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Compiling to WASM with llvm on macOS

howto-wasm-minimal by Zalka Ernő (my fork here) is a neat demo of a minimal WASM module. It uses C++ to define functions for simple image manipulation including blurring an image, compiles it to WASM using llvm/clang++, then uses JavaScript to run those functions against an image loaded into a <canvas> element.

I decided to try compiling it myself:

cd /tmp
git clone https://github.com/ern0/howto-wasm-minimal
cd howto-wasm-minimal
./build.sh

This gave me the following error:

error: unable to create target: 'No available targets are compatible with triple "wasm32"'

Searching for the error lead me to this comment:

You need to install llvm from Homebrew. Xcode's clang doesn't have support for WebAssembly.

So I installed llvm from Homebrew:

% brew install llvm
...
To use the bundled libc++ please add the following LDFLAGS:
  LDFLAGS="-L/usr/local/opt/llvm/lib -Wl,-rpath,/usr/local/opt/llvm/lib"

llvm is keg-only, which means it was not symlinked into /usr/local,
because macOS already provides this software and installing another version in
parallel can cause all kinds of trouble.

If you need to have llvm first in your PATH, run:
  echo 'export PATH="/usr/local/opt/llvm/bin:$PATH"' >> ~/.zshrc

For compilers to find llvm you may need to set:
  export LDFLAGS="-L/usr/local/opt/llvm/lib"
  export CPPFLAGS="-I/usr/local/opt/llvm/include"
...

I've quoted the relevant output. I'm not ready to permanently replace the system llvm with the one from Homebrew, so I ran the build script like this instead:

% PATH="/usr/local/opt/llvm/bin:$PATH" ./build.sh 
0000000 00 61 73 6d 01 00 00 00 01 14 04 60 00 00 60 01

That created a inc.wasm file in my current folder.

I tried opening index.html in Firefox and got the following error:

Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at file:///private/tmp/howto-wasm-minimal/inc.wasm. (Reason: CORS request not http).

So I ran a localhost web server instead using this Python one-liner:

% python3 -m http.server 8004
Serving HTTP on :: port 8004 (http://[::]:8004/) ...

This worked! http://localhost:8004/ displayed the working demo:

A blurry picture of a car

For reference, here's that build.sh script in full:

#!/bin/bash
set -e

clang++ \
	--target=wasm32 \
	-nostdlib \
	-O3 \
	-Wl,--no-entry \
	-Wl,--export-all \
	-Wl,--lto-O3 \
	-Wl,--allow-undefined \
	-Wl,--import-memory \
	-o inc.wasm \
	inc.cpp

hexdump inc.wasm | head -n 1
#wasm-objdump -x inc.wasm

Bonus: Web Assembly Binary Toolkit

The last commented-out line of that script caught my attention:

wasm-objdump -x inc.wasm

I searched, and wasm-objdump is part of the tools provided by the WebAssembly Binary Toolkit .

You can install that with Homebrew by running brew install wabt.

Then this worked:

% wasm-objdump -x inc.wasm

inc.wasm:	file format wasm 0x1

Section Details:

Type[4]:
 - type[0] () -> nil
 - type[1] (i32) -> i32
 - type[2] (i32, i32) -> nil
 - type[3] (i32, i32, i32) -> nil
Import[1]:
 - memory[0] pages: initial=2 <- env.memory
Function[7]:
 - func[0] sig=0 <__wasm_call_ctors>
 - func[1] sig=1 <inc>
 - func[2] sig=0 <incmem>
 - func[3] sig=2 <gray>
 - func[4] sig=2 <swaprg>
 - func[5] sig=3 <swap_red>
 - func[6] sig=2 <blur>
Global[7]:
 - global[0] i32 mutable=1 <__stack_pointer> - init i32=66560
 - global[1] i32 mutable=0 <__dso_handle> - init i32=1024
 - global[2] i32 mutable=0 <__data_end> - init i32=1024
 - global[3] i32 mutable=0 <__global_base> - init i32=1024
 - global[4] i32 mutable=0 <__heap_base> - init i32=66560
 - global[5] i32 mutable=0 <__memory_base> - init i32=0
 - global[6] i32 mutable=0 <__table_base> - init i32=1
Export[13]:
 - func[0] <__wasm_call_ctors> -> "__wasm_call_ctors"
 - func[1] <inc> -> "inc"
 - func[2] <incmem> -> "incmem"
 - func[3] <gray> -> "gray"
 - func[4] <swaprg> -> "swaprg"
 - func[5] <swap_red> -> "swap_red"
 - func[6] <blur> -> "blur"
 - global[1] -> "__dso_handle"
 - global[2] -> "__data_end"
 - global[3] -> "__global_base"
 - global[4] -> "__heap_base"
 - global[5] -> "__memory_base"
 - global[6] -> "__table_base"
Code[7]:
 - func[0] size=2 <__wasm_call_ctors>
 - func[1] size=7 <inc>
 - func[2] size=69 <incmem>
 - func[3] size=101 <gray>
 - func[4] size=215 <swaprg>
 - func[5] size=1332 <swap_red>
 - func[6] size=615 <blur>
Custom:
 - name: "name"
 - func[0] <__wasm_call_ctors>
 - func[1] <inc>
 - func[2] <incmem>
 - func[3] <gray>
 - func[4] <swaprg>
 - func[5] <swap_red>
 - func[6] <blur>
 - global[0] <__stack_pointer>
Custom:
 - name: "producers"

Also fun: wasm-decompile to "decompile a wasm binary into readable C-like syntax":

% wasm-decompile inc.wasm
import memory env_memory;

global stack_pointer:int = 66560;
export global dso_handle:int = 1024;
export global data_end:int = 1024;
export global global_base:int = 1024;
export global heap_base:int = 66560;
...

And wasm-opcodecnt to "count opcode usage for instructions":

% wasm-opcodecnt inc.wasm 
Total opcodes: 1188

Opcode counts:
local.get: 263
i32.const: 240
i32.add: 172
local.set: 88
...

More in the README: https://github.com/WebAssembly/wabt/blob/main/README.md