Issue #2183: Simplify and optimize bytecode for list comprehensions.

3 files changed, 18 insertions, 33 deletions

diff --git a/Lib/compiler/pycodegen.py b/Lib/compiler/pycodegen.py
index 61b9fe9..5d5dca0 100644
--- a/Lib/compiler/pycodegen.py
+++ b/Lib/compiler/pycodegen.py
@@ -570,16 +570,10 @@ class CodeGenerator:
             self.nextBlock(end)
 
     # list comprehensions
-    __list_count = 0
-
     def visitListComp(self, node):
         self.set_lineno(node)
         # setup list
-        tmpname = "$list%d" % self.__list_count
-        self.__list_count = self.__list_count + 1
         self.emit('BUILD_LIST', 0)
-        self.emit('DUP_TOP')
-        self._implicitNameOp('STORE', tmpname)
 
         stack = []
         for i, for_ in zip(range(len(node.quals)), node.quals):
@@ -591,9 +585,8 @@ class CodeGenerator:
                 self.visit(if_, cont)
             stack.insert(0, (start, cont, anchor))
 
-        self._implicitNameOp('LOAD', tmpname)
         self.visit(node.expr)
-        self.emit('LIST_APPEND')
+        self.emit('LIST_APPEND', len(node.quals) + 1)
 
         for start, cont, anchor in stack:
             if cont:
@@ -604,9 +597,6 @@ class CodeGenerator:
                 self.nextBlock(skip_one)
             self.emit('JUMP_ABSOLUTE', start)
             self.startBlock(anchor)
-        self._implicitNameOp('DELETE', tmpname)
-
-        self.__list_count = self.__list_count - 1
 
     def visitListCompFor(self, node):
         start = self.newBlock()
diff --git a/Lib/opcode.py b/Lib/opcode.py
index cee5057..d41383b 100644
--- a/Lib/opcode.py
+++ b/Lib/opcode.py
@@ -58,7 +58,6 @@ def_op('UNARY_CONVERT', 13)
 
 def_op('UNARY_INVERT', 15)
 
-def_op('LIST_APPEND', 18)
 def_op('BINARY_POWER', 19)
 def_op('BINARY_MULTIPLY', 20)
 def_op('BINARY_DIVIDE', 21)
@@ -128,7 +127,7 @@ name_op('STORE_NAME', 90)       # Index in name list
 name_op('DELETE_NAME', 91)      # ""
 def_op('UNPACK_SEQUENCE', 92)   # Number of tuple items
 jrel_op('FOR_ITER', 93)
-
+def_op('LIST_APPEND', 94)
 name_op('STORE_ATTR', 95)       # Index in name list
 name_op('DELETE_ATTR', 96)      # ""
 name_op('STORE_GLOBAL', 97)     # ""
diff --git a/Lib/test/test_dis.py b/Lib/test/test_dis.py
index fd508a9..6fa437b 100644
--- a/Lib/test/test_dis.py
+++ b/Lib/test/test_dis.py
@@ -54,29 +54,25 @@ def bug1333982(x=[]):
 
 dis_bug1333982 = """\
  %-4d         0 LOAD_CONST               1 (0)
-              3 JUMP_IF_TRUE            41 (to 47)
+              3 JUMP_IF_TRUE            33 (to 39)
               6 POP_TOP
               7 LOAD_GLOBAL              0 (AssertionError)
              10 BUILD_LIST               0
-             13 DUP_TOP
-             14 STORE_FAST               1 (_[1])
-             17 LOAD_FAST                0 (x)
-             20 GET_ITER
-        >>   21 FOR_ITER                13 (to 37)
-             24 STORE_FAST               2 (s)
-             27 LOAD_FAST                1 (_[1])
-             30 LOAD_FAST                2 (s)
-             33 LIST_APPEND
-             34 JUMP_ABSOLUTE           21
-        >>   37 DELETE_FAST              1 (_[1])
-
- %-4d        40 LOAD_CONST               2 (1)
-             43 BINARY_ADD
-             44 RAISE_VARARGS            2
-        >>   47 POP_TOP
-
- %-4d        48 LOAD_CONST               0 (None)
-             51 RETURN_VALUE
+             13 LOAD_FAST                0 (x)
+             16 GET_ITER
+        >>   17 FOR_ITER                12 (to 32)
+             20 STORE_FAST               1 (s)
+             23 LOAD_FAST                1 (s)
+             26 LIST_APPEND              2
+             29 JUMP_ABSOLUTE           17
+
+ %-4d   >>   32 LOAD_CONST               2 (1)
+             35 BINARY_ADD
+             36 RAISE_VARARGS            2
+        >>   39 POP_TOP
+
+ %-4d        40 LOAD_CONST               0 (None)
+             43 RETURN_VALUE
 """%(bug1333982.func_code.co_firstlineno + 1,
      bug1333982.func_code.co_firstlineno + 2,
      bug1333982.func_code.co_firstlineno + 3)