Tolerate recursively-useless phi-nodes

Also, make to_ssa more deterministic.

bril-ts/brili.ts

@@ -654,8 +654,17 @@ function evalInstr(instr: bril.Instruction, state: State): Action {
       // Last label not handled. Leave uninitialized.
       state.env.delete(instr.dest);
     } else {
-      let val = getArgument(instr, state.env, idx);
-      state.env.set(instr.dest, val);
+      // Copy the right argument (including an undefined one).
+      if (!instr.args || idx >= instr.args.length) {
+        throw error(`phi node needed at least ${idx+1} arguments`);
+      }
+      let src = instr.args[idx];
+      let val = state.env.get(src);
+      if (val === undefined) {
+        state.env.delete(instr.dest);
+      } else {
+        state.env.set(instr.dest, val);
+      }
     }
     return NEXT;
   }

examples/to_ssa.py

@@ -76,7 +76,7 @@ def _rename(block):
                     phi_args[s][p].append((block, stack[p][0]))
 
         # Recursive calls.
-        for b in domtree[block]:
+        for b in sorted(domtree[block]):
             _rename(b)
 
         # Restore stacks.
@@ -90,7 +90,7 @@ def _rename(block):
 
 def insert_phis(blocks, phi_args, phi_dests, types):
     for block, instrs in blocks.items():
-        for dest, pairs in phi_args[block].items():
+        for dest, pairs in sorted(phi_args[block].items()):
             phi = {
                 'op': 'phi',
                 'dest': phi_dests[block][dest],