[llvm-dev] Retrieving numeric value of instruction operand

Sun Apr 24 22:46:52 PDT 2016

foo.c;
int foo(int a, int b){
  if (b==1)
    return 0;
  return a+b;
}

$ clang -c foo.c -emit-llvm
$ llvm-dis foo.bc
$ cat foo.ll
...
  %5 = icmp eq i32 %4, 1
  br i1 %5, label %6, label %7
...
$ llc -march=cpp foo.bc
$ cat foo.cpp
...
  ICmpInst* int1_14 = new ICmpInst(*label_4, ICmpInst::ICMP_EQ,
int32_13, const_int32_2, "");
  BranchInst::Create(label_5, label_6, int1_14, label_4);
...

You can get a rough idea of what to do from clang and llvm itself.

On Mon, Apr 25, 2016 at 2:07 PM, Ammar Naqvi <ammarnaqvi92 at gmail.com> wrote:
> 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
>> >
>
>