[llvm-dev] What is relationship between Operator and Instruction?

[周书林 via llvm-dev]

Hi Craig,

Thank you very much!

Sincerely,
Shulin

Craig Topper <craig.topper at gmail.com> 于2021年6月11日周五 上午2:50写道：

> Operator is a subclass of User that contains methods that expose common
> functionality between Instruction and ConstantExpr. For example, having an
> opcode and operands. This allows a lot of code to treat them the same way
> without carrying about the differences. A GEPOperator is either a GEP
> Instruction or a GEP ConstantExpr. GEPOperator lets you not care which it
> really is. An Instruction can have a ConstantExpr as an operand, but a
> ConstantExpr cannot have an Instruction as an operand.
>
> The real class hierarchy is such that Instructions and ConstantExpr don't
> have the same immediate base class though they both inherit from User.
> ConstantExpr inherits from Constant which inherits from User. Constant
> doesn't have an opcode(other than the one from Value) operands, but
> ConstantExpr does. Nothing inherits from Operator and it is
> never instantiated. Because it doesn't have any data members that aren't
> part of User we can static_cast both Instruction and ConstantExpr to it.
> All of the methods exposed by Operator look at member variables available
> in User to determine if it is an Instruction or a ConstantExpr and call the
> appropriate method by static_casting to Instruction or ConstantExpr first.
>
> Hope that helps.
>
> ~Craig
>
>
> On Thu, Jun 10, 2021 at 4:29 AM jeniffer lesley via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> I second this question. I never undersrood this in LLVM IR. LangRef fails
>> to explain this.
>>
>> On Thu, Jun 10, 2021, 4:46 PM 周书林 via llvm-dev <llvm-dev at lists.llvm.org>
>> wrote:
>>
>>> Hi,
>>>
>>> Thank you for your suggestions.
>>> I have learnt the basic IR structures in simple programs, but not all
>>> the objects could be generated. As I mentioned, about the Operators and
>>> ConstantExprs. I only got GEPOperator objects from .bc of MySQL. So is
>>> there any reference to learn about the relationship between those classes
>>> in IR?
>>>
>>> Sincerely,
>>> Shulin
>>>
>>> rahul utkoor <rahulutkoor4887 at gmail.com> 于2021年6月10日周四 下午5:25写道：
>>>
>>>> Hello,
>>>> Since you are starting to learn IR, I recommend you to start with one
>>>> simple program, compile the program, genreate CFG and understand the
>>>> hierarchy of types/operands/instructions.
>>>> It helped me to understand the compilation pipeline in a quick way.
>>>>
>>>>
>>>> Thanks,
>>>> Rahul Utkoor
>>>>
>>>>
>>>> On Thu, 10 Jun, 2021, 2:08 pm 周书林 via llvm-dev, <
>>>> llvm-dev at lists.llvm.org> wrote:
>>>>
>>>>> Hello everyone,
>>>>>
>>>>> I am learning the IR in LLVM and trying to do some analysis. Now I am
>>>>> doing a dataflow analysis via the def-use chain provided by Users.
>>>>>
>>>>> In my opinion, the Users of a Value should be an instruction. However,
>>>>> when I iterate the User of a Value, sometimes I get an Operator, like
>>>>> GEPOperator, or an ConstantExpr.
>>>>> I am quite confused about these situations. My question is, what is
>>>>> Operator in IR? and what is the difference between Operator and
>>>>> Instruction? Why can I get a GEPOperator as an Operand in a instruction
>>>>> rather than two instruction( a gep instruction and the next User
>>>>> instruction) ?
>>>>>
>>>>> Best regards,
>>>>> Shulin
>>>>> _______________________________________________
>>>>> LLVM Developers mailing list
>>>>> llvm-dev at lists.llvm.org
>>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>
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>
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