Add more comments explaining ghost instructions.

lib/codegen/meta-python/cdsl/instructions.py

@@ -92,7 +92,8 @@ class Instruction(object):
     :param is_indirect_branch: This is an indirect branch instruction.
     :param is_call: This is a call instruction.
     :param is_return: This is a return instruction.
-    :param is_ghost: This is a ghost instruction.
+    :param is_ghost: This is a ghost instruction, which has no encoding and no
+                     other register allocation constraints.
     :param can_trap: This instruction can trap.
     :param can_load: This instruction can load from memory.
     :param can_store: This instruction can store to memory.

lib/codegen/src/regalloc/coloring.rs

@@ -166,6 +166,8 @@ impl<'a> Context<'a> {
         while let Some(inst) = self.cur.next_inst() {
             self.cur.use_srcloc(inst);
             if !self.cur.func.dfg[inst].opcode().is_ghost() {
+                // This is an instruction which either has an encoding or carries ABI-related
+                // register allocation constraints.
                 let enc = self.cur.func.encodings[inst];
                 let constraints = self.encinfo.operand_constraints(enc);
                 if self.visit_inst(inst, constraints, tracker, &mut regs) {
@@ -175,7 +177,7 @@ impl<'a> Context<'a> {
                     self.cur.goto_inst(inst);
                 }
             } else {
-                // This is a ghost instruction with no encoding.
+                // This is a ghost instruction with no encoding and no extra constraints.
                 let (_throughs, kills) = tracker.process_ghost(inst);
                 self.process_ghost_kills(kills, &mut regs);
             }

lib/codegen/src/regalloc/reload.rs

@@ -126,10 +126,14 @@ impl<'a> Context<'a> {
         // visit_ebb_header() places us at the first interesting instruction in the EBB.
         while let Some(inst) = self.cur.current_inst() {
             if !self.cur.func.dfg[inst].opcode().is_ghost() {
+                // This instruction either has an encoding or has ABI constraints, so visit it to
+                // insert spills and fills as needed.
                 let encoding = self.cur.func.encodings[inst];
                 self.visit_inst(ebb, inst, encoding, tracker);
                 tracker.drop_dead(inst);
             } else {
+                // This is a ghost instruction with no encoding and no extra constraints, so we can
+                // just skip over it.
                 self.cur.next_inst();
             }
         }