Compiling to LLVM IR

[Yugesh Kothari]

Hi,

I am trying to look for a way to compile C/C++ source written with emsripten headers to LLVM IR (.ll or .bc) and then subsequently use smack (https://github.com/smackers/smack) to get it to boogie. I use the command :

emsdk/upstream/bin/clang++ --target=wasm64 `em++ --cflags` -emit-llvm -S <file>.cpp -o <file>.ll

which runs. But then when I try to use smack I get this error:
"Exception: Problem reading input bitcode/IR: Invalid record".

I run into the same problem if I try to compile down the above llvm to an executable using llc.

I suspect this has something to do with how I originally compiled my C++ source. Can someone suggest what is the "right" way to get LLVM IR and .js+.wasm instead of just the latter?

Thanks!

[Thomas Lively]

What version of LLVM does Smack support? Since Emscripten uses tip-of-tree LLVM, my guess is that the LLVM IR you're getting is too new for Smack to read. It's hard to say from just that error message, though.

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[Yugesh Kothari]

Ah yes. This could be the reason as well.
smack uses LLVM 10.0.1 and i just saw that my emsdk has llvm version 12.0.0git.

How can I install emsdk for a specific version of LLVM? Are particular versions of the sdk tied to particular versions of LLVM?

[Sam Clegg]

Firstly you won't want to be using `--target=wasm64` at all right now, it is experimental and not supported by any engines yet.    Lucky `em++ --cflags` is going to contain its own `--target=wasm32-unknown-emscripten` so that is probably unrelated to your current failure.

Secondly, if you want emcc to produce bitcode you can simply add `-emit-llvm` to you normal emcc command:

  em++ -c -emit-llvm -S <file>.cpp -o <file>.ll

or 

  em++ -c -emit-llvm <file>.cpp -o <file>.bc

If you want to combine the bitcode files into a single large bitcode file we do also still have legacy support for that via the `-r` + `-flto` flags.

  em++ -r -flto  <file1>.bc <file2>.bc -o whole-program.bc

But this kind of bitcode linking has some caveats and I would not recommend it.  Avoid it if you can.

cheers,

sam

On Tue, Nov 3, 2020 at 4:36 PM Yugesh Kothari <kothar...@gmail.com> wrote:

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