llvm dev - Jan 2015 - [LLVMdev] AllocaInst for FunctionType?

Hi,
I'm trying to get my head around c++ - IR - c++ API and getting used 
tramform manual information to code.


The manual states alloca is defined for <type>. FunstionType is a type, so
alloca for functionType should be possible? Not?


If we have a valid Module *m  

we can get an allocate instruction allocating space for a non-argumented 
function as follows:

AllocaInst* pa2 = new AllocaInst( FunctionType::get( 
            Type::getVoidTy(m->getContext()), false  )
   , 1, "myName");


but how about nonvar-argumented functions? E.g. I32,I16

std::vector<Type*>FuncTy_args;
FuncTy_args.push_back(IntegerType::get(mod->getContext(), 32));
FuncTy_args.push_back(IntegerType::get(mod->getContext(), 16));


AllocaInst* pa3 = new AllocaInst( FunctionType::get( 
    Type::getVoidTy(m->getContext()), FuncTy_args ,false  )
    , 1, "MyName");


does this make sense?
I guess this might not be correct, because we explicitely introduce 
FuncTy_args, where we only would like to know the 'necessary memory 
footprint' of it, for alloca to be able to allocate it, not?

Alex

> On Jan 15, 2015, at 3:05 AM, Alexander Poddey <alexander.poddey at
gmx.net> wrote:
> 
> Hi,
> I'm trying to get my head around c++ - IR - c++ API and getting used 
> tramform manual information to code.
> 
> 
> The manual states alloca is defined for <type>. FunstionType is a
type, so
> alloca for functionType should be possible? Not?
I may not understand what you try to achieve because it does not make much sense
to me :)

As far as I understand, an alloca instruction allocate space for a variable. It
does not really make sense for it to be used with a function type (do you want
to store a pointer to a function of this type?).

Maybe this would help: http://llvm.org/docs/tutorial/LangImpl7.html
(and in any case you can check how clang is using alloca to emit llvm IR)

OTH

Mehdi


> 
> 
> If we have a valid Module *m  
> 
> we can get an allocate instruction allocating space for a non-argumented 
> function as follows:
> 
> AllocaInst* pa2 = new AllocaInst( FunctionType::get( 
>            Type::getVoidTy(m->getContext()), false  )
>   , 1, "myName");
> 
> 
> but how about nonvar-argumented functions? E.g. I32,I16
> 
> std::vector<Type*>FuncTy_args;
> FuncTy_args.push_back(IntegerType::get(mod->getContext(), 32));
> FuncTy_args.push_back(IntegerType::get(mod->getContext(), 16));
> 
> 
> AllocaInst* pa3 = new AllocaInst( FunctionType::get( 
>    Type::getVoidTy(m->getContext()), FuncTy_args ,false  )
>    , 1, "MyName");
> 
> 
> does this make sense?
> I guess this might not be correct, because we explicitely introduce 
> FuncTy_args, where we only would like to know the 'necessary memory 
> footprint' of it, for alloca to be able to allocate it, not?
> 
> Alex
> 
> 
> 
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

> As far as I understand, an alloca instruction allocate space for a
> variable. It does not really make sense for it to be used with a function
> type (do you want to store a pointer to a function of this type?).
Yes. if I undertand correctly, alloca allocates space for a variable.
I could then want to allocate space for a function pointer.

I can now see code similar to my example by using c function pointers and 
emitting c++API code:


When translating c++ code using function pointers, the emitted llvm for 

int (*foo2)(double,float);

looks like

%foo2 = alloca i32 (double, float)*, align 8

and the c++ API code (generated by llc -march=cpp) 



std::vector<Type*>FuncTy_49_args;
 FuncTy_49_args.push_back(Type::getDoubleTy(mod->getContext()));
 FuncTy_49_args.push_back(Type::getFloatTy(mod->getContext()));
 FunctionType* FuncTy_49 = FunctionType::get(
  /*Result=*/IntegerType::get(mod->getContext(), 32),
  /*Params=*/FuncTy_49_args,
  /*isVarArg=*/false);
 
PointerType* PointerTy_48 = PointerType::get(FuncTy_49, 0);
 
AllocaInst* ptr_foo2 = new AllocaInst(PointerTy_48, "foo2",
label_entry_71);


This is what I had in my 1st post. So this seems to be the way to do it.
This vice versa transformation between source, IR and API is quite cool!


Alex