Communicate exceptions through global memory

Instead of allocating space on the stack and returning a pointer we
should be able to use a single global memory location to communicate
this error payload information. This shouldn't run into any reentrancy
issues since it's only stored just before returning to wasm and it's
always read just after returning from wasm.

Author: alexcrichton

crates/backend/src/codegen.rs

@@ -974,28 +974,12 @@ impl TryToTokens for ast::ImportFunction {
         }
 
         let mut exceptional_ret = quote!();
-        let exn_data = if self.catch {
-            let exn_data = Ident::new("exn_data", Span::call_site());
-            let exn_data_ptr = Ident::new("exn_data_ptr", Span::call_site());
-            abi_argument_names.push(exn_data_ptr.clone());
-            abi_arguments.push(quote! { #exn_data_ptr: *mut u32 });
+        if self.catch {
             convert_ret = quote! { Ok(#convert_ret) };
             exceptional_ret = quote! {
-                if #exn_data[0] == 1 {
-                    return Err(
-                        <
-                            wasm_bindgen::JsValue as wasm_bindgen::convert::FromWasmAbi
-                        >::from_abi(#exn_data[1], &mut wasm_bindgen::convert::GlobalStack::new())
-                    )
-                }
+                wasm_bindgen::__rt::take_last_exception()?;
             };
-            quote! {
-                let mut #exn_data = [0; 2];
-                let #exn_data_ptr = #exn_data.as_mut_ptr();
-            }
-        } else {
-            quote!()
-        };
+        }
 
         let rust_name = &self.rust_name;
         let import_name = &self.shim;
@@ -1055,7 +1039,6 @@ impl TryToTokens for ast::ImportFunction {
                 #extern_fn
 
                 unsafe {
-                    #exn_data
                     let #ret_ident = {
                         let mut __stack = wasm_bindgen::convert::GlobalStack::new();
                         #(#arg_conversions)*

crates/cli-support/src/js/mod.rs

@@ -1419,27 +1419,23 @@ impl<'a> Context<'a> {
         if !self.should_write_global("handle_error") {
             return Ok(());
         }
-        self.expose_uint32_memory();
+        self.require_internal_export("__wbindgen_exn_store")?;
         if self.config.anyref {
             self.expose_add_to_anyref_table()?;
             self.global(
                 "
-                function handleError(exnptr, e) {
+                function handleError(e) {
                     const idx = addToAnyrefTable(e);
-                    const view = getUint32Memory();
-                    view[exnptr / 4] = 1;
-                    view[exnptr / 4 + 1] = idx;
+                    wasm.__wbindgen_exn_store(idx);
                 }
                 ",
             );
         } else {
             self.expose_add_heap_object();
             self.global(
                 "
-                function handleError(exnptr, e) {
-                    const view = getUint32Memory();
-                    view[exnptr / 4] = 1;
-                    view[exnptr / 4 + 1] = addHeapObject(e);
+                function handleError(e) {
+                    wasm.__wbindgen_exn_store(addHeapObject(e));
                 }
                 ",
             );
@@ -2115,7 +2111,7 @@ impl<'a> Context<'a> {
 
     fn export_function_table(&mut self) -> Result<(), Error> {
         if !self.should_write_global("wbg-function-table") {
-            return Ok(())
+            return Ok(());
         }
         let id = match self.module.tables.main_function_table()? {
             Some(id) => id,
@@ -2192,7 +2188,7 @@ impl ExportedClass {
         field: &str,
         js: &str,
         prefix: &str,
-        ret_ty: &str
+        ret_ty: &str,
     ) -> &mut bool {
         self.contents.push_str(docs);
         self.contents.push_str(prefix);
@@ -2215,7 +2211,8 @@ impl ExportedClass {
 /// generated output is deterministic and we do so by ensuring that iteration of
 /// hash maps is consistently sorted.
 fn sorted_iter<K, V>(map: &HashMap<K, V>) -> impl Iterator<Item = (&K, &V)>
-    where K: Ord,
+where
+    K: Ord,
 {
     let mut pairs = map.iter().collect::<Vec<_>>();
     pairs.sort_by_key(|(k, _)| *k);

crates/cli-support/src/js/rust2js.rs

@@ -928,7 +928,7 @@ impl<'a, 'b> Rust2Js<'a, 'b> {
                 try {{\n\
                     {}
                 }} catch (e) {{\n\
-                    handleError(exnptr, e);\n\
+                    handleError(e);\n\
                 }}\
                 ",
                 &invoc
@@ -973,12 +973,6 @@ impl<'a, 'b> Rust2Js<'a, 'b> {
         let mut ret = String::new();
         ret.push_str("function(");
         ret.push_str(&self.shim_arguments.join(", "));
-        if self.catch {
-            if self.shim_arguments.len() > 0 {
-                ret.push_str(", ")
-            }
-            ret.push_str("exnptr");
-        }
         ret.push_str(") {\n");
         ret.push_str(&self.prelude);
 

src/lib.rs

@@ -1067,6 +1067,28 @@ pub mod __rt {
     pub fn link_mem_intrinsics() {
         crate::anyref::link_intrinsics();
     }
+
+    static mut GLOBAL_EXNDATA: [u32; 2] = [0; 2];
+
+    #[no_mangle]
+    pub unsafe extern "C" fn __wbindgen_exn_store(idx: u32) {
+        assert_eq!(GLOBAL_EXNDATA[0], 0);
+        GLOBAL_EXNDATA[0] = 1;
+        GLOBAL_EXNDATA[1] = idx;
+    }
+
+    pub fn take_last_exception() -> Result<(), super::JsValue> {
+        unsafe {
+            let ret = if GLOBAL_EXNDATA[0] == 1 {
+                Err(super::JsValue:: _new(GLOBAL_EXNDATA[1]))
+            } else {
+                Ok(())
+            };
+            GLOBAL_EXNDATA[0] = 0;
+            GLOBAL_EXNDATA[1] = 0;
+            return ret;
+        }
+    }
 }
 
 /// A wrapper type around slices and vectors for binding the `Uint8ClampedArray`