[llvm-dev] Value

[Evgeny Astigeevich via llvm-dev]

You think in terms of the textual representation of IR and are trying to work on IR in these terms.

See:

http://www.llvm.org/docs/ProgrammersManual.html#coreclasses

http://llvm.org/doxygen/classllvm_1_1Value.html


In the LLVM representation Value is a thing which can be used as an operand to instructions. In API there is no such thing as the named right part. An instruction itself is a value which is produced as result of execution of the instruction. Look at the example:


Instr 1:  add i32 1, 2



Instr 2:  add  i32 <result of Instr1>, 10


We have a def-use connection between Instr 2 and Instr 1. We don't need names. We can traverse def-use connection in this way (http://www.llvm.org/docs/ProgrammersManual.html#helpful-hints-for-common-operations):


Instruction *pi = ...;

for (Use &U : pi->operands()) {
  Value *v = U.get();
  // ...
}


Your case:


> %a = alloca i32, align 4 - %a here

>, but I don't quite understand the difference between Instruction object and Value object of a, which is used further , and in this case :


Instr1: alloca i32, align 4

If there are users of Instr1 then can access them calling user_begin() on Instr1. You can access Instr1 from users as in the example above. Of course you need to keep the pointer to Instr1 somewhere to check that it's used.


> %1 = alloca i32, align 4 - I also wanted to use %1 and in this case the only possibility is Instruction object.


You traverse basic blocks and process the instructions. Processing can include any type of actions: analysis, optimization etc.

So what is your use case?


-Evgeny

________________________________
From: Anastasiya Ruzhanskaya <anastasiya.ruzhanskaya at frtk.ru>
Sent: Thursday, July 20, 2017 3:02:26 PM
To: Evgeny Astigeevich
Cc: llvm-dev at lists.llvm.org; nd
Subject: Re: [llvm-dev] Value

Thank you! I wanted to use the right part of the instruction ,
 %a = alloca i32, align 4 - %a here , but I don't quite understand the difference between Instruction object and Value object of a, which is used further , and in this case :
%1 = alloca i32, align 4 - I also wanted to use %1 and in this case the only possibility is Instruction object.

2017-07-20 15:32 GMT+02:00 Evgeny Astigeevich <Evgeny.Astigeevich at arm.com<mailto:Evgeny.Astigeevich at arm.com>>:

Hi Anastasiya,


What do you mean 'identifiable'? You have Module, Function, BasicBlock etc. You can access Values and Users. They form a data-flow graph. You can store pointers to Values and Users if you wish. If you want a name for any Value there is ValueSymbolTable (http://www.llvm.org/docs/ProgrammersManual.html#advanced-topics) which provides a symbol table that the Function and Module classes use for naming value definitions. You should take into account that some Values might not have names and passes can change names of Values.


Thanks,

Evgeny Astigevich

________________________________
From: llvm-dev <llvm-dev-bounces at lists.llvm.org<mailto:llvm-dev-bounces at lists.llvm.org>> on behalf of Anastasiya Ruzhanskaya via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
Sent: Thursday, July 20, 2017 2:21:44 PM
To: LLVM Developers Mailing List
Subject: [llvm-dev] Value

Hello,
I am trying to write my own pass for some purposes, and found out that there is no way to work with such variables as %1 and so on. How they are identifiable? Only by the Value object, without any possibility for visual representation as for example %a, %b values?

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170720/a8fff547/attachment-0001.html>