http://bugs.python.org/issue4715

This patch by Antoine Pitrou optimizes the bytecode for conditional branches by
merging the following "POP_TOP" instruction into the conditional jump.  For
example, the list comprehension "[x for x in l if not x]" produced the
following bytecode:

  1           0 BUILD_LIST               0
              3 LOAD_FAST                0 (.0)
        >>    6 FOR_ITER                23 (to 32)
              9 STORE_FAST               1 (x)
             12 LOAD_FAST                1 (x)
             15 JUMP_IF_TRUE            10 (to 28)
             18 POP_TOP
             19 LOAD_FAST                1 (x)
             22 LIST_APPEND              2
             25 JUMP_ABSOLUTE            6
        >>   28 POP_TOP
             29 JUMP_ABSOLUTE            6
        >>   32 RETURN_VALUE

but after the patch it produces the following bytecode:

  1           0 BUILD_LIST               0
              3 LOAD_FAST                0 (.0)
        >>    6 FOR_ITER                18 (to 27)
              9 STORE_FAST               1 (x)
             12 LOAD_FAST                1 (x)
             15 POP_JUMP_IF_TRUE         6
             18 LOAD_FAST                1 (x)
             21 LIST_APPEND              2
             24 JUMP_ABSOLUTE            6
        >>   27 RETURN_VALUE

Notice that not only the code is shorter, but the conditional jump
(POP_JUMP_IF_TRUE) jumps right to the start of the loop instead of going through
the JUMP_ABSOLUTE at the end. "continue" statements are helped
similarly.

Furthermore, the old jump opcodes (JUMP_IF_FALSE, JUMP_IF_TRUE) have been
replaced by two new opcodes:
- JUMP_IF_TRUE_OR_POP, which jumps if true and pops otherwise
- JUMP_IF_FALSE_OR_POP, which jumps if false and pops otherwise

1 files changed, 65 insertions, 15 deletions

diff --git a/Python/ceval.c b/Python/ceval.c
index e0ce92e..d1d6d1d 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -1295,7 +1295,6 @@ PyEval_EvalFrameEx(PyFrameObject *f, int throwflag)
 			SETLOCAL(oparg, v);
 			FAST_DISPATCH();
 
-		PREDICTED(POP_TOP);
 		TARGET(POP_TOP)
 			v = POP();
 			Py_DECREF(v);
@@ -2204,8 +2203,8 @@ PyEval_EvalFrameEx(PyFrameObject *f, int throwflag)
 			Py_DECREF(w);
 			SET_TOP(x);
 			if (x == NULL) break;
-			PREDICT(JUMP_IF_FALSE);
-			PREDICT(JUMP_IF_TRUE);
+			PREDICT(POP_JUMP_IF_FALSE);
+			PREDICT(POP_JUMP_IF_TRUE);
 			DISPATCH();
 
 		TARGET(IMPORT_NAME)
@@ -2282,41 +2281,45 @@ PyEval_EvalFrameEx(PyFrameObject *f, int throwflag)
 			JUMPBY(oparg);
 			FAST_DISPATCH();
 
-		PREDICTED_WITH_ARG(JUMP_IF_FALSE);
-		TARGET(JUMP_IF_FALSE)
-			w = TOP();
+		PREDICTED_WITH_ARG(POP_JUMP_IF_FALSE);
+		TARGET(POP_JUMP_IF_FALSE)
+			w = POP();
 			if (w == Py_True) {
-				PREDICT(POP_TOP);
+				Py_DECREF(w);
 				FAST_DISPATCH();
 			}
 			if (w == Py_False) {
-				JUMPBY(oparg);
+				Py_DECREF(w);
+				JUMPTO(oparg);
 				FAST_DISPATCH();
 			}
 			err = PyObject_IsTrue(w);
+			Py_DECREF(w);
 			if (err > 0)
 				err = 0;
 			else if (err == 0)
-				JUMPBY(oparg);
+				JUMPTO(oparg);
 			else
 				break;
 			DISPATCH();
 
-		PREDICTED_WITH_ARG(JUMP_IF_TRUE);
-		TARGET(JUMP_IF_TRUE)
-			w = TOP();
+		PREDICTED_WITH_ARG(POP_JUMP_IF_TRUE);
+		TARGET(POP_JUMP_IF_TRUE)
+			w = POP();
 			if (w == Py_False) {
-				PREDICT(POP_TOP);
+				Py_DECREF(w);
 				FAST_DISPATCH();
 			}
 			if (w == Py_True) {
-				JUMPBY(oparg);
+				Py_DECREF(w);
+				JUMPTO(oparg);
 				FAST_DISPATCH();
 			}
 			err = PyObject_IsTrue(w);
+			Py_DECREF(w);
 			if (err > 0) {
 				err = 0;
-				JUMPBY(oparg);
+				JUMPTO(oparg);
 			}
 			else if (err == 0)
 				;
@@ -2324,6 +2327,53 @@ PyEval_EvalFrameEx(PyFrameObject *f, int throwflag)
 				break;
 			DISPATCH();
 
+		TARGET(JUMP_IF_FALSE_OR_POP)
+			w = TOP();
+			if (w == Py_True) {
+				STACKADJ(-1);
+				Py_DECREF(w);
+				FAST_DISPATCH();
+			}
+			if (w == Py_False) {
+				JUMPTO(oparg);
+				FAST_DISPATCH();
+			}
+			err = PyObject_IsTrue(w);
+			if (err > 0) {
+				STACKADJ(-1);
+				Py_DECREF(w);
+				err = 0;
+			}
+			else if (err == 0)
+				JUMPTO(oparg);
+			else
+				break;
+			DISPATCH();
+
+		TARGET(JUMP_IF_TRUE_OR_POP)
+			w = TOP();
+			if (w == Py_False) {
+				STACKADJ(-1);
+				Py_DECREF(w);
+				FAST_DISPATCH();
+			}
+			if (w == Py_True) {
+				JUMPTO(oparg);
+				FAST_DISPATCH();
+			}
+			err = PyObject_IsTrue(w);
+			if (err > 0) {
+				err = 0;
+				JUMPTO(oparg);
+			}
+			else if (err == 0) {
+				STACKADJ(-1);
+				Py_DECREF(w);
+			}
+			else
+				break;
+			DISPATCH();
+
 		PREDICTED_WITH_ARG(JUMP_ABSOLUTE);
 		TARGET(JUMP_ABSOLUTE)
 			JUMPTO(oparg);