Passing struct by value in static compiling

General Usage
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1

We know in Julia structs are passed by reference, and this is also the case in StaticCompiler.jl:

using StaticTools
using StaticCompiler

struct StudentInfo 
    name::StaticString
    id::Cint
end

id(student::StudentInfo) = student.id

compile_shlib(id, (StudentInfo,))

# The following line is wrong: Julia functions only accept pointers to structs 
# It likely gives you some nonsense like 1461676824
@ccall "./id.so".id(StudentInfo(c"Lucas", 14)::StudentInfo)::Cint

# The following line is right
@ccall "./id.so".id(Ref(StudentInfo(c"Lucas", 14))::Ref{StudentInfo})::Cint

Now suppose I’m to write a library that is static enough to be compiled by StaticCompiler and might be called by non-Julia codes, like a C++ project. Suppose I want to pass a 3D vector (a composite type, but a small one) to a function in the library. In Julia this can be easily done as in my_function(SVector{3, Float64}(1.0, 2.0, 3.0)); in C/C++, however, you can’t write my_function(Point{1.0, 2.0, 3.0}) where Point is appropriately defined: you have to pass a pointer to my_function, which makes the code verbose.

Is there any way to let a Julia function recognize an incoming immutable composite type variable as a value, not a pointer? If this is not supported currently (it seems to be not supported), is there any inherent obstruction that prevents this from being done in the future?

2

Are they? There are many mentions of passing by value here.

3

What you mentioned here is the behavior of ccall, and in @ccall "my-library.so".my_function(somet_struct), some_struct is indeed passed by value. But this is calling C function from Julia: when a Julia function is defined, it seems to accept a pointer to the struct being passed.

This can also be directly verified by @code_llvm:


using StaticArrays

function first(s::SVector{3, Int})
s[1]
end

arr = SVector{3, Int}([1, 2, 3])
@code_llvm first(arr)



; @ REPL[9]:1 within `first`
define i64 @julia_first_909([1 x [3 x i64]]* nocapture noundef nonnull readonly align 8 dereferenceable(24) %0) #0 {
top:
; @ REPL[9]:2 within `first`
; ┌ @ /home/jinyuan/.julia/packages/StaticArrays/yXGNL/src/SArray.jl:62 within `getindex` @ tuple.jl:29
%1 = getelementptr inbounds [1 x [3 x i64]], [1 x [3 x i64]]* %0, i64 0, i64 0, i64 0
; └
%2 = load i64, i64* %1, align 8
ret i64 %2
}

Note that * after [1 x [3 x i64]], which is SVector{3, Int64} (a tuple of three 64bit integers in a struct).

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