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

3 files changed, 10 insertions, 29 deletions

diff --git a/Python/ceval.c b/Python/ceval.c
index bd35185..09501af 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -1294,9 +1294,8 @@ PyEval_EvalFrameEx(PyFrameObject *f, int throwflag)
 
 		case LIST_APPEND:
 			w = POP();
-			v = POP();
+			v = stack_pointer[-oparg];
 			err = PyList_Append(v, w);
-			Py_DECREF(v);
 			Py_DECREF(w);
 			if (err == 0) {
 				PREDICT(JUMP_ABSOLUTE);
diff --git a/Python/compile.c b/Python/compile.c
index 88d54ab..756a903 100644
--- a/Python/compile.c
+++ b/Python/compile.c
@@ -693,7 +693,7 @@ opcode_stack_effect(int opcode, int oparg)
 			return 0;
 
 		case LIST_APPEND:
-			return -2;
+			return -1;
 
 		case BINARY_POWER:
 		case BINARY_MULTIPLY:
@@ -2599,9 +2599,8 @@ compiler_call(struct compiler *c, expr_ty e)
 }
 
 static int
-compiler_listcomp_generator(struct compiler *c, PyObject *tmpname,
-			    asdl_seq *generators, int gen_index, 
-			    expr_ty elt)
+compiler_listcomp_generator(struct compiler *c, asdl_seq *generators,
+			    int gen_index, expr_ty elt)
 {
 	/* generate code for the iterator, then each of the ifs,
 	   and then write to the element */
@@ -2638,16 +2637,13 @@ compiler_listcomp_generator(struct compiler *c, PyObject *tmpname,
 	} 
 
 	if (++gen_index < asdl_seq_LEN(generators))
-	    if (!compiler_listcomp_generator(c, tmpname, 
-					     generators, gen_index, elt))
+	    if (!compiler_listcomp_generator(c, generators, gen_index, elt))
 		return 0;
 
 	/* only append after the last for generator */
 	if (gen_index >= asdl_seq_LEN(generators)) {
-	    if (!compiler_nameop(c, tmpname, Load))
-		return 0;
 	    VISIT(c, expr, elt);
-	    ADDOP(c, LIST_APPEND);
+	    ADDOP_I(c, LIST_APPEND, gen_index+1);
 
 	    compiler_use_next_block(c, skip);
 	}
@@ -2659,10 +2655,6 @@ compiler_listcomp_generator(struct compiler *c, PyObject *tmpname,
 	} 
 	ADDOP_JABS(c, JUMP_ABSOLUTE, start);
 	compiler_use_next_block(c, anchor);
-	/* delete the temporary list name added to locals */
-	if (gen_index == 1)
-	    if (!compiler_nameop(c, tmpname, Del))
-		return 0;
 	
 	return 1;
 }
@@ -2670,21 +2662,10 @@ compiler_listcomp_generator(struct compiler *c, PyObject *tmpname,
 static int
 compiler_listcomp(struct compiler *c, expr_ty e)
 {
-	identifier tmp;
-	int rc = 0;
-	asdl_seq *generators = e->v.ListComp.generators;
-
 	assert(e->kind == ListComp_kind);
-	tmp = compiler_new_tmpname(c);
-	if (!tmp)
-		return 0;
 	ADDOP_I(c, BUILD_LIST, 0);
-	ADDOP(c, DUP_TOP);
-	if (compiler_nameop(c, tmp, Store))
-	    rc = compiler_listcomp_generator(c, tmp, generators, 0, 
-					     e->v.ListComp.elt);
-	Py_DECREF(tmp);
-	return rc;
+	return compiler_listcomp_generator(c, e->v.ListComp.generators, 0,
+					   e->v.ListComp.elt);
 }
 
 static int
diff --git a/Python/import.c b/Python/import.c
index 781bb48..40fc018 100644
--- a/Python/import.c
+++ b/Python/import.c
@@ -73,9 +73,10 @@ extern time_t PyOS_GetLastModificationTime(char *, FILE *);
        Python 2.5c2: 62131 (fix wrong code: for x, in ... in listcomp/genexp)
        Python 2.6a0: 62151 (peephole optimizations and STORE_MAP opcode)
        Python 2.6a1: 62161 (WITH_CLEANUP optimization)
+       Python 2.7a0: 62171 (optimize list comprehensions/change LIST_APPEND)
 .
 */
-#define MAGIC (62161 | ((long)'\r'<<16) | ((long)'\n'<<24))
+#define MAGIC (62171 | ((long)'\r'<<16) | ((long)'\n'<<24))
 
 /* Magic word as global; note that _PyImport_Init() can change the
    value of this global to accommodate for alterations of how the