[llvm-dev] Retrieving numeric value of instruction operand

[Ammar Naqvi via llvm-dev]

Hi John,

Thank you so much for the valuable answer! :)

However, i have one tough issue that i need to solve, in my pass :

let's say here

%s = add %a, %b

I need to check if the value of %a or %b is 2 or 4 and then apply a
transformation to the other operand accordingly,

for example if the value of %a = 2 the i'll multiply %b by 2 in my
transform pass.

I'm having a hard time getting the integer value of variables since it is
so simple in regular languages, it can't be possible that LLVM does not
have a way for that, I've also tried casting the operand to a ConstantInt
which wasn't successful.

I'm a beginner with LLVM and i do apologize for my naivety.

Best Regards,
Ammar

On 24 April 2016 at 10:51, John Criswell <jtcriswel at gmail.com> wrote:

> On 4/24/16 1:27 PM, Ammar Naqvi wrote:
>
> hey john,
>
> yes indeed, that's what I'm trying, retreiving the values of %a and %b in
> an LLVM pass,  sorry about the confusion.
>
>
> If you want to add an instruction that uses the values %a and %b, then
> your solution is easy: in the LLVM IR, a value and the instruction that
> creates it are one and the same, and they are both represented by the same
> object.  For example, in the following instruction:
>
> %s = add %a, %b
>
> The "%s" and the "add %a, %b" are the same thing (because SSA only allows
> one assignment to a virtual register).  Therefore, in memory, there is one
> object (a sub-class of the Value class) that has name "%s" and represents
> the instruction "add %a, %b."
>
> So, let's say you have the following code to get an operand from an object
> of class Instruction:
>
> Instruction * i = <whatever>;
> Value * Va = i->getOperand(0);
>
> The variable Va points to the object representing the first operand of the
> instruction (in your case, %a).  You can then create new instructions that
> use this value as an operand:
>
> BinaryOperator * Sub = BinaryOperator::CreateNeg (Va, "name", InsertPt);
>
> The above would create the following instruction (which is an integer
> negation instruction):
>
> %name = sub 0, %a
>
> Regards,
>
> John Criswell
>
>
> On Apr 24, 2016 7:18 AM, "John Criswell" <jtcriswel at gmail.com> wrote:
>
>> Dear Ammar,
>>
>> It is not clear what you are asking.  %a and %b in your code below are
>> not constants; there is no way, at compile time, to determine what numeric
>> values they will hold at run-time.
>>
>> Are you asking how to write an LLVM pass that will add code to summ() to
>> print out its result?
>>
>> Regards,
>>
>> John Criswell
>>
>> On 4/24/16 2:27 AM, Ammar Naqvi via llvm-dev wrote:
>>
>> Hello Everyone,
>>
>> I need some help in retrieving the numeric value of an instruction
>> operand from LLVM IR.
>>
>> this is what the IR looks like for a simple add function that adds two
>> i32 integers
>>
>> define i32 @summ(i32 %a, i32 %b) #0 {
>> entry:
>>   %add = add nsw i32 %b, %a
>>   ret i32 %add
>> }
>>
>> i would like to know the integer value of %a and %b.
>>
>> I've tried
>>
>> -i->getOpcodeName() which gives me the string add
>>
>> -i->getOperand(0) which gives me the string i32 %b
>>
>> -i->getOperand(0)->getName() which gives me the string a
>>
>> what method exists to get the actual integer value of the operands?
>> for example we called summ(1,2), how to retrieve the values 1 and 2 held
>> in the operands?
>>
>> Any help and guidance is greatly appreciated! :)
>>
>> Best,
>> Ammar
>>
>>
>> _______________________________________________
>> LLVM Developers mailing listllvm-dev at lists.llvm.orghttp://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>>
>> --
>> John Criswell
>> Assistant Professor
>> Department of Computer Science, University of Rochesterhttp://www.cs.rochester.edu/u/criswell
>>
>>
>
> --
> John Criswell
> Assistant Professor
> Department of Computer Science, University of Rochesterhttp://www.cs.rochester.edu/u/criswell
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20160424/9590446e/attachment.html>