Question

Is it possible to pass structure by parameter ?

Is it compatible with the C abi ?

[edit]

Basically, I would like to have a C++ POD which would contain two members (the structure would be a fat pointer, with a pointer and an integer), and be able to pass this structure as function parameter in call instructions (even when calling C code).

I’m not using fat pointer now (the pointer and the integer are each in a different function parameter), and I would like to know if it’s possible before starting a pretty big refactoring !

Answer 1

You can do this.

You can figure out what the LLVM code is for sample C by copying and pasting the C code into LLVM’s online demo at http://llvm.org/demo/index.cgi.

If you copy and paste the code at codepad.org in, you’ll see that LLVM generates the following for myFunction:

define void @_Z10myFunction10MyStruct_t(i8* %myStructAsParam.coerce0, i32     %myStructAsParam.coerce1) nounwind uwtable {
  %1 = tail call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([23 x i8]* @.str, i64 0, i64 0), i8* %myStructAsParam.coerce0, i32 %myStructAsParam.coerce1)
  ret void
}

Of course, if you look at the call you’ll notice that no copy is being made. It’s up to the calling function to do that. If we write a small C function:

void myCallingFunction(MyStruct_t *foobar)
{
  myFunction(*foobar);
}

We can see that the LLVM bitcode generated for myCallingFunction is:

define void @_Z17myCallingFunctionP10MyStruct_t(%struct.MyStruct_t* nocapture %foobar)   nounwind uwtable {
  %foobar.0 = getelementptr inbounds %struct.MyStruct_t* %foobar, i64 0, i32 0
  %tmp = load i8** %foobar.0, align 8
  %foobar.1 = getelementptr inbounds %struct.MyStruct_t* %foobar, i64 0, i32 1
  %tmp1 = load i32* %foobar.1, align 8
  %1 = tail call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([23 x i8]* @.str, i64 0, i64 0), i8* %tmp, i32 %tmp1) nounwind
  ret void
}

The calling function makes a copy of the struct, and then passes in the address of the copy.