How to link C++ code through the new compiler in V12?

Question

The Advanced Compilation Topics talk in the WTC 2018 shows that one can easily link C++ code through the new compiler technology. This feature seems very useful as one can easily load existing C++ code into Mathematica without extensive modifications.

I followed the instruction but unable to make it work:

Needs["Compile`"]

Create the C++ source file

code="extern \"C\"
double square(double a)
{return a * a;}
";
WriteString["/tmp/e.cpp",code]

Compile the C++ code to llvm

Run["clang -S -Wno-everything -O3 -o /tmp/e.ll -emit-llvm -march=skylake /tmp/e.cpp"]

Import the llvm

llvmStr = Import["/tmp/e.ll", "Text"];
prog = Program[{
    LLVMString[llvmStr],
    Function[{Typed[arg1, "Real64"]}, 
     Native`PrimitiveFunction["square"][arg1]]
    }];

Compile to external program

fun = CompileToExternalFunction[prog]

I'm getting an error

Compile: CompilerException. Type kind is not handled

I'm using Mathematica V12 on ubuntu 16.04.

Any ideas about the error and the correct way to compile?

Answer 1

Too long for a comment, but this (undocumented) syntax works in version 12.3 with a minor modification and taking into account a renamed function, for example:

Needs["Compile`"]

llvmStr = "define double @square(double %0) local_unnamed_addr #0 {
  %2 = fmul double %0, %0
  ret double %2
}";

fun = CompileToCodeFunction[
   Program[{LLVMString[llvmStr]}, 
    Function[{Typed[arg1, "Real64"]}, 
     Native`PrimitiveFunction["square"][arg1]]]];

fun[12.]

(* 144. *)

For creating a DLL:

CompileToLibrary[
 Program[{LLVMString[llvmStr]}, 
  Function[{Typed[arg1, "Real64"]}, 
   Native`PrimitiveFunction["square"][arg1]]], 
 "LibraryName" -> "square"]