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.