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Scott Ricketts

2009-May-21 02:31 UTC

[LLVMdev] Passing a pointer to a function

I recently began hacking around with my first LLVM pass. The big
picture is that I would like to insert function calls for each
instruction type, and pass some parameters based on the instruction
type. Then I will link the output to some C file that implements those
functions.

Things were going well until I started trying to make function calls
with a pointer as a parameter. For example, I would like to do
something like:

void
visitLoadInst(LoadInst &I) {
  Value *P;
  P = I.getPointerOperand();

  CallInst::Create(load_fcall, P, "", &I);
}

Where load_fcall is build using a call to Module::getOrInsertFunction
as in the example here:

http://wiki.llvm.org/HowTo:_Insert_a_function_call

My question is, what do I pass as the argument type for P above? The
following seems to work, as long as there are no floating point ops:

PointerType::getUnqual(IntegerType::get(32))

So I tried using just a void pointer type, as in:
PointerType::getUnqual(Type::VoidTy)

But then Type.cpp throws this assertion.
Assertion `ValueType != Type::VoidTy && "Pointer to void is not
valid,
use sbyte* instead!"' failed.

I can try checking the type before building the function, as in:

void
visitLoadInst(LoadInst &I) {
  Value *P;
  P = I.getPointerOperand();

  PointerType* PtrTy = (PointerType*)(P);

  Constant* fcall;

  if (PtrTy->getElementType()->isInteger()) {
    fcall = buildFcallMem(*theModule, "load_int", Type::Int32Ty);
    CallInst::Create(fcall, P, "", &I);
  } else if (PtrTy->getElementType()->isInteger()){
    fcall = buildFcallMem(*theModule, "load_float", Type::FloatTy);
    CallInst::Create(fcall, P, "", &I);
  }
}

But then I get segfaults or assertions fail in Type.h (it's a huge
mess). I could be more specific, but I have tried several different
approaches and am trying to be brief here.

Perhaps I am overlooking an obvious approach. I will continue snooping
through the code to attempt to learn what is going on, but if anyone
could nudge me in the right direction or point out where I am
completely off, that would probably be helpful.

Thanks,
Scott

Eli Friedman

2009-May-21 02:45 UTC

head link

[LLVMdev] Passing a pointer to a function

On Wed, May 20, 2009 at 7:31 PM, Scott Ricketts <sricketts at
maxentric.com> wrote:> Where load_fcall is build using a call to Module::getOrInsertFunction
> as in the example here:
>
> http://wiki.llvm.org/HowTo:_Insert_a_function_call
>
> My question is, what do I pass as the argument type for P above? The
> following seems to work, as long as there are no floating point ops:
I would suggest bit-casting p to an i8*.
> So I tried using just a void pointer type, as in:
> PointerType::getUnqual(Type::VoidTy)
>
> But then Type.cpp throws this assertion.
> Assertion `ValueType != Type::VoidTy && "Pointer to void is
not valid,
> use sbyte* instead!"' failed.
Yeah, void* in C == i8* in LLVM.  VoidTy is only used for function return types.

I'm not sure why you're getting crashes with the last approach; it
looks like it should work.

-Eli

Scott Ricketts

2009-May-21 15:17 UTC

head link

[LLVMdev] Passing a pointer to a function

Eli: Thanks for the quick and helpful response.
> I would suggest bit-casting p to an i8*.
Yup, that did the trick:

void
visitLoadInst(LoadInst &I) {
  Value *P = I.getPointerOperand();

  const Type *PtrTy = PointerType::getUnqual(Type::Int8Ty);

  BasicBlock::iterator it(I);
  ++it;

  CastInst *CI     CastInst::Create(Instruction::BitCast, P, PtrTy,
"", it);

  CallInst::Create(load_fcall, CI, "", it);
}
> I'm not sure why you're getting crashes with the last approach; it
> looks like it should work.
In that particular case, it encounters a pointer to an element of type
label and then segfaults on the call to isInteger(). I didn't look
into this bug because of the bitcast solution works for now.

Thanks,
Scott

Anthony Danalis

2009-May-22 18:05 UTC

head link

[LLVMdev] Passing a pointer to a function

I am not an expert in this, but maybe what you are missing is a  
bitcast.  I'm attaching a simple function I use to pass strings to  
functions I've inserted in target programs:

     //////////////////////////////////////////////////////////////////////
     // convert an STL std::string into an LLVM GlobalVariable holding a
     // ConstantArray and then cast it to a PointerType so we can pass  
it
     // to a function call as an actual.
     Value *stringToPtr(std::string str, Module *mdl, Instruction  
*instr){
         Constant *strConstant;
         GlobalVariable *gv;

         strConstant = ConstantArray::get(std::string(str));
         gv =  new llvm::GlobalVariable(strConstant->getType(),
                                        true,
                                        GlobalValue::InternalLinkage,
                                        strConstant,
                                        "",
                                        mdl,
                                        false);
         PointerType *strPtrTy = PointerType::getUnqual(Type::Int8Ty);
         return CastInst::Create(Instruction::BitCast, gv, strPtrTy,  
"", instr);
     }


I hope it helps,
Anthony


On May 20, 2009, at 10:31 PM, Scott Ricketts wrote:
> I recently began hacking around with my first LLVM pass. The big
> picture is that I would like to insert function calls for each
> instruction type, and pass some parameters based on the instruction
> type. Then I will link the output to some C file that implements those
> functions.
>
> Things were going well until I started trying to make function calls
> with a pointer as a parameter. For example, I would like to do
> something like:
>
> void
> visitLoadInst(LoadInst &I) {
>  Value *P;
>  P = I.getPointerOperand();
>
>  CallInst::Create(load_fcall, P, "", &I);
> }
>
> Where load_fcall is build using a call to Module::getOrInsertFunction
> as in the example here:
>
> http://wiki.llvm.org/HowTo:_Insert_a_function_call
>
> My question is, what do I pass as the argument type for P above? The
> following seems to work, as long as there are no floating point ops:
>
> PointerType::getUnqual(IntegerType::get(32))
>
> So I tried using just a void pointer type, as in:
> PointerType::getUnqual(Type::VoidTy)
>
> But then Type.cpp throws this assertion.
> Assertion `ValueType != Type::VoidTy && "Pointer to void is
not valid,
> use sbyte* instead!"' failed.
>
> I can try checking the type before building the function, as in:
>
> void
> visitLoadInst(LoadInst &I) {
>  Value *P;
>  P = I.getPointerOperand();
>
>  PointerType* PtrTy = (PointerType*)(P);
>
>  Constant* fcall;
>
>  if (PtrTy->getElementType()->isInteger()) {
>    fcall = buildFcallMem(*theModule, "load_int", Type::Int32Ty);
>    CallInst::Create(fcall, P, "", &I);
>  } else if (PtrTy->getElementType()->isInteger()){
>    fcall = buildFcallMem(*theModule, "load_float",
Type::FloatTy);
>    CallInst::Create(fcall, P, "", &I);
>  }
> }
>
> But then I get segfaults or assertions fail in Type.h (it's a huge
> mess). I could be more specific, but I have tried several different
> approaches and am trying to be brief here.
>
> Perhaps I am overlooking an obvious approach. I will continue snooping
> through the code to attempt to learn what is going on, but if anyone
> could nudge me in the right direction or point out where I am
> completely off, that would probably be helpful.
>
> Thanks,
> Scott
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

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