[llvm-dev] Function arguments pass by value or by reference.

[John Criswell via llvm-dev]

On 5/17/16 4:13 AM, PeiLIU via llvm-dev wrote:
> Now, I am using LLVM-3.3 do some process with functions, however there 
> are some difficult things I can't handle by myself. So,
>
> I want get your help to get it down properly.
>
> Q1. There is a function declaration:
>
> call i32 @create(i64* %tid, %union.t* %pab, i8* (i8*)* @worker, i8* 
> null) // callInst
>
> Store instruction goes like this:
>
> store i8* (i32, double, i32*)* %fp, i8* (i32, double, i32*)** %fp.addr 
> // storeInst
>
> I want to determine the type of the operands are function pointer or 
> not? (That's what I want)
>
> However, callInst->getOperand(2)->getType()->getTypeID() always return 
> 14 that'a the enum number of
>
> PointerTypeID. For the store instruction, first operand is the same as 
> the operation with call instruction.
>
> How can I get the function pointer properly?

Instead of looking at the Type ID, you should use isa<FunctionType> to 
determine if the value has function type:

if (isa<FunctionType>(callInst->getOperand(2)->getType()) {
   ... <second operand has function type>
}

In LLVM, functions are global variables and therefore are a pointer type.

>
> Q2. Function arguments can be passed by value or by pointer in 
> C-programming language.
>
> int arr[10];
> struct node {
> int a;
> int att[10];
> double ul;
> };
>
> struct node Node;
>
> testStruct(Node);
>
> testStructPointer(&Node);
>
> After compiled and change it to .ll file, it looks like this:
>
> call void @testStruct(%struct.node* byval align 8 @Node), !dbg !160
>
> call void @testStructPointer(%struct.node* @Node), !dbg !161
>
> You can see that function named testStruction's parameter is passed by 
> value while the testStructPointer passed by pointer.
>
> I used callInst->getOperand(0)->getType()->dump(), it always return 
> the same %struct.node*.
>
> I want to know is there some properly library functions can be used to 
> get the precise parameters type?
>
> I want to know the functions arguments are passed by value or by pointer?

In LLVM, all parameters are passed by value unless they have the byval 
attribute.  There's probably a method in the Argument class that will 
tell you whether the argument has the byval attribute.  If it's not an 
attribute of the Argument class, it's probably part of the type of the 
Function.

Just out of curiosity, are you using the doxygen documentation?  The 
doxygen documentation is invaluable; if you have a basic understanding 
of how LLVM uses its class hierarchy to represent the IR, you can 
usually find what you need via the doxygen docs.

Regards,

John Criswell
>
> Any advice would be appreciate. Thanks a lot.:)
>
>
>
>
>
> _______________________________________________
> 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

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20160517/271668e1/attachment.html>