Better generation of code at tail of routines.

HISTORY.md

@@ -14,6 +14,9 @@ History of SixtyPical
 *   Added `--prune-unreachable-routines` option, which causes
     the compiler to in fact omit routines determined to be
     unreachable as described above.
+*   Code generation now performs modest peephole optimization,
+    generating better code for `goto`s and `if` blocks at the
+    end of a routine.
 *   The `dcc6502-adapter` test adapter was updated to conform
     to the output of the latest version of `dcc6502`.
 

TODO.md

@@ -90,11 +90,6 @@ if the block is in tail position.  The constraints should iron out the same both
 
 As long as the routine has consistent type context every place it exits, that should be fine.
 
-### Branch optimization in `if`
-
-Currently the `if` generator is not smart enough to avoid generating silly
-jump instructions.  (See the Fallthru tests.)  Improve it.
-
 Implementation
 --------------
 

src/sixtypical/compiler.py

@@ -9,7 +9,7 @@ from sixtypical.model import (
     TableType, PointerType, RoutineType, VectorType,
     REG_A, REG_X, REG_Y, FLAG_C
 )
-from sixtypical.emitter import Byte, Word, Table, Label, Offset, LowAddressByte, HighAddressByte
+from sixtypical.emitter import Byte, Word, Table, Label, Offset, LowAddressByte, HighAddressByte, Emitter
 from sixtypical.gen6502 import (
     Immediate, Absolute, AbsoluteX, AbsoluteY, ZeroPage, Indirect, IndirectY, Relative,
     LDA, LDX, LDY, STA, STX, STY,
@@ -157,17 +157,31 @@ class Compiler(object):
                 self.emitter.resolve_bss_label(label)
 
     def compile_routine(self, routine, next_routine=None):
-        self.current_routine = routine
-        self.skip_final_goto = (next_routine is not None)
-        self.final_goto_seen = False
         assert isinstance(routine, Routine)
+
+        self.current_routine = routine
+        saved_emitter = self.emitter
+        self.emitter = Emitter(saved_emitter.addr)
         if routine.block:
             self.emitter.resolve_label(self.get_label(routine.name))
             self.compile_block(routine.block)
-            if not self.final_goto_seen:
+
+            needs_rts = True
+            if self.emitter.accum:
+                last_op = self.emitter.accum[-1]
+                if isinstance(last_op, JMP):
+                    needs_rts = False
+                    if isinstance(last_op.operand, Absolute):
+                        if isinstance(last_op.operand.value, Label):
+                            if next_routine and last_op.operand.value.name == next_routine.name:
+                                self.emitter.retract()
+
+            if needs_rts:
                 self.emitter.emit(RTS())
+
+        saved_emitter.emit(*self.emitter.accum)
+        self.emitter = saved_emitter
         self.current_routine = None
-        self.skip_final_goto = False
 
     def compile_block(self, block):
         assert isinstance(block, Block)
@@ -441,19 +455,15 @@ class Compiler(object):
             raise NotImplementedError(location_type)
 
     def compile_goto(self, instr):
-        self.final_goto_seen = True
-        if self.skip_final_goto:
-            pass
+        location = instr.location
+        label = self.get_label(instr.location.name)
+        location_type = self.get_type(location)
+        if isinstance(location_type, RoutineType):
+            self.emitter.emit(JMP(Absolute(label)))
+        elif isinstance(location_type, VectorType):
+            self.emitter.emit(JMP(Indirect(label)))
         else:
-            location = instr.location
-            label = self.get_label(instr.location.name)
-            location_type = self.get_type(location)
-            if isinstance(location_type, RoutineType):
-                self.emitter.emit(JMP(Absolute(label)))
-            elif isinstance(location_type, VectorType):
-                self.emitter.emit(JMP(Indirect(label)))
-            else:
-                raise NotImplementedError(location_type)
+            raise NotImplementedError(location_type)
 
     def compile_cmp(self, instr, src, dest):
         """`instr` is only for reporting purposes"""
@@ -666,12 +676,17 @@ class Compiler(object):
         else_label = Label('else_label')
         self.emitter.emit(cls(Relative(else_label)))
         self.compile_block(instr.block1)
+
         if instr.block2 is not None:
-            end_label = Label('end_label')
-            self.emitter.emit(JMP(Absolute(end_label)))
-            self.emitter.resolve_label(else_label)
-            self.compile_block(instr.block2)
-            self.emitter.resolve_label(end_label)
+            if instr.block1.shallow_contains_goto:
+                self.emitter.resolve_label(else_label)
+                self.compile_block(instr.block2)
+            else:
+                end_label = Label('end_label')
+                self.emitter.emit(JMP(Absolute(end_label)))
+                self.emitter.resolve_label(else_label)
+                self.compile_block(instr.block2)
+                self.emitter.resolve_label(end_label)
         else:
             self.emitter.resolve_label(else_label)
 

src/sixtypical/emitter.py

@@ -171,6 +171,10 @@ class Emitter(object):
             self.accum.append(thing)
             self.addr += thing.size()
 
+    def retract(self):
+        thing = self.accum.pop()
+        self.addr -= thing.size()
+
     def serialize_to(self, stream):
         """`stream` should be a file opened in binary mode."""
         addr = self.start_addr

tests/SixtyPical Compilation.md

@@ -610,7 +610,6 @@ Compiling `repeat forever`.
     = $080D   LDY #$41
     = $080F   INY
     = $0810   JMP $080F
-    = $0813   RTS
 
 The body of `repeat forever` can be empty.
 
@@ -620,7 +619,6 @@ The body of `repeat forever` can be empty.
     |     } forever
     | }
     = $080D   JMP $080D
-    = $0810   RTS
 
 Compiling `for ... up to`.