From d99783fc91e1ad9f11f83a0ecb02c227f522d85e Mon Sep 17 00:00:00 2001
From: Trevor Elliott <telliott@fastly.com>
Date: Fri, 10 Feb 2023 08:53:30 -0800
Subject: [PATCH] Move default blocks into jump tables (#5756)

Move the default block off of the br_table instrution, and into the JumpTable that it references.
---
 cranelift/codegen/meta/src/shared/entities.rs |  10 --
 cranelift/codegen/meta/src/shared/formats.rs  |   1 -
 .../codegen/meta/src/shared/instructions.rs   |   3 +-
 cranelift/codegen/src/inst_predicates.rs      |  18 +--
 cranelift/codegen/src/ir/function.rs          |  12 +-
 cranelift/codegen/src/ir/jumptable.rs         | 111 ++++++++++--------
 cranelift/codegen/src/isa/aarch64/lower.isle  |   2 +-
 cranelift/codegen/src/isa/aarch64/mod.rs      |   9 +-
 cranelift/codegen/src/isa/riscv64/inst.isle   |   2 +-
 cranelift/codegen/src/isa/s390x/lower.isle    |   2 +-
 cranelift/codegen/src/isa/x64/lower.isle      |   2 +-
 cranelift/codegen/src/isa/x64/mod.rs          |   9 +-
 cranelift/codegen/src/verifier/mod.rs         |  26 +---
 cranelift/codegen/src/write.rs                |   7 +-
 cranelift/frontend/src/frontend.rs            |  12 +-
 cranelift/frontend/src/ssa.rs                 |  19 +--
 cranelift/frontend/src/switch.rs              |   4 +-
 cranelift/fuzzgen/src/function_generator.rs   |   4 +-
 cranelift/interpreter/src/step.rs             |   7 +-
 cranelift/reader/src/parser.rs                |  19 ++-
 cranelift/wasm/src/code_translator.rs         |  14 +--
 21 files changed, 118 insertions(+), 175 deletions(-)

diff --git a/cranelift/codegen/meta/src/shared/entities.rs b/cranelift/codegen/meta/src/shared/entities.rs
index e713bc92ec..374e61f416 100644
--- a/cranelift/codegen/meta/src/shared/entities.rs
+++ b/cranelift/codegen/meta/src/shared/entities.rs
@@ -15,10 +15,6 @@ pub(crate) struct EntityRefs {
     /// This is primarily used in control flow instructions.
     pub(crate) block_call: OperandKind,
 
-    /// A reference to a basic block in the same function.
-    /// This is primarily used in control flow instructions.
-    pub(crate) label: OperandKind,
-
     /// A reference to a basic block in the same function, with its arguments provided.
     /// This is primarily used in control flow instructions.
     pub(crate) block_then: OperandKind,
@@ -63,12 +59,6 @@ impl EntityRefs {
                 "a basic block in the same function, with its arguments provided.",
             ),
 
-            label: new(
-                "destination",
-                "ir::Block",
-                "a basic block in the same function.",
-            ),
-
             block_then: new(
                 "block_then",
                 "ir::BlockCall",
diff --git a/cranelift/codegen/meta/src/shared/formats.rs b/cranelift/codegen/meta/src/shared/formats.rs
index b44ecb83e2..35ea2e7f8f 100644
--- a/cranelift/codegen/meta/src/shared/formats.rs
+++ b/cranelift/codegen/meta/src/shared/formats.rs
@@ -117,7 +117,6 @@ impl Formats {
 
             branch_table: Builder::new("BranchTable")
                 .value()
-                .imm(&entities.label)
                 .imm(&entities.jump_table)
                 .build(),
 
diff --git a/cranelift/codegen/meta/src/shared/instructions.rs b/cranelift/codegen/meta/src/shared/instructions.rs
index 014e3b4661..575caa4437 100755
--- a/cranelift/codegen/meta/src/shared/instructions.rs
+++ b/cranelift/codegen/meta/src/shared/instructions.rs
@@ -19,7 +19,6 @@ fn define_control_flow(
 ) {
     let block_call = &Operand::new("block_call", &entities.block_call)
         .with_doc("Destination basic block, with its arguments provided");
-    let label = &Operand::new("label", &entities.label).with_doc("Destination basic block");
 
     ig.push(
         Inst::new(
@@ -93,7 +92,7 @@ fn define_control_flow(
         "#,
                 &formats.branch_table,
             )
-            .operands_in(vec![x, label, JT])
+            .operands_in(vec![x, JT])
             .branches(),
         );
     }
diff --git a/cranelift/codegen/src/inst_predicates.rs b/cranelift/codegen/src/inst_predicates.rs
index 96bd63e602..e6bc6e6664 100644
--- a/cranelift/codegen/src/inst_predicates.rs
+++ b/cranelift/codegen/src/inst_predicates.rs
@@ -190,16 +190,16 @@ pub(crate) fn visit_block_succs<F: FnMut(Inst, Block, bool)>(
                 visit(inst, block_else.block(&f.dfg.value_lists), false);
             }
 
-            ir::InstructionData::BranchTable {
-                table,
-                destination: dest,
-                ..
-            } => {
-                // The default block is reached via a direct conditional branch,
-                // so it is not part of the table.
-                visit(inst, *dest, false);
+            ir::InstructionData::BranchTable { table, .. } => {
+                let table = &f.stencil.dfg.jump_tables[*table];
 
-                for &dest in f.stencil.dfg.jump_tables[*table].as_slice() {
+                // The default block is reached via a direct conditional branch,
+                // so it is not part of the table. We visit the default block first
+                // explicitly, as some callers of visit_block_succs depend on that
+                // ordering.
+                visit(inst, table.default_block(), false);
+
+                for &dest in table.as_slice() {
                     visit(inst, dest, true);
                 }
             }
diff --git a/cranelift/codegen/src/ir/function.rs b/cranelift/codegen/src/ir/function.rs
index 7e5176c013..3e0a00b177 100644
--- a/cranelift/codegen/src/ir/function.rs
+++ b/cranelift/codegen/src/ir/function.rs
@@ -295,19 +295,11 @@ impl FunctionStencil {
                 }
             }
 
-            InstructionData::BranchTable {
-                table,
-                destination: default_dest,
-                ..
-            } => {
-                self.dfg.jump_tables[*table].iter_mut().for_each(|entry| {
+            InstructionData::BranchTable { table, .. } => {
+                for entry in self.dfg.jump_tables[*table].all_branches_mut() {
                     if *entry == old_dest {
                         *entry = new_dest;
                     }
-                });
-
-                if *default_dest == old_dest {
-                    *default_dest = new_dest;
                 }
             }
 
diff --git a/cranelift/codegen/src/ir/jumptable.rs b/cranelift/codegen/src/ir/jumptable.rs
index f92ae1162c..70d0b1ddde 100644
--- a/cranelift/codegen/src/ir/jumptable.rs
+++ b/cranelift/codegen/src/ir/jumptable.rs
@@ -14,6 +14,12 @@ use serde::{Deserialize, Serialize};
 /// Contents of a jump table.
 ///
 /// All jump tables use 0-based indexing and are densely populated.
+///
+/// The default block for the jump table is stored as the last element of the underlying vector,
+/// and is not included in the length of the jump table. It can be accessed through the
+/// `default_block` and `default_block_mut` functions. All blocks may be iterated using the
+/// `all_branches` and `all_branches_mut` functions, which will both iterate over the default block
+/// last.
 #[derive(Clone, PartialEq, Hash)]
 #[cfg_attr(feature = "enable-serde", derive(Serialize, Deserialize))]
 pub struct JumpTableData {
@@ -22,72 +28,71 @@ pub struct JumpTableData {
 }
 
 impl JumpTableData {
-    /// Create a new empty jump table.
-    pub fn new() -> Self {
-        Self { table: Vec::new() }
-    }
-
-    /// Create a new empty jump table with the specified capacity.
-    pub fn with_capacity(capacity: usize) -> Self {
-        Self {
-            table: Vec::with_capacity(capacity),
-        }
-    }
     /// Create a new jump table with the provided blocks
-    pub fn with_blocks(table: Vec<Block>) -> Self {
+    pub fn new(def: Block, mut table: Vec<Block>) -> Self {
+        table.push(def);
         Self { table }
     }
 
-    /// Get the number of table entries.
-    pub fn len(&self) -> usize {
-        self.table.len()
+    /// Fetch the default block for this jump table.
+    pub fn default_block(&self) -> Block {
+        *self.table.last().unwrap()
     }
 
-    /// Append a table entry.
-    pub fn push_entry(&mut self, dest: Block) {
-        self.table.push(dest)
+    /// Mutable access to the default block of this jump table.
+    pub fn default_block_mut(&mut self) -> &mut Block {
+        self.table.last_mut().unwrap()
     }
 
-    /// Checks if any of the entries branch to `block`.
-    pub fn branches_to(&self, block: Block) -> bool {
-        self.table.iter().any(|target_block| *target_block == block)
-    }
-
-    /// Access the whole table as a slice.
-    pub fn as_slice(&self) -> &[Block] {
+    /// The jump table and default block as a single slice. The default block will always be last.
+    pub fn all_branches(&self) -> &[Block] {
         self.table.as_slice()
     }
 
-    /// Access the whole table as a mutable slice.
-    pub fn as_mut_slice(&mut self) -> &mut [Block] {
+    /// The jump table and default block as a single mutable slice. The default block will always
+    /// be last.
+    pub fn all_branches_mut(&mut self) -> &mut [Block] {
         self.table.as_mut_slice()
     }
 
-    /// Returns an iterator over the table.
+    /// Access the jump table as a slice. This excludes the default block.
+    pub fn as_slice(&self) -> &[Block] {
+        let last = self.table.len() - 1;
+        &self.table.as_slice()[0..last]
+    }
+
+    /// Access the jump table as a mutable slice. This excludes the default block.
+    pub fn as_mut_slice(&mut self) -> &mut [Block] {
+        let last = self.table.len() - 1;
+        &mut self.table.as_mut_slice()[0..last]
+    }
+
+    /// Returns an iterator to the jump table, excluding the default block.
+    #[deprecated(since = "7.0.0", note = "please use `.as_slice()` instead")]
     pub fn iter(&self) -> Iter<Block> {
-        self.table.iter()
+        self.as_slice().iter()
     }
 
-    /// Returns an iterator that allows modifying each value.
+    /// Returns an iterator that allows modifying each value, excluding the default block.
+    #[deprecated(since = "7.0.0", note = "please use `.as_mut_slice()` instead")]
     pub fn iter_mut(&mut self) -> IterMut<Block> {
-        self.table.iter_mut()
+        self.as_mut_slice().iter_mut()
     }
 
-    /// Clears all entries in this jump table.
+    /// Clears all entries in this jump table, except for the default block.
     pub fn clear(&mut self) {
-        self.table.clear();
+        self.table.drain(0..self.table.len() - 1);
     }
 }
 
 impl Display for JumpTableData {
     fn fmt(&self, fmt: &mut Formatter) -> fmt::Result {
-        write!(fmt, "[")?;
-        match self.table.first() {
-            None => (),
-            Some(first) => write!(fmt, "{}", first)?,
-        }
-        for block in self.table.iter().skip(1) {
-            write!(fmt, ", {}", block)?;
+        write!(fmt, "{}, [", self.default_block())?;
+        if let Some((first, rest)) = self.as_slice().split_first() {
+            write!(fmt, "{}", first)?;
+            for block in rest {
+                write!(fmt, ", {}", block)?;
+            }
         }
         write!(fmt, "]")
     }
@@ -102,31 +107,33 @@ mod tests {
 
     #[test]
     fn empty() {
-        let jt = JumpTableData::new();
+        let def = Block::new(0);
+
+        let jt = JumpTableData::new(def, vec![]);
+
+        assert_eq!(jt.all_branches().get(0), Some(&def));
 
         assert_eq!(jt.as_slice().get(0), None);
         assert_eq!(jt.as_slice().get(10), None);
 
-        assert_eq!(jt.to_string(), "[]");
+        assert_eq!(jt.to_string(), "block0, []");
 
-        let v = jt.as_slice();
-        assert_eq!(v, []);
+        assert_eq!(jt.all_branches(), [def]);
+        assert_eq!(jt.as_slice(), []);
     }
 
     #[test]
     fn insert() {
+        let def = Block::new(0);
         let e1 = Block::new(1);
         let e2 = Block::new(2);
 
-        let mut jt = JumpTableData::new();
+        let jt = JumpTableData::new(def, vec![e1, e2, e1]);
 
-        jt.push_entry(e1);
-        jt.push_entry(e2);
-        jt.push_entry(e1);
+        assert_eq!(jt.default_block(), def);
+        assert_eq!(jt.to_string(), "block0, [block1, block2, block1]");
 
-        assert_eq!(jt.to_string(), "[block1, block2, block1]");
-
-        let v = jt.as_slice();
-        assert_eq!(v, [e1, e2, e1]);
+        assert_eq!(jt.all_branches(), [e1, e2, e1, def]);
+        assert_eq!(jt.as_slice(), [e1, e2, e1]);
     }
 }
diff --git a/cranelift/codegen/src/isa/aarch64/lower.isle b/cranelift/codegen/src/isa/aarch64/lower.isle
index eccda08b60..c42bc2c411 100644
--- a/cranelift/codegen/src/isa/aarch64/lower.isle
+++ b/cranelift/codegen/src/isa/aarch64/lower.isle
@@ -2515,7 +2515,7 @@
 
 ;; `targets` contains the default target with the list of branch targets
 ;; concatenated.
-(rule (lower_branch (br_table idx _ _) targets)
+(rule (lower_branch (br_table idx _) targets)
       (let ((jt_size u32 (targets_jt_size targets))
             (_ InstOutput (side_effect
                   (emit_island (targets_jt_space targets))))
diff --git a/cranelift/codegen/src/isa/aarch64/mod.rs b/cranelift/codegen/src/isa/aarch64/mod.rs
index d7a2630ae8..8345491028 100644
--- a/cranelift/codegen/src/isa/aarch64/mod.rs
+++ b/cranelift/codegen/src/isa/aarch64/mod.rs
@@ -361,11 +361,10 @@ mod test {
         let mut pos = FuncCursor::new(&mut func);
 
         pos.insert_block(bb0);
-        let mut jt_data = JumpTableData::new();
-        jt_data.push_entry(bb1);
-        jt_data.push_entry(bb2);
-        let jt = pos.func.create_jump_table(jt_data);
-        pos.ins().br_table(arg0, bb3, jt);
+        let jt = pos
+            .func
+            .create_jump_table(JumpTableData::new(bb3, vec![bb1, bb2]));
+        pos.ins().br_table(arg0, jt);
 
         pos.insert_block(bb1);
         let v1 = pos.ins().iconst(I32, 1);
diff --git a/cranelift/codegen/src/isa/riscv64/inst.isle b/cranelift/codegen/src/isa/riscv64/inst.isle
index ef6f8dd379..10bcbd193b 100644
--- a/cranelift/codegen/src/isa/riscv64/inst.isle
+++ b/cranelift/codegen/src/isa/riscv64/inst.isle
@@ -1942,7 +1942,7 @@
 (extern constructor lower_br_table lower_br_table)
 
 (rule
-  (lower_branch (br_table index _ _) targets)
+  (lower_branch (br_table index _) targets)
   (lower_br_table index targets))
 
 (decl load_ra () Reg)
diff --git a/cranelift/codegen/src/isa/s390x/lower.isle b/cranelift/codegen/src/isa/s390x/lower.isle
index 1eed87da07..f1768f95d5 100644
--- a/cranelift/codegen/src/isa/s390x/lower.isle
+++ b/cranelift/codegen/src/isa/s390x/lower.isle
@@ -3725,7 +3725,7 @@
 
 ;; Jump table.  `targets` contains the default target followed by the
 ;; list of branch targets per index value.
-(rule (lower_branch (br_table val_idx _ _) targets)
+(rule (lower_branch (br_table val_idx _) targets)
       (let ((idx Reg (put_in_reg_zext64 val_idx))
             ;; Bounds-check the index and branch to default.
             ;; This is an internal branch that is not a terminator insn.
diff --git a/cranelift/codegen/src/isa/x64/lower.isle b/cranelift/codegen/src/isa/x64/lower.isle
index 9adc4e2fdb..e8fd01f840 100644
--- a/cranelift/codegen/src/isa/x64/lower.isle
+++ b/cranelift/codegen/src/isa/x64/lower.isle
@@ -2942,7 +2942,7 @@
 
 ;; Rules for `br_table` ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
-(rule (lower_branch (br_table idx @ (value_type ty) _ _) (jump_table_targets default_target jt_targets))
+(rule (lower_branch (br_table idx @ (value_type ty) _) (jump_table_targets default_target jt_targets))
       (emit_side_effect (jmp_table_seq ty idx default_target jt_targets)))
 
 ;; Rules for `select_spectre_guard` ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
diff --git a/cranelift/codegen/src/isa/x64/mod.rs b/cranelift/codegen/src/isa/x64/mod.rs
index a567ddf214..a7bda0a466 100644
--- a/cranelift/codegen/src/isa/x64/mod.rs
+++ b/cranelift/codegen/src/isa/x64/mod.rs
@@ -408,11 +408,10 @@ mod test {
         let mut pos = FuncCursor::new(&mut func);
 
         pos.insert_block(bb0);
-        let mut jt_data = JumpTableData::new();
-        jt_data.push_entry(bb1);
-        jt_data.push_entry(bb2);
-        let jt = pos.func.create_jump_table(jt_data);
-        pos.ins().br_table(arg0, bb3, jt);
+        let jt = pos
+            .func
+            .create_jump_table(JumpTableData::new(bb3, vec![bb1, bb2]));
+        pos.ins().br_table(arg0, jt);
 
         pos.insert_block(bb1);
         let v1 = pos.ins().iconst(I32, 1);
diff --git a/cranelift/codegen/src/verifier/mod.rs b/cranelift/codegen/src/verifier/mod.rs
index e9fff15ff4..4572a5a2ec 100644
--- a/cranelift/codegen/src/verifier/mod.rs
+++ b/cranelift/codegen/src/verifier/mod.rs
@@ -579,10 +579,7 @@ impl<'a> Verifier<'a> {
                 self.verify_block(inst, block_then.block(&self.func.dfg.value_lists), errors)?;
                 self.verify_block(inst, block_else.block(&self.func.dfg.value_lists), errors)?;
             }
-            BranchTable {
-                table, destination, ..
-            } => {
-                self.verify_block(inst, destination, errors)?;
+            BranchTable { table, .. } => {
                 self.verify_jump_table(inst, table, errors)?;
             }
             Call {
@@ -852,7 +849,7 @@ impl<'a> Verifier<'a> {
                 format!("invalid jump table reference {}", j),
             ))
         } else {
-            for &block in self.func.stencil.dfg.jump_tables[j].as_slice() {
+            for &block in self.func.stencil.dfg.jump_tables[j].all_branches() {
                 self.verify_block(inst, block, errors)?;
             }
             Ok(())
@@ -1322,23 +1319,8 @@ impl<'a> Verifier<'a> {
                 let args_else = block_else.args_slice(&self.func.dfg.value_lists);
                 self.typecheck_variable_args_iterator(inst, iter, args_else, errors)?;
             }
-            ir::InstructionData::BranchTable {
-                table,
-                destination: block,
-                ..
-            } => {
-                let arg_count = self.func.dfg.num_block_params(*block);
-                if arg_count != 0 {
-                    return errors.nonfatal((
-                        inst,
-                        self.context(inst),
-                        format!(
-                            "takes no arguments, but had target {} with {} arguments",
-                            block, arg_count,
-                        ),
-                    ));
-                }
-                for block in self.func.stencil.dfg.jump_tables[*table].iter() {
+            ir::InstructionData::BranchTable { table, .. } => {
+                for block in self.func.stencil.dfg.jump_tables[*table].all_branches() {
                     let arg_count = self.func.dfg.num_block_params(*block);
                     if arg_count != 0 {
                         return errors.nonfatal((
diff --git a/cranelift/codegen/src/write.rs b/cranelift/codegen/src/write.rs
index 0b4c1d9da4..3f77fff50d 100644
--- a/cranelift/codegen/src/write.rs
+++ b/cranelift/codegen/src/write.rs
@@ -421,12 +421,7 @@ pub fn write_operands(w: &mut dyn Write, dfg: &DataFlowGraph, inst: Inst) -> fmt
             write!(w, " {}, {}", arg, block_then.display(pool))?;
             write!(w, ", {}", block_else.display(pool))
         }
-        BranchTable {
-            arg,
-            destination,
-            table,
-            ..
-        } => write!(w, " {}, {}, {}", arg, destination, jump_tables[table]),
+        BranchTable { arg, table, .. } => write!(w, " {}, {}", arg, jump_tables[table]),
         Call {
             func_ref, ref args, ..
         } => write!(w, " {}({})", func_ref, DisplayValues(args.as_slice(pool))),
diff --git a/cranelift/frontend/src/frontend.rs b/cranelift/frontend/src/frontend.rs
index db9875602b..6f405757f6 100644
--- a/cranelift/frontend/src/frontend.rs
+++ b/cranelift/frontend/src/frontend.rs
@@ -129,9 +129,7 @@ impl<'short, 'long> InstBuilderBase<'short> for FuncInstBuilder<'short, 'long> {
                 }
             }
 
-            ir::InstructionData::BranchTable {
-                table, destination, ..
-            } => {
+            ir::InstructionData::BranchTable { table, .. } => {
                 // Unlike all other jumps/branches, jump tables are
                 // capable of having the same successor appear
                 // multiple times, so we must deduplicate.
@@ -144,9 +142,12 @@ impl<'short, 'long> InstBuilderBase<'short> for FuncInstBuilder<'short, 'long> {
                     .jump_tables
                     .get(*table)
                     .expect("you are referencing an undeclared jump table")
-                    .iter()
-                    .filter(|&dest_block| unique.insert(*dest_block))
+                    .all_branches()
                 {
+                    if !unique.insert(*dest_block) {
+                        continue;
+                    }
+
                     // Call `declare_block_predecessor` instead of `declare_successor` for
                     // avoiding the borrow checker.
                     self.builder
@@ -154,7 +155,6 @@ impl<'short, 'long> InstBuilderBase<'short> for FuncInstBuilder<'short, 'long> {
                         .ssa
                         .declare_block_predecessor(*dest_block, inst);
                 }
-                self.builder.declare_successor(*destination, inst);
             }
 
             inst => debug_assert!(!inst.opcode().is_branch()),
diff --git a/cranelift/frontend/src/ssa.rs b/cranelift/frontend/src/ssa.rs
index d6c852e2f6..c388ee425a 100644
--- a/cranelift/frontend/src/ssa.rs
+++ b/cranelift/frontend/src/ssa.rs
@@ -591,27 +591,18 @@ impl SSABuilder {
                 }
                 None
             }
-            InstructionData::BranchTable {
-                table: jt,
-                destination,
-                ..
-            } => {
+            InstructionData::BranchTable { table: jt, .. } => {
                 // In the case of a jump table, the situation is tricky because br_table doesn't
                 // support arguments. We have to split the critical edge.
                 let middle_block = dfg.blocks.add();
                 func.stencil.layout.append_block(middle_block);
 
-                let table = &mut dfg.jump_tables[*jt];
-                for block in table.iter_mut() {
+                for block in dfg.jump_tables[*jt].all_branches_mut() {
                     if *block == dest_block {
                         *block = middle_block;
                     }
                 }
 
-                if *destination == dest_block {
-                    *destination = middle_block;
-                }
-
                 let mut cur = FuncCursor::new(func).at_bottom(middle_block);
                 let middle_jump_inst = cur.ins().jump(dest_block, &[val]);
                 Some((middle_block, middle_jump_inst))
@@ -1001,9 +992,6 @@ mod tests {
         let block0 = func.dfg.make_block();
         let block1 = func.dfg.make_block();
         let block2 = func.dfg.make_block();
-        let mut jump_table = JumpTableData::new();
-        jump_table.push_entry(block2);
-        jump_table.push_entry(block1);
         {
             let mut cur = FuncCursor::new(&mut func);
             cur.insert_block(block0);
@@ -1022,9 +1010,10 @@ mod tests {
         ssa.def_var(x_var, x1, block0);
         ssa.use_var(&mut func, x_var, I32, block0).0;
         let br_table = {
+            let jump_table = JumpTableData::new(block2, vec![block2, block1]);
             let jt = func.create_jump_table(jump_table);
             let mut cur = FuncCursor::new(&mut func).at_bottom(block0);
-            cur.ins().br_table(x1, block2, jt)
+            cur.ins().br_table(x1, jt)
         };
 
         // block1
diff --git a/cranelift/frontend/src/switch.rs b/cranelift/frontend/src/switch.rs
index f7ff7f8743..7788283286 100644
--- a/cranelift/frontend/src/switch.rs
+++ b/cranelift/frontend/src/switch.rs
@@ -220,7 +220,7 @@ impl Switch {
             "Jump tables bigger than 2^32-1 are not yet supported"
         );
 
-        let jt_data = JumpTableData::with_blocks(Vec::from(blocks));
+        let jt_data = JumpTableData::new(otherwise, Vec::from(blocks));
         let jump_table = bx.create_jump_table(jt_data);
 
         let discr = if first_index == 0 {
@@ -256,7 +256,7 @@ impl Switch {
             _ => discr,
         };
 
-        bx.ins().br_table(discr, otherwise, jump_table);
+        bx.ins().br_table(discr, jump_table);
     }
 
     /// Build the switch
diff --git a/cranelift/fuzzgen/src/function_generator.rs b/cranelift/fuzzgen/src/function_generator.rs
index 4ffc3f6794..1c4b68b187 100644
--- a/cranelift/fuzzgen/src/function_generator.rs
+++ b/cranelift/fuzzgen/src/function_generator.rs
@@ -1614,12 +1614,12 @@ where
             }
             BlockTerminator::BrTable(default, targets) => {
                 // Create jump tables on demand
-                let jt = builder.create_jump_table(JumpTableData::with_blocks(targets));
+                let jt = builder.create_jump_table(JumpTableData::new(default, targets));
 
                 // br_table only supports I32
                 let val = builder.use_var(self.get_variable_of_type(I32)?);
 
-                builder.ins().br_table(val, default, jt);
+                builder.ins().br_table(val, jt);
             }
             BlockTerminator::Switch(_type, default, entries) => {
                 let mut switch = Switch::new();
diff --git a/cranelift/interpreter/src/step.rs b/cranelift/interpreter/src/step.rs
index 7973b846e6..98b699b1ca 100644
--- a/cranelift/interpreter/src/step.rs
+++ b/cranelift/interpreter/src/step.rs
@@ -373,10 +373,7 @@ where
             }
         }
         Opcode::BrTable => {
-            if let InstructionData::BranchTable {
-                table, destination, ..
-            } = inst
-            {
+            if let InstructionData::BranchTable { table, .. } = inst {
                 let jt_data = &state.get_current_function().stencil.dfg.jump_tables[table];
 
                 // Convert to usize to remove negative indexes from the following operations
@@ -384,7 +381,7 @@ where
                     .ok()
                     .and_then(|i| jt_data.as_slice().get(i))
                     .copied()
-                    .unwrap_or(destination);
+                    .unwrap_or(jt_data.default_block());
 
                 ControlFlow::ContinueAt(jump_target, SmallVec::new())
             } else {
diff --git a/cranelift/reader/src/parser.rs b/cranelift/reader/src/parser.rs
index 2154219bb4..b0cb2baac5 100644
--- a/cranelift/reader/src/parser.rs
+++ b/cranelift/reader/src/parser.rs
@@ -1767,15 +1767,15 @@ impl<'a> Parser<'a> {
     //
     // jump-table-lit ::= "[" block {"," block } "]"
     //                  | "[]"
-    fn parse_jump_table(&mut self) -> ParseResult<JumpTableData> {
+    fn parse_jump_table(&mut self, def: Block) -> ParseResult<JumpTableData> {
         self.match_token(Token::LBracket, "expected '[' before jump table contents")?;
 
-        let mut data = JumpTableData::new();
+        let mut data = Vec::new();
 
         match self.token() {
             Some(Token::Block(dest)) => {
                 self.consume();
-                data.push_entry(dest);
+                data.push(dest);
 
                 loop {
                     match self.token() {
@@ -1783,7 +1783,7 @@ impl<'a> Parser<'a> {
                             self.consume();
                             if let Some(Token::Block(dest)) = self.token() {
                                 self.consume();
-                                data.push_entry(dest);
+                                data.push(dest);
                             } else {
                                 return err!(self.loc, "expected jump_table entry");
                             }
@@ -1799,7 +1799,7 @@ impl<'a> Parser<'a> {
 
         self.consume();
 
-        Ok(data)
+        Ok(JumpTableData::new(def, data))
     }
 
     // Parse a constant decl.
@@ -2571,14 +2571,9 @@ impl<'a> Parser<'a> {
                 self.match_token(Token::Comma, "expected ',' between operands")?;
                 let block_num = self.match_block("expected branch destination block")?;
                 self.match_token(Token::Comma, "expected ',' between operands")?;
-                let table_data = self.parse_jump_table()?;
+                let table_data = self.parse_jump_table(block_num)?;
                 let table = ctx.function.dfg.jump_tables.push(table_data);
-                InstructionData::BranchTable {
-                    opcode,
-                    arg,
-                    destination: block_num,
-                    table,
-                }
+                InstructionData::BranchTable { opcode, arg, table }
             }
             InstructionFormat::TernaryImm8 => {
                 let lhs = self.match_value("expected SSA value first operand")?;
diff --git a/cranelift/wasm/src/code_translator.rs b/cranelift/wasm/src/code_translator.rs
index 22032a0f29..1d7fd2d959 100644
--- a/cranelift/wasm/src/code_translator.rs
+++ b/cranelift/wasm/src/code_translator.rs
@@ -515,7 +515,7 @@ pub fn translate_operator<FE: FuncEnvironment + ?Sized>(
                 }
             };
             let val = state.pop1();
-            let mut data = JumpTableData::with_capacity(targets.len() as usize);
+            let mut data = Vec::with_capacity(targets.len() as usize);
             if jump_args_count == 0 {
                 // No jump arguments
                 for depth in targets.targets() {
@@ -526,16 +526,16 @@ pub fn translate_operator<FE: FuncEnvironment + ?Sized>(
                         frame.set_branched_to_exit();
                         frame.br_destination()
                     };
-                    data.push_entry(block);
+                    data.push(block);
                 }
-                let jt = builder.create_jump_table(data);
                 let block = {
                     let i = state.control_stack.len() - 1 - (default as usize);
                     let frame = &mut state.control_stack[i];
                     frame.set_branched_to_exit();
                     frame.br_destination()
                 };
-                builder.ins().br_table(val, block, jt);
+                let jt = builder.create_jump_table(JumpTableData::new(block, data));
+                builder.ins().br_table(val, jt);
             } else {
                 // Here we have jump arguments, but Cranelift's br_table doesn't support them
                 // We then proceed to split the edges going out of the br_table
@@ -552,7 +552,7 @@ pub fn translate_operator<FE: FuncEnvironment + ?Sized>(
                             *entry.insert(block)
                         }
                     };
-                    data.push_entry(branch_block);
+                    data.push(branch_block);
                 }
                 let default_branch_block = match dest_block_map.entry(default as usize) {
                     hash_map::Entry::Occupied(entry) => *entry.get(),
@@ -562,8 +562,8 @@ pub fn translate_operator<FE: FuncEnvironment + ?Sized>(
                         *entry.insert(block)
                     }
                 };
-                let jt = builder.create_jump_table(data);
-                builder.ins().br_table(val, default_branch_block, jt);
+                let jt = builder.create_jump_table(JumpTableData::new(default_branch_block, data));
+                builder.ins().br_table(val, jt);
                 for (depth, dest_block) in dest_block_sequence {
                     builder.switch_to_block(dest_block);
                     builder.seal_block(dest_block);