Support wasm exception handling for Emscripten target

[hoodmane]

Currently the Rust Emscripten target supports js exception handling. I've been trying to get it to support also wasm exception handling. The patch to get it to exclusively use wasm exception handling is quite small. The following suffices:

Patch

diff --git a/compiler/rustc_codegen_llvm/src/llvm_util.rs b/compiler/rustc_codegen_llvm/src/llvm_util.rs
index a0a640473eb..cd88648911a 100644
--- a/compiler/rustc_codegen_llvm/src/llvm_util.rs
+++ b/compiler/rustc_codegen_llvm/src/llvm_util.rs
@@ -110,7 +110,7 @@ fn llvm_arg_to_arg_name(full_arg: &str) -> &str {
         }
 
         if sess.target.os == "emscripten" && sess.panic_strategy() == PanicStrategy::Unwind {
-            add("-enable-emscripten-cxx-exceptions", false);
+            add("-wasm-enable-sjlj", false);
         }
 
         // HACK(eddyb) LLVM inserts `llvm.assume` calls to preserve align attributes
diff --git a/compiler/rustc_codegen_ssa/src/back/link.rs b/compiler/rustc_codegen_ssa/src/back/link.rs
index 94acdea894b..2909401836d 100644
--- a/compiler/rustc_codegen_ssa/src/back/link.rs
+++ b/compiler/rustc_codegen_ssa/src/back/link.rs
@@ -2057,12 +2057,11 @@ fn add_order_independent_options(
     }
 
     if sess.target.os == "emscripten" {
-        cmd.arg("-s");
-        cmd.arg(if sess.panic_strategy() == PanicStrategy::Abort {
-            "DISABLE_EXCEPTION_CATCHING=1"
+        if sess.panic_strategy() == PanicStrategy::Abort {
+            cmd.arg("-sDISABLE_EXCEPTION_CATCHING=1");
         } else {
-            "DISABLE_EXCEPTION_CATCHING=0"
-        });
+            cmd.arg("-fwasm-exceptions");
+        }
     }
 
     if flavor == LinkerFlavor::PtxLinker {

I'm not sure what the best way is to add support for a choice between these two options.

@aheejin Is there any way to turn -enable-emscripten-cxx-exceptions back off by adding another argument? It would be convenient to be able to inject -mllvm -disable-emscripten-cxx-exceptions -mllvm -wasm-enable-sjlj and get wasm exceptions to work...

[hoodmane]

If a target has more than one stack unwinding ABI is there some standard way to switch between them?

[hoodmane]

Ah it seems that the following may work to disable js exceptions and enable wasm ones:

	-C link-arg=-fwasm-exceptions \
	-C link-arg=-sDISABLE_EXCEPTION_CATCHING=1 \
	-C llvm-args=-enable-emscripten-cxx-exceptions=0 \
    -C llvm-args=-wasm-enable-sjlj 

[Jules-Bertholet]

@rustbot label O-wasm

[rustbot]

Error: Label o-wasm can only be set by Rust team members

Please file an issue on GitHub at triagebot if there's a problem with this bot, or reach out on #t-infra on Zulip.

[aheejin]

@aheejin Is there any way to turn -enable-emscripten-cxx-exceptions back off by adding another argument? It would be convenient to be able to inject -mllvm -disable-emscripten-cxx-exceptions -mllvm -wasm-enable-sjlj and get wasm exceptions to work...

Which tools do you call? clang? And the option you want to use is either -enable-emscripten-cxx-exceptions -mllvm -enable-emscripten-sjlj (for Emscripten (JS) exceptions/SjLj) or -fwasm-exceptions -mllvm -wasm-enable-sjlj (for Wasm exceptions/SjLj). In which case do you need -disable-emscripten-cxx-exceptions? Only specifying -fwasm-exceptions will enable Wasm EH and disable Emscripten EH, and specifying -enable-emscripten-cxx-exceptions will enable Emscripten EH and disable Wasm EH. If you give both -enable-emscripten-cxx-exceptions and -fwasm-exceptions at the same time it will error out.

[hoodmane]

The issue is that Rust always adds -enable-emscripten-cxx-exceptions, so if I want to use wasm-exceptions I have to turn it back off somehow. But -enable-emscripten-cxx-exceptions=0 seems to do that... at least some of the time.

[hoodmane]

I made a bit more progress over here:
emscripten-core/emscripten#19612

It seems that we need to adjust the ir generated for the try intrinsic and I have a guess of what the IR should be. But I would probably need help from someone who understands rustc_codegen_llvm a bit better.

[purplesyringa]

Are there any plans on how wasm vs JS exceptions are going to be configured? (-Z emscripten_wasm_eh is funny, but I suppose there's going to be a -C option?) Would this option be readable via a cfg?

[hoodmane]

Well if you look at the MCP and [the PR] (#131830) you'll see in the discussion that we aren't really thinking about stabilizing it to a -C flag but eventually turning it on always and then later removing the option to turn it off. I think we'll need more discussion before each of these changes though. Do you expect to need the legacy exception handling going forward?

There is a cfg flag for it but you have to enable an internal feature to access it. I'm not sure if you can enable internal features outside the standard library though.

[purplesyringa]

I do not necessarily need legacy exception handling myself, but I'm maintaining a crate that needs to use different code for different EH ABIs, hence I'm interested in knowing what the way forward is going to be so that I can update the code preemptively. I understand this whole crate idea is based on thin ice, so I'll just hardcode cfg(emscripten_wasm_eh) for the time being, but if there's a public cfg, I'll happily use it instead.

[hoodmane]

Cool! I appreciate that you are making the effort to support emscripten.

[hoodmane]

@workingjubilee should we close this as resolved via the -Zemscripten-wasm-eh flag?

[purplesyringa]

Wasm EH is still utterly broken, see e.g. #135665. Please don't.