Add support for new exception handling proposal

The exception handling proposal switched its design from
an exnref-based approach with first-class exception reference
values to a version with tagged catch blocks and implicit
exception references.

This commit adds support for this new semantics and revises
tests to match.

It does not yet remove exnref, as that can be done in a separate
patch.

Note: it does not add the `delegate` instruction in the new
proposal yet, as it does not yet have an opcode assigned.

Author: takikawa

crates/wasmparser/src/binary_reader.rs

@@ -1101,15 +1101,16 @@ impl<'a> BinaryReader<'a> {
             0x06 => Operator::Try {
                 ty: self.read_blocktype()?,
             },
-            0x07 => Operator::Catch,
+            0x07 => Operator::Catch {
+                index: self.read_var_u32()?,
+            },
             0x08 => Operator::Throw {
                 index: self.read_var_u32()?,
             },
-            0x09 => Operator::Rethrow,
-            0x0a => Operator::BrOnExn {
+            0x09 => Operator::Rethrow {
                 relative_depth: self.read_var_u32()?,
-                index: self.read_var_u32()?,
             },
+            0x0a => Operator::Unwind,
             0x0b => Operator::End,
             0x0c => Operator::Br {
                 relative_depth: self.read_var_u32()?,

crates/wasmparser/src/operators_validator.rs

@@ -112,6 +112,8 @@ enum FrameKind {
     Loop,
     Try,
     Catch,
+    CatchAll,
+    Unwind,
 }
 
 impl OperatorValidator {
@@ -567,10 +569,23 @@ impl OperatorValidator {
             }
             Operator::Else => {
                 let frame = self.pop_ctrl(resources)?;
-                if frame.kind != FrameKind::If {
-                    bail_op_err!("else found outside of an `if` block");
+                // The `catch_all` instruction shares an opcode with `else`,
+                // so we check the frame to see how it's interpreted.
+                if self.features.exceptions
+                    && (frame.kind == FrameKind::Try || frame.kind == FrameKind::Catch)
+                {
+                    self.control.push(Frame {
+                        kind: FrameKind::CatchAll,
+                        block_type: frame.block_type,
+                        height: self.operands.len(),
+                        unreachable: false,
+                    });
+                } else {
+                    if frame.kind != FrameKind::If {
+                        bail_op_err!("else found outside of an `if` block");
+                    }
+                    self.push_ctrl(FrameKind::Else, frame.block_type, resources)?;
                 }
-                self.push_ctrl(FrameKind::Else, frame.block_type, resources)?;
             }
             Operator::Try { ty } => {
                 self.check_exceptions_enabled()?;
@@ -580,10 +595,10 @@ impl OperatorValidator {
                 }
                 self.push_ctrl(FrameKind::Try, ty, resources)?;
             }
-            Operator::Catch => {
+            Operator::Catch { index } => {
                 self.check_exceptions_enabled()?;
                 let frame = self.pop_ctrl(resources)?;
-                if frame.kind != FrameKind::Try {
+                if frame.kind != FrameKind::Try && frame.kind != FrameKind::Catch {
                     bail_op_err!("catch found outside of an `try` block");
                 }
                 // Start a new frame and push `exnref` value.
@@ -593,7 +608,11 @@ impl OperatorValidator {
                     height: self.operands.len(),
                     unreachable: false,
                 });
-                self.push_operand(Type::ExnRef)?;
+                // Push exception argument types.
+                let ty = func_type_at(&resources, index)?;
+                for ty in ty.inputs() {
+                    self.push_operand(ty)?;
+                }
             }
             Operator::Throw { index } => {
                 self.check_exceptions_enabled()?;
@@ -607,24 +626,30 @@ impl OperatorValidator {
                 }
                 self.unreachable();
             }
-            Operator::Rethrow => {
+            Operator::Rethrow { relative_depth } => {
                 self.check_exceptions_enabled()?;
-                self.pop_operand(Some(Type::ExnRef))?;
+                // This is not a jump, but we need to check that the `rethrow`
+                // targets an actual `catch` to get the exception.
+                let (_, kind) = self.jump(relative_depth)?;
+                if kind != FrameKind::Catch && kind != FrameKind::CatchAll {
+                    bail_op_err!("rethrow target was not a `catch` block");
+                }
                 self.unreachable();
             }
-            Operator::BrOnExn {
-                relative_depth,
-                index,
-            } => {
+            Operator::Unwind => {
                 self.check_exceptions_enabled()?;
-                let (ty, kind) = self.jump(relative_depth)?;
-                self.pop_operand(Some(Type::ExnRef))?;
-                // Check the exception's argument values with target block's.
-                let exn_args = func_type_at(&resources, index)?;
-                if Iterator::ne(exn_args.inputs(), label_types(ty, resources, kind)?) {
-                    bail_op_err!("target block types do not match");
+                // Switch from `try` to an `unwind` frame, so we can check that
+                // the result type is empty.
+                let frame = self.pop_ctrl(resources)?;
+                if frame.kind != FrameKind::Try {
+                    bail_op_err!("unwind found outside of an `try` block");
                 }
-                self.push_operand(Type::ExnRef)?;
+                self.control.push(Frame {
+                    kind: FrameKind::Unwind,
+                    block_type: TypeOrFuncType::Type(Type::EmptyBlockType),
+                    height: self.operands.len(),
+                    unreachable: false,
+                });
             }
             Operator::End => {
                 let mut frame = self.pop_ctrl(resources)?;

crates/wasmparser/src/primitives.rs

@@ -347,10 +347,10 @@ pub enum Operator<'a> {
     If { ty: TypeOrFuncType },
     Else,
     Try { ty: TypeOrFuncType },
-    Catch,
+    Catch { index: u32 },
     Throw { index: u32 },
-    Rethrow,
-    BrOnExn { relative_depth: u32, index: u32 },
+    Rethrow { relative_depth: u32 },
+    Unwind,
     End,
     Br { relative_depth: u32 },
     BrIf { relative_depth: u32 },

crates/wasmprinter/src/lib.rs

@@ -665,7 +665,7 @@ impl Printer {
                     // `else`/`catch` are special in that it's printed at
                     // the previous indentation, but it doesn't actually change
                     // our nesting level.
-                    Operator::Else | Operator::Catch => {
+                    Operator::Else | Operator::Catch { .. } | Operator::Unwind => {
                         self.nesting -= 1;
                         self.newline();
                         self.nesting += 1;
@@ -729,24 +729,21 @@ impl Printer {
                 self.print_blockty(ty)?;
                 write!(self.result, "  ;; label = @{}", cur_label)?;
             }
-            Catch => self.result.push_str("catch"),
+            Catch { index } => {
+                write!(self.result, "catch {}", index)?;
+            }
             Throw { index } => {
                 write!(self.result, "throw {}", index)?;
             }
-            Rethrow => self.result.push_str("rethrow"),
-            BrOnExn {
-                relative_depth,
-                index,
-            } => {
+            Rethrow { relative_depth } => {
                 write!(
                     self.result,
-                    "br_on_exn {} (;{};)",
+                    "rethrow {} (;{};)",
                     relative_depth,
-                    label(*relative_depth),
+                    label(*relative_depth)
                 )?;
-                write!(self.result, " {}", index)?;
             }
-
+            Unwind => self.result.push_str("unwind"),
             End => self.result.push_str("end"),
             Br { relative_depth } => {
                 write!(

crates/wast/src/ast/expr.rs

@@ -84,7 +84,9 @@ enum If<'a> {
 enum Try<'a> {
     /// Next thing to parse is the `do` block.
     Do(Instruction<'a>),
-    /// Next thing to parse is the `catch` block.
+    /// Next thing to parse is `catch`/`catch_all`, or `unwind`.
+    CatchOrUnwind,
+    /// Next thing to parse is a `catch` block or `catch_all`.
     Catch,
     /// This `try` statement has finished parsing and if anything remains it's a
     /// syntax error.
@@ -195,8 +197,8 @@ impl<'a> ExpressionParser<'a> {
                     Level::Try(Try::Do(_)) => {
                         return Err(parser.error("previous `try` had no `do`"));
                     }
-                    Level::Try(Try::Catch) => {
-                        return Err(parser.error("previous `try` had no `catch`"));
+                    Level::Try(Try::CatchOrUnwind) => {
+                        return Err(parser.error("previous `try` had no `catch`, `catch_all`, or `unwind`"));
                     }
                     Level::Try(_) => {
                         self.instrs.push(Instruction::End(None));
@@ -305,7 +307,7 @@ impl<'a> ExpressionParser<'a> {
     /// than an `if` as the syntactic form is:
     ///
     /// ```wat
-    /// (try (do $do) (catch $catch))
+    /// (try (do $do) (catch $event $catch))
     /// ```
     ///
     /// where the `do` and `catch` keywords are mandatory, even for an empty
@@ -328,7 +330,7 @@ impl<'a> ExpressionParser<'a> {
             if parser.parse::<Option<kw::r#do>>()?.is_some() {
                 // The state is advanced here only if the parse succeeds in
                 // order to strictly require the keyword.
-                *i = Try::Catch;
+                *i = Try::CatchOrUnwind;
                 self.stack.push(Level::TryArm);
                 return Ok(true);
             }
@@ -338,17 +340,50 @@ impl<'a> ExpressionParser<'a> {
             return Ok(false);
         }
 
-        // `catch` handled similar to `do`, including requiring the keyword.
-        if let Try::Catch = i {
-            self.instrs.push(Instruction::Catch);
+        // After a try's `do`, there are several possible kinds of handlers.
+        if let Try::CatchOrUnwind = i {
+            // `catch` may be followed by more `catch`s or `catch_all`.
             if parser.parse::<Option<kw::catch>>()?.is_some() {
+                let evt = parser.parse::<ast::Index<'a>>()?;
+                self.instrs.push(Instruction::Catch(evt));
+                *i = Try::Catch;
+                self.stack.push(Level::TryArm);
+                return Ok(true);
+            }
+            // `catch_all` can only come at the end and has no argument.
+            if parser.parse::<Option<kw::catch_all>>()?.is_some() {
+                self.instrs.push(Instruction::CatchAll);
+                *i = Try::End;
+                self.stack.push(Level::TryArm);
+                return Ok(true);
+            }
+            // `unwind` is similar to `catch_all`.
+            if parser.parse::<Option<kw::unwind>>()?.is_some() {
+                self.instrs.push(Instruction::Unwind);
                 *i = Try::End;
                 self.stack.push(Level::TryArm);
                 return Ok(true);
             }
             return Ok(false);
         }
 
+        if let Try::Catch = i {
+            if parser.parse::<Option<kw::catch>>()?.is_some() {
+                let evt = parser.parse::<ast::Index<'a>>()?;
+                self.instrs.push(Instruction::Catch(evt));
+                *i = Try::Catch;
+                self.stack.push(Level::TryArm);
+                return Ok(true);
+            }
+            if parser.parse::<Option<kw::catch_all>>()?.is_some() {
+                self.instrs.push(Instruction::CatchAll);
+                *i = Try::End;
+                self.stack.push(Level::TryArm);
+                return Ok(true);
+            }
+            return Err(parser.error("unexpected items after `catch`"));
+        }
+
         Err(parser.error("too many payloads inside of `(try)`"))
     }
 }
@@ -997,11 +1032,12 @@ instructions! {
         V128Load64Zero(MemArg<8>) : [0xfd, 0xfd] : "v128.load64_zero",
 
         // Exception handling proposal
+        CatchAll : [0x05] : "catch_all", // Reuses the else opcode.
         Try(BlockType<'a>) : [0x06] : "try",
-        Catch : [0x07] : "catch",
+        Catch(ast::Index<'a>) : [0x07] : "catch",
         Throw(ast::Index<'a>) : [0x08] : "throw",
-        Rethrow : [0x09] : "rethrow",
-        BrOnExn(BrOnExn<'a>) : [0x0a] : "br_on_exn",
+        Rethrow(ast::Index<'a>) : [0x09] : "rethrow",
+        Unwind : [0x0a] : "unwind",
     }
 }
 

crates/wast/src/ast/mod.rs

@@ -346,6 +346,7 @@ pub mod kw {
     custom_keyword!(binary);
     custom_keyword!(block);
     custom_keyword!(catch);
+    custom_keyword!(catch_all);
     custom_keyword!(code);
     custom_keyword!(data);
     custom_keyword!(declare);
@@ -416,6 +417,7 @@ pub mod kw {
     custom_keyword!(table);
     custom_keyword!(then);
     custom_keyword!(r#try = "try");
+    custom_keyword!(unwind);
     custom_keyword!(v128);
 }
 

crates/wast/src/resolve/names.rs

@@ -1950,10 +1950,13 @@ impl<'a, 'b> ExprResolver<'a, 'b> {
             Throw(i) => {
                 self.module.resolve(i, Ns::Event)?;
             }
-            BrOnExn(b) => {
-                self.resolve_label(&mut b.label)?;
-                self.module.resolve(&mut b.exn, Ns::Event)?;
+            Rethrow(i) => {
+                self.resolve_label(i)?;
             }
+            Catch(i) => {
+                self.module.resolve(i, Ns::Event)?;
+            }
+
             BrOnCast(b) => {
                 self.resolve_label(&mut b.label)?;
                 self.module.resolve_heaptype(&mut b.val)?;

tests/local/exception-handling.wast

@@ -1,29 +1,57 @@
 ;; --enable-exceptions --enable-multi-value
 (module 
   (type (func (param i32 i64)))
+  (type (func (param i32)))
   (event (type 0))
-  (func $check-br_on_exn-rethrow (param exnref)
-    block $l (result i32 i64)
-        local.get 0
-        ;; exnref $e is on the stack at this point
-        br_on_exn $l 0 ;; branch to $l with $e's arguments
-        rethrow
-    end
-    drop
-    drop
-  )
+  (event (type 1))
   (func $check-throw
     i32.const 1
     i64.const 2
     throw 0
   )
-  (func $check-try-w-calls (result i32)
-    try (result i32)
+  (func $check-try-catch-rethrow
+    try (result i32 i64)
       call $check-throw
-      i32.const 0
-    catch
-      call $check-br_on_exn-rethrow
+      unreachable
+    catch 0
+      ;; the exception arguments are on the stack at this point
+    catch 1
+      i64.const 2
+    catch_all
+      rethrow 0
+    end
+    drop
+    drop
+  )
+  (func $check-unwind (local i32)
+    try
       i32.const 1
+      local.set 0
+      call $check-throw
+    unwind
+      i32.const 0
+      local.set 0
     end
   )
 )
+
+(assert_invalid
+  (module
+    (func try catch_all catch_all end))
+  ;; we can't distinguish between `catch_all` and `else` in error cases
+  "else found outside of an `if` block")
+
+(assert_invalid
+  (module
+    (func try catch_all catch 0 end))
+  "catch found outside of an `try` block")
+
+(assert_invalid
+  (module
+    (func try unwind i32.const 1 end))
+  "type mismatch: values remaining on stack at end of block")
+
+(assert_invalid
+  (module
+    (func block try catch_all rethrow 1 end end))
+  "rethrow target was not a `catch` block")