Don't require that the fallthrough instruction has an encoding.

A fallthrough jump is actually represented as 0 bytes, so no encoding is
needed.

Also allow for unencoded instructions in the generated emit_inst
implementations. The verifier has stricter rules for when this is
allowed.

filetests/wasm/control.cton

@@ -32,3 +32,14 @@ ebb1(v2: i32):
 ebb2:
     jump ebb1(v0)
 }
+
+function %br_if_fallthrough(i32) -> i32 {
+ebb0(v0: i32):
+    v1 = iconst.i32 1
+    brz v0, ebb1(v1)
+    ; This jump gets converted to a fallthrough.
+    jump ebb1(v0)
+
+ebb1(v2: i32):
+    return v2
+}

lib/cretonne/meta/gen_binemit.py

@@ -135,14 +135,18 @@ def gen_isa(isa, fmt):
                 'pub fn emit_inst<CS: CodeSink + ?Sized>'
                 '(func: &Function, inst: Inst, '
                 'divert: &mut RegDiversions, sink: &mut CS) {', '}'):
-            fmt.line('let bits = func.encodings[inst].bits();')
+            fmt.line('let encoding = func.encodings[inst];')
+            fmt.line('let bits = encoding.bits();')
             with fmt.indented('match func.encodings[inst].recipe() {', '}'):
                 for i, recipe in enumerate(isa.all_recipes):
                     fmt.comment(recipe.name)
                     with fmt.indented('{} => {{'.format(i), '}'):
                         gen_recipe(recipe, fmt)
                 fmt.line('_ => {}')
-            fmt.line('bad_encoding(func, inst);')
+            # Allow for un-encoded ghost instructions.
+            # Verifier checks the details.
+            with fmt.indented('if encoding.is_legal() {', '}'):
+                fmt.line('bad_encoding(func, inst);')
 
 
 def generate(isas, out_dir):

lib/cretonne/src/verifier/mod.rs

@@ -57,7 +57,7 @@ use flowgraph::ControlFlowGraph;
 use ir::entities::AnyEntity;
 use ir::instructions::{InstructionFormat, BranchInfo, ResolvedConstraint, CallInfo};
 use ir::{types, Function, ValueDef, Ebb, Inst, SigRef, FuncRef, ValueList, JumpTable, StackSlot,
-         Value, Type};
+         Value, Type, Opcode};
 use isa::TargetIsa;
 use std::error as std_error;
 use std::fmt::{self, Display, Formatter};
@@ -711,6 +711,12 @@ impl<'a> Verifier<'a> {
         // Instructions with side effects are not allowed to be ghost instructions.
         let opcode = self.func.dfg[inst].opcode();
 
+        // The `fallthrough` instruction is marked as a terminator and a branch, but it is not
+        // required to have an encoding.
+        if opcode == Opcode::Fallthrough {
+            return Ok(());
+        }
+
         if opcode.is_branch() {
             return err!(inst, "Branch must have an encoding");
         }