llvm dev - Dec 2012 - [LLVMdev] GetElementPtrInst question

Hi all,

How can I create an llvm::GetElementPtrInst in which the pointer and
the index are in registers previously loaded with llvm::LoadInst ? I
mean, the generated instruction will be like this:

  %1 = getelementptr i8* %myreg1, i32 %myreg2

here, %myreg1 and %myreg2 are previously created by load instructions
(llvm::LoadInst).

Please, let me know if there is an example of something similar.

Thanks for the help,
Eduardo

Hi,

You'd use an IRBuilder, like you did to create your LoadInsts.

IRBuilder<> IRB(BB);
IRB.CreateGEP(myreg1, myreg2);

I assume because you asked this question, something went wrong when using the
above method. What was it? :)

Cheers,

James
________________________________________
From: llvmdev-bounces at cs.uiuc.edu [llvmdev-bounces at cs.uiuc.edu] On Behalf
Of Eduardo [erocha.ssa at gmail.com]
Sent: 02 December 2012 15:37
To: llvmdev at cs.uiuc.edu
Subject: [LLVMdev] GetElementPtrInst question

Hi all,

How can I create an llvm::GetElementPtrInst in which the pointer and
the index are in registers previously loaded with llvm::LoadInst ? I
mean, the generated instruction will be like this:

  %1 = getelementptr i8* %myreg1, i32 %myreg2

here, %myreg1 and %myreg2 are previously created by load instructions
(llvm::LoadInst).

Please, let me know if there is an example of something similar.

Thanks for the help,
Eduardo
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Hi James,

Thanks for your quick reply.
> I assume because you asked this question, something went wrong when using
the above method. What was it? :)
No, I am not using this method. I was trying to create a
llvm::GetElementPtrInst . I didn't create IRBuilder. I am writing a
ModulePass that insert new instruction to an existing Module.

Besides, how can you get a reference to myreg1 ? Looking at your
snippet this variable would be a llvm::Value* (I saw that in the
documentation of IRBuilder). All I have in my code is something like:

  llvm::LoadInst *inst2 = new llvm::LoadInst( oneGlobalVariable,
"myreg1", inst1);

Probably I can get a Value pointer to the output register from the
inst2 variable, but I don't know how (I couldn't figure out myself by
just reading the documentation).

Any help is very welcome,
Eduardo

On 12/2/2012 9:37 AM, Eduardo wrote:
> Hi all,
>
> How can I create an llvm::GetElementPtrInst in which the pointer and
> the index are in registers previously loaded with llvm::LoadInst ? I
> mean, the generated instruction will be like this:
>
>    %1 = getelementptr i8* %myreg1, i32 %myreg2
>
> here, %myreg1 and %myreg2 are previously created by load instructions
> (llvm::LoadInst).
Other than using the IR builder, you can create this instruction 
directly, using one of the "Create" functions, for example:

   static GetElementPtrInst *Create(Value *Ptr, ArrayRef<Value *> IdxList,
                                    const Twine &NameStr,
                                    Instruction *InsertBefore = 0) {
     GetElementPtrInst *GEP = Create(Ptr, IdxList, NameStr, InsertBefore);
     GEP->setIsInBounds(true);
     return GEP;
   }



The "Ptr" would be your "myreg1", and you'll need to
create an ArrayRef
that holds a single element: "myreg2".  Conveniently, ArrayRef has a 
constructor that creates a "singleton" array:

     /// Construct an ArrayRef from a single element.
     /*implicit*/ ArrayRef(const T &OneElt)
       : Data(&OneElt), Length(1) {}

Since it's not an explicit constructor, it may actually work if you do
   GEP = GetElementPtrInst::Create(myreg1, myreg2, "name.of.my.gep", 
where-to-insert);

Otherwise you'll need to create an ArrayRef that represents the 
arguments to the GetElemPtr instruction.


-Krzysztof



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