Implement a poor man's jump table.

We will eventually support real jump tables, but for now just expand
br_table into a sequence of conditional branches.

cranelift/filetests/wasm/control.cton

@@ -48,3 +48,18 @@ function %undefined() {
 ebb0:
     trap user0
 }
+
+function %br_table(i32) {
+jt0 = jump_table ebb3, ebb1, 0, ebb2
+
+ebb0(v0: i32):
+    br_table v0, jt0
+    trap oob
+
+ebb1:
+    return
+ebb2:
+    return
+ebb3:
+    return
+}

lib/cretonne/meta/base/legalize.py

@@ -53,6 +53,7 @@ expand.custom_legalize(insts.heap_addr, 'expand_heap_addr')
 # TODO: Add sufficient XForm syntax that we don't need to hand-code these.
 expand.custom_legalize(insts.trapz, 'expand_cond_trap')
 expand.custom_legalize(insts.trapnz, 'expand_cond_trap')
+expand.custom_legalize(insts.br_table, 'expand_br_table')
 
 # Custom expansions for floating point constants.
 # These expansions require bit-casting or creating constant pool entries.

lib/cretonne/src/ir/jumptable.rs

@@ -39,6 +39,11 @@ impl JumpTableData {
         }
     }
 
+    /// Get the number of table entries.
+    pub fn len(&self) -> usize {
+        self.table.len()
+    }
+
     /// Set a table entry.
     ///
     /// The table will grow as needed to fit `idx`.

lib/cretonne/src/legalizer/mod.rs

@@ -147,6 +147,38 @@ fn expand_cond_trap(inst: ir::Inst, func: &mut ir::Function, cfg: &mut ControlFl
     cfg.recompute_ebb(pos.func, new_ebb);
 }
 
+/// Jump tables.
+fn expand_br_table(inst: ir::Inst, func: &mut ir::Function, cfg: &mut ControlFlowGraph) {
+    use ir::condcodes::IntCC;
+
+    let (arg, table) = match func.dfg[inst] {
+        ir::InstructionData::BranchTable {
+            opcode: ir::Opcode::BrTable,
+            arg,
+            table,
+        } => (arg, table),
+        _ => panic!("Expected br_table: {}", func.dfg.display_inst(inst, None)),
+    };
+
+    // This is a poor man's jump table using just a sequence of conditional branches.
+    // TODO: Lower into a jump table load and indirect branch.
+    let table_size = func.jump_tables[table].len();
+    let mut pos = FuncCursor::new(func).at_inst(inst);
+    pos.use_srcloc(inst);
+
+    for i in 0..table_size {
+        if let Some(dest) = pos.func.jump_tables[table].get_entry(i) {
+            let t = pos.ins().icmp_imm(IntCC::Equal, arg, i as i64);
+            pos.ins().brnz(t, dest, &[]);
+        }
+    }
+
+    // `br_table` falls through when nothing matches.
+    let ebb = pos.current_ebb().unwrap();
+    pos.remove_inst();
+    cfg.recompute_ebb(pos.func, ebb);
+}
+
 /// Expand illegal `f32const` and `f64const` instructions.
 fn expand_fconst(inst: ir::Inst, func: &mut ir::Function, _cfg: &mut ControlFlowGraph) {
     let ty = func.dfg.value_type(func.dfg.first_result(inst));