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

[mats petersson via llvm-dev]

On 17 May 2016 at 16:57, mats petersson <mats at planetcatfish.com> wrote:

>
>
> On 17 May 2016 at 15:45, John Criswell via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> 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.
>>
> I think you mean that they are "by reference unless they have a byval"
> attribute?
>
> The "ByVal" is an attribute on the call instruction.
>
> Here's where my compiler sets that:
> https://github.com/Leporacanthicus/lacsap/blob/master/expr.cpp#L1260
> (The "ByVal" attribute itself is stored in the attribute container on line
> 1235)
>

And also matching attributes in the function itself, see:
https://github.com/Leporacanthicus/lacsap/blob/master/expr.cpp#L1386

--
Mats

>
> --
> Mats
>
>>
>> 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 listllvm-dev at lists.llvm.orghttp://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>>
>> --
>> John Criswell
>> Assistant Professor
>> Department of Computer Science, University of Rochesterhttp://www.cs.rochester.edu/u/criswell
>>
>>
>> _______________________________________________
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>>
>>
>
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