lopes
> ------------------------------------
> Best Regards,
> José Pedro Lopes
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