[llvm-dev] Retrieving numeric value of instruction operand

Sun Apr 24 21:37:17 PDT 2016

Hey Jeremy,

thank you for the response,I see what you're saying,

The simplified version of the question is can i compare a Value to check if
it is 1, is there a way to do *if(i32 %b==1) *in LLVM through a pass?

On 24 April 2016 at 19:36, Jeremy Lakeman <Jeremy.Lakeman at gmail.com> wrote:

> Let me repeat the above;
> %a is a variable. It could have any value.
> There could be any number of other calls to @summ(a,b) in other
> translation units, or linked at runtime, which pass any value at all.
>
> However, if you have a call site which calls @summ with constant
> arguments, the method could be inlined at that call site...
>
> There is a very big difference between compiling a method, and
> interpreting a method.
>
> You can write an LLVM pass to modify the method and change what it
> does. For example you can add code to inspect the values of a and b at
> run time.
>
> It might help to think about what the final version of your function
> would look like if you wrote it in your source language. eg;
>
> int summ(int a, int b) {return a+b;} ==> int summ(int a, int b) {if
> (a==2) return b*2; else return a+b;}
>
> Then compile both versions to LLVM so you can start thinking about
> what LLVM operations need to be performed to turn one into the other.
>
>
> On Mon, Apr 25, 2016 at 5:45 AM, Ammar Naqvi via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > 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 list
> >>> llvm-dev at lists.llvm.org
> >>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >>>
> >>>
> >>>
> >>> --
> >>> John Criswell
> >>> Assistant Professor
> >>> Department of Computer Science, University of Rochester
> >>> http://www.cs.rochester.edu/u/criswell
> >>
> >>
> >>
> >> --
> >> John Criswell
> >> Assistant Professor
> >> Department of Computer Science, University of Rochester
> >> http://www.cs.rochester.edu/u/criswell
> >
> >
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
>
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