mfournial/openmp-wasm-poc

POC to Compile OpenMP code to Wasm without linking libomp

Clang OpenMP output to WASM Proof of Concept

This repo is a simple proof of concept that is it possible to take OpenMP code, and have a compiler doing the OpenMP to C translation according to the OpenMP specifications. And then compile the resulting code to Wasm without having to compile the actual libomp library to Wasm as well.

Requirements

• LLVM >8 (I'm running on 9 as you'll see later). My LLVM install is using the Ubuntu nightly repo (if you have Ubuntu and frequently find yourself downloading specific LLVM versions, this is great). But that means I've had to create the first simlink of the output below (clang-9 doesn't prepend -9 to linker invocation).

ls -l /usr/bin/wasm-*
>  /usr/bin/wasm-ld -> wasm-ld-9
>  /usr/bin/wasm-ld-9 -> ../lib/llvm-9/bin/wasm-ld

• emscripten
• wasi-sdk:
  • wasi-sdk called sysroot by clang
  • libclang rt: put it in the right place, for Ubuntu nightly:

sudo cp lib/wasi/libclang_rt.builtins-wasm32.a <path/to/llvm-9>/lib/clang/9.0.0/lib/wasi/libclang_rt.builtins-wasm32.a

• WABT: Wasm tooling (e.g. wasm2wat)
• WAVM, Wasmer (and Wasmtime)

POC

If you check the outputs folder, you'll see the resulting (faulty but existent Wasm modules) from the procedure described below.

Steps taken

We will be trying tp compile this minimum OpenMP example

Compiling OpenMP program to Wasm object files

Of course compiling straight away to Wasm fails because the linker cannot find the libomp runtime symbols.

SYSROOT=<path/to/wasi-sdk>
clang-9 --target=wasm32-wasi --sysroot=$SYSROOT -O2 -o easy.wasm -fopenmp easy.cpp
> wasm-ld: error: /tmp/easy-288a60.o: undefined symbol: __kmpc_fork_call

So let's instead compile to an object file and see exactly what is going on:

clang-9 --target=wasm32-unknwon-wasi --sysroot=$SYSROOT -O2 -fopenmp -c easy.cpp

llvm-nm-9 easy.o
> 00000000 d .L.str
> 00000018 d .L__unnamed_1
> 00000052 t .omp_outlined.
>             U __kmpc_fork_call
> 00000001 T __original_main
>             U __stack_pointer
> 00000062 T main

Compiling a fake library to Wasm object files

This is really the hello world of OpenMP it seems, there is only one function call to provide (__kmpc_fork_call). Let's see what this function is in the LLVM codebase:

llvm-project/openmp > git grep -n "void.*kmpc_fork_call"                                                                                                                    
kmp.h:3614:KMP_EXPORT void __kmpc_fork_call(ident_t *, kmp_int32 nargs,
kmp_csupport.cpp:265:void __kmpc_fork_call(ident_t *loc, kmp_int32 argc, kmpc_micro microtask, ...) {

Umm this is a variadic function with many complex types. I renounced to import the header with the proper typedefs in the mini-library for now. It was importing too many things that would have cluttered the output anyway.
Instead, I checked the linker errors to so how many arguments the variadic fork call was taking and which types they were. It gave me something of the form fork(i32, i32, i32, i32) so I just went for 4 ints like you can see in the mini-runtime.c file and compiled the object file:

clang-9 --target=wasm32-unknwon-wasi --sysroot=$SYSROOT -O2 -c mini-runtime.c

Linked everything together

wasm-ld-9 easy.o mini-runtime.o --no-entry -o test.wasmr

Which produces

wasm2wat test.wasm
> (module
>   (table (;0;) 1 1 funcref)
>   (memory (;0;) 2)
>   (global (;0;) (mut i32) (i32.const 66560))
>   (export "memory" (memory 0)))

To get an emscriptem version, which unlike the first one will run to a segfa:

emcc easy.o mini-runtime.o -s EMIT_EMSCRIPTEN_METADATA=1 -o viaem.wasm

is not "correct" in terms of OpenMP but valid Wasm output (passes the Wasm validation) which was the simple goal here! The wasm files are included in this repo if you can't reproduce the steps above but want to look at how they act on WAVM, Wastime etc. it's interesting for the least.

I'll need to look at why the Wasm code is being trapped.