Modifications

docs/declass_algo.txt

@@ -36,23 +36,50 @@ which in the example is:
 REPLACE it with:
 
     [6] = Fclosure8 Const(1) [],
-    [8] = Fclosure8 Const(0) [],
     [18] = GetField Atom(atom("prototype")) [6],
+    [8] = Fclosure8 Const(0) [],
     [] = PutField Atom(file_atom[5]) [8, 18],
+    [] = Nop None [],
+    [] = Nop None [],
+    [] = Nop None [],
+    [] = Nop None [],
 
 INSTRUCTION BY INSTRUCTION:
 
-    PushConst8 Const(1) + DefineClass [5,4]   ->  Fclosure8 Const(1), keep dst [6]
-    Fclosure8 Const(0)                        ->  unchanged (the method closure)
-    DefineMethod atom=A [8,7]                  ->  GetField "prototype" [6]
-                                                   then PutField atom=A [8,18]
-    Fclosure8 Const(2)                         ->  deleted
-    SetHomeObject                              ->  deleted
-    PutLoc1 [12]                               ->  deleted
-    Drop [11]                                  ->  deleted
+    PushConst8 Const(1) ; DefineClass [5,4]
+        -> Fclosure8 Const(1), which keeps the DefineClass dst [6].
 
-    [18] is any fresh value id.
-    Repeat the DefineMethod line for each method in the class.
+    GetField "prototype" [6]
+        -> emitted once for the whole class, into a fresh value id [18].
+           It is NOT emitted per method: the prototype cannot change
+           between the PutFields below. A class with no methods does not
+           emit it at all.
+
+    Fclosure8 Const(0)
+        -> unchanged. It is the method closure.
+
+    DefineMethod atom=A [8,7]
+        -> PutField atom=A [8, 18], reading the [18] from above.
+           Repeat the Fclosure8/PutField pair for each method.
+
+    Fclosure8 Const(2) ; SetHomeObject ; PutLoc1 [12] ; Drop [11]
+        -> deleted. This is the field-initialiser tail.
+
+    Nop
+        -> padding, described below.
+
+LENGTH, and why the rewrite is written in place:
+
+    matched  = 2 + 2n instructions, plus 4 more if the field-initialiser
+               tail is there (n = the number of methods).
+    emitted  = 2 + 2n instructions, or 1 when n is 0.
+
+    So the emission is never longer than what it replaces. It is written
+    straight over the matched range and the slack is padded with Nop,
+    which pops and pushes nothing. Every instruction outside a class
+    build keeps its position, so no other range has to be adjusted.
+
+    In the example: 8 matched, 4 emitted, 4 Nop.
 
 REMEMBER for step 2 and 3:
 
@@ -200,7 +227,17 @@ NOT TOUCHED (passed through unchanged)
 
 
 ====================================================================
-LEFT BEHIND (removed by a later pass)
+LEFT BEHIND (an empty function)
 ====================================================================
 
-    FnById(4), the fields function. Nothing references it anymore.
+    FnById(4), the fields function. Step 1 deleted the fclosure8 that
+    built it, and step 2 moved its writes into the constructor, so
+    nothing references it anymore.
+
+    Step 2 empties it:
+
+        code: [],
+
+    It is NOT removed from the function list. A function id is an index
+    into that list, so removing one would renumber every function after
+    it and break every FnById that points past it.