From ae54fbf1668959e50632034495631b0c5952ae9d Mon Sep 17 00:00:00 2001
From: Damien George <damien.p.george@gmail.com>
Date: Sat, 22 Apr 2017 14:58:01 +1000
Subject: [PATCH] py/compile: Add COMP_RETURN_IF_EXPR option to enable
return-if-else opt.
With this optimisation enabled the compiler optimises the if-else
expression within a return statement. The optimisation reduces bytecode
size by 2 bytes for each use of such a return-if-else statement. Since
such a statement is not often used, and costs bytes for the code, the
feature is disabled by default.
For example the following code:
def f(x):
return 1 if x else 2
compiles to this bytecode with the optimisation disabled (left column is
bytecode offset in bytes):
00 LOAD_FAST 0
01 POP_JUMP_IF_FALSE 8
04 LOAD_CONST_SMALL_INT 1
05 JUMP 9
08 LOAD_CONST_SMALL_INT 2
09 RETURN_VALUE
and to this bytecode with the optimisation enabled:
00 LOAD_FAST 0
01 POP_JUMP_IF_FALSE 6
04 LOAD_CONST_SMALL_INT 1
05 RETURN_VALUE
06 LOAD_CONST_SMALL_INT 2
07 RETURN_VALUE
So the JUMP to RETURN_VALUE is optimised and replaced by RETURN_VALUE,
saving 2 bytes and making the code a bit faster.
---
py/compile.c | 3 ++-
py/mpconfig.h | 6 ++++++
2 files changed, 8 insertions(+), 1 deletion(-)
diff --git a/py/compile.c b/py/compile.c
index 3aa70e8bf..ae5c3b2dc 100644
--- a/py/compile.c
+++ b/py/compile.c
@@ -980,7 +980,8 @@ STATIC void compile_return_stmt(compiler_t *comp, mp_parse_node_struct_t *pns) {
if (MP_PARSE_NODE_IS_NULL(pns->nodes[0])) {
// no argument to 'return', so return None
EMIT_ARG(load_const_tok, MP_TOKEN_KW_NONE);
- } else if (MP_PARSE_NODE_IS_STRUCT_KIND(pns->nodes[0], PN_test_if_expr)) {
+ } else if (MICROPY_COMP_RETURN_IF_EXPR
+ && MP_PARSE_NODE_IS_STRUCT_KIND(pns->nodes[0], PN_test_if_expr)) {
// special case when returning an if-expression; to match CPython optimisation
mp_parse_node_struct_t *pns_test_if_expr = (mp_parse_node_struct_t*)pns->nodes[0];
mp_parse_node_struct_t *pns_test_if_else = (mp_parse_node_struct_t*)pns_test_if_expr->nodes[1];
diff --git a/py/mpconfig.h b/py/mpconfig.h
index b6e24d205..9122c9916 100644
--- a/py/mpconfig.h
+++ b/py/mpconfig.h
@@ -353,6 +353,12 @@
#define MICROPY_COMP_TRIPLE_TUPLE_ASSIGN (0)
#endif
+// Whether to enable optimisation of: return a if b else c
+// Costs about 80 bytes (Thumb2) and saves 2 bytes of bytecode for each use
+#ifndef MICROPY_COMP_RETURN_IF_EXPR
+#define MICROPY_COMP_RETURN_IF_EXPR (0)
+#endif
+
/*****************************************************************************/
/* Internal debugging stuff */
--
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