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Using branch prediction hints (or not)
|
From: |
Ludovic Courtès |
|
Subject: |
Using branch prediction hints (or not) |
|
Date: |
Tue, 04 Dec 2007 21:35:04 +0100 |
|
User-agent: |
Gnus/5.11 (Gnus v5.11) Emacs/22.1 (gnu/linux) |
Hi,
A long time ago, Aubbrey Jaffer wrote this page about his experiments
with branch prediction in SCM:
http://www-swiss.ai.mit.edu/~jaffer/CNS/interpreter-branch.html
I played with GCC's `__builtin_expect' (patch attached) to provide GCC
with branch prediction hints in the most obvious situations:
`SCM_ASSERT'-like macros, `scm_wrong_num_args ()' situations in the
evaluator, and a few others.
It provides a little improvement on my x86 machine, less than 5% for
this program:
(let loop ((x 20000000))
(and (> x 0)
(loop (1- x))))
This made me wonder whether it's even worth it. OTOH, it doesn't hurt.
So, should we apply it?
Thanks,
Ludovic.
--- orig/libguile/__scm.h
+++ mod/libguile/__scm.h
@@ -3,7 +3,7 @@
#ifndef SCM___SCM_H
#define SCM___SCM_H
-/* Copyright (C) 1995,1996,1998,1999,2000,2001,2002,2003, 2006 Free Software
Foundation, Inc.
+/* Copyright (C) 1995,1996,1998,1999,2000,2001,2002,2003, 2006, 2007 Free
Software Foundation, Inc.
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
@@ -109,6 +109,20 @@
#endif
+/* The SCM_EXPECT macros provide branch prediction hints to the compiler. To
+ * use only in places where the result of the expression under "normal"
+ * circumstances is known. */
+#if defined(__GNUC__) && (__GNUC__ >= 3)
+# define SCM_EXPECT __builtin_expect
+#else
+# define SCM_EXPECT(_expr, _value) (_expr)
+#endif
+
+#define SCM_EXPECT_TRUE(_expr) SCM_EXPECT ((_expr), 1)
+#define SCM_EXPECT_FALSE(_expr) SCM_EXPECT ((_expr), 0)
+
+
+
/* {Supported Options}
*
* These may be defined or undefined.
@@ -500,14 +514,14 @@
#define SCM_ASSERT_TYPE(_cond, _arg, _pos, _subr, _msg)
#define SCM_ASRTGO(_cond, _label)
#else
-#define SCM_ASSERT(_cond, _arg, _pos, _subr) \
- do { if (!(_cond)) \
+#define SCM_ASSERT(_cond, _arg, _pos, _subr) \
+ do { if (SCM_EXPECT_FALSE (!(_cond))) \
scm_wrong_type_arg (_subr, _pos, _arg); } while (0)
-#define SCM_ASSERT_TYPE(_cond, _arg, _pos, _subr, _msg) \
- do { if (!(_cond)) \
+#define SCM_ASSERT_TYPE(_cond, _arg, _pos, _subr, _msg)
\
+ do { if (SCM_EXPECT_FALSE (!(_cond))) \
scm_wrong_type_arg_msg(_subr, _pos, _arg, _msg); } while (0)
-#define SCM_ASRTGO(_cond, _label) \
- do { if (!(_cond)) \
+#define SCM_ASRTGO(_cond, _label) \
+ do { if (SCM_EXPECT_FALSE (!(_cond))) \
goto _label; } while (0)
#endif
@@ -526,8 +540,9 @@
return (SCM_UNPACK (gf) \
? scm_call_generic_0 ((gf)) \
: (scm_error_num_args_subr ((subr)), SCM_UNSPECIFIED))
-#define SCM_GASSERT0(cond, gf, subr) \
- if (!(cond)) SCM_WTA_DISPATCH_0((gf), (subr))
+#define SCM_GASSERT0(cond, gf, subr) \
+ if (SCM_EXPECT_FALSE(!(cond))) \
+ SCM_WTA_DISPATCH_0((gf), (subr))
SCM_API SCM scm_call_generic_1 (SCM gf, SCM a1);
@@ -535,8 +550,9 @@
return (SCM_UNPACK (gf) \
? scm_call_generic_1 ((gf), (a1)) \
: (scm_wrong_type_arg ((subr), (pos), (a1)), SCM_UNSPECIFIED))
-#define SCM_GASSERT1(cond, gf, a1, pos, subr) \
- if (!(cond)) SCM_WTA_DISPATCH_1((gf), (a1), (pos), (subr))
+#define SCM_GASSERT1(cond, gf, a1, pos, subr) \
+ if (SCM_EXPECT_FALSE (!(cond))) \
+ SCM_WTA_DISPATCH_1((gf), (a1), (pos), (subr))
SCM_API SCM scm_call_generic_2 (SCM gf, SCM a1, SCM a2);
@@ -546,8 +562,9 @@
: (scm_wrong_type_arg ((subr), (pos), \
(pos) == SCM_ARG1 ? (a1) : (a2)), \
SCM_UNSPECIFIED))
-#define SCM_GASSERT2(cond, gf, a1, a2, pos, subr) \
- if (!(cond)) SCM_WTA_DISPATCH_2((gf), (a1), (a2), (pos), (subr))
+#define SCM_GASSERT2(cond, gf, a1, a2, pos, subr) \
+ if (SCM_EXPECT_FALSE (!(cond))) \
+ SCM_WTA_DISPATCH_2((gf), (a1), (a2), (pos), (subr))
SCM_API SCM scm_apply_generic (SCM gf, SCM args);
@@ -558,8 +575,9 @@
scm_list_ref ((args), \
scm_from_int ((pos) - 1))), \
SCM_UNSPECIFIED))
-#define SCM_GASSERTn(cond, gf, args, pos, subr) \
- if (!(cond)) SCM_WTA_DISPATCH_n((gf), (args), (pos), (subr))
+#define SCM_GASSERTn(cond, gf, args, pos, subr) \
+ if (SCM_EXPECT_FALSE (!(cond))) \
+ SCM_WTA_DISPATCH_n((gf), (args), (pos), (subr))
#ifndef SCM_MAGIC_SNARFER
/* Let these macros pass through if
--- orig/libguile/eval.c
+++ mod/libguile/eval.c
@@ -1,4 +1,4 @@
-/* Copyright (C) 1995,1996,1997,1998,1999,2000,2001,2002,2003,2004,2005,2006
+/* Copyright (C)
1995,1996,1997,1998,1999,2000,2001,2002,2003,2004,2005,2006,2007
* Free Software Foundation, Inc.
*
* This library is free software; you can redistribute it and/or
@@ -299,10 +299,12 @@
/* Shortcut macros to simplify syntax error handling. */
-#define ASSERT_SYNTAX(cond, message, form) \
- { if (!(cond)) syntax_error (message, form, SCM_UNDEFINED); }
-#define ASSERT_SYNTAX_2(cond, message, form, expr) \
- { if (!(cond)) syntax_error (message, form, expr); }
+#define ASSERT_SYNTAX(cond, message, form) \
+ { if (SCM_EXPECT_FALSE (!(cond))) \
+ syntax_error (message, form, SCM_UNDEFINED); }
+#define ASSERT_SYNTAX_2(cond, message, form, expr) \
+ { if (SCM_EXPECT_FALSE (!(cond))) \
+ syntax_error (message, form, expr); }
--- orig/libguile/eval.i.c
+++ mod/libguile/eval.i.c
@@ -681,7 +681,7 @@
#ifdef DEVAL
debug.info->a.args = arg1;
#endif
- if (scm_badargsp (formals, arg1))
+ if (SCM_EXPECT_FALSE (scm_badargsp (formals, arg1)))
scm_wrong_num_args (proc);
ENTER_APPLY;
/* Copy argument list */
@@ -1143,7 +1143,7 @@
case scm_tcs_closures:
{
const SCM formals = SCM_CLOSURE_FORMALS (proc);
- if (scm_is_pair (formals))
+ if (SCM_EXPECT_FALSE (scm_is_pair (formals)))
goto wrongnumargs;
x = SCM_CLOSURE_BODY (proc);
env = SCM_EXTEND_ENV (formals, SCM_EOL, SCM_ENV (proc));
@@ -1187,7 +1187,7 @@
/* must handle macros by here */
x = SCM_CDR (x);
- if (scm_is_pair (x))
+ if (SCM_EXPECT_TRUE (scm_is_pair (x)))
arg1 = EVALCAR (x, env);
else
scm_wrong_num_args (proc);
@@ -1316,7 +1316,7 @@
goto badfun;
}
}
- if (scm_is_pair (x))
+ if (SCM_EXPECT_TRUE (scm_is_pair (x)))
arg2 = EVALCAR (x, env);
else
scm_wrong_num_args (proc);
@@ -1440,7 +1440,7 @@
}
}
}
- if (!scm_is_pair (x))
+ if (SCM_EXPECT_FALSE (!scm_is_pair (x)))
scm_wrong_num_args (proc);
#ifdef DEVAL
debug.info->a.args = scm_cons2 (arg1, arg2,
@@ -1518,7 +1518,7 @@
}
#else /* DEVAL */
case scm_tc7_subr_3:
- if (!scm_is_null (SCM_CDR (x)))
+ if (SCM_EXPECT_FALSE (!scm_is_null (SCM_CDR (x))))
scm_wrong_num_args (proc);
else
RETURN (SCM_SUBRF (proc) (arg1, arg2, EVALCAR (x, env)));
@@ -1709,37 +1709,38 @@
switch (SCM_TYP7 (proc))
{
case scm_tc7_subr_2o:
- if (SCM_UNBNDP (arg1))
+ if (SCM_EXPECT_FALSE (SCM_UNBNDP (arg1)))
scm_wrong_num_args (proc);
if (scm_is_null (args))
args = SCM_UNDEFINED;
else
{
- if (! scm_is_null (SCM_CDR (args)))
+ if (SCM_EXPECT_FALSE (! scm_is_null (SCM_CDR (args))))
scm_wrong_num_args (proc);
args = SCM_CAR (args);
}
RETURN (SCM_SUBRF (proc) (arg1, args));
case scm_tc7_subr_2:
- if (scm_is_null (args) || !scm_is_null (SCM_CDR (args)))
+ if (SCM_EXPECT_FALSE (scm_is_null (args) ||
+ !scm_is_null (SCM_CDR (args))))
scm_wrong_num_args (proc);
args = SCM_CAR (args);
RETURN (SCM_SUBRF (proc) (arg1, args));
case scm_tc7_subr_0:
- if (!SCM_UNBNDP (arg1))
+ if (SCM_EXPECT_FALSE (!SCM_UNBNDP (arg1)))
scm_wrong_num_args (proc);
else
RETURN (SCM_SUBRF (proc) ());
case scm_tc7_subr_1:
- if (SCM_UNBNDP (arg1))
+ if (SCM_EXPECT_FALSE (SCM_UNBNDP (arg1)))
scm_wrong_num_args (proc);
case scm_tc7_subr_1o:
- if (!scm_is_null (args))
+ if (SCM_EXPECT_FALSE (!scm_is_null (args)))
scm_wrong_num_args (proc);
else
RETURN (SCM_SUBRF (proc) (arg1));
case scm_tc7_dsubr:
- if (SCM_UNBNDP (arg1) || !scm_is_null (args))
+ if (SCM_EXPECT_FALSE (SCM_UNBNDP (arg1) || !scm_is_null (args)))
scm_wrong_num_args (proc);
if (SCM_I_INUMP (arg1))
{
@@ -1760,13 +1761,13 @@
SCM_WTA_DISPATCH_1 (*SCM_SUBR_GENERIC (proc), arg1,
SCM_ARG1, scm_i_symbol_chars (SCM_SNAME (proc)));
case scm_tc7_cxr:
- if (SCM_UNBNDP (arg1) || !scm_is_null (args))
+ if (SCM_EXPECT_FALSE (SCM_UNBNDP (arg1) || !scm_is_null (args)))
scm_wrong_num_args (proc);
RETURN (scm_i_chase_pairs (arg1, (scm_t_bits) SCM_SUBRF (proc)));
case scm_tc7_subr_3:
- if (scm_is_null (args)
- || scm_is_null (SCM_CDR (args))
- || !scm_is_null (SCM_CDDR (args)))
+ if (SCM_EXPECT_FALSE (scm_is_null (args)
+ || scm_is_null (SCM_CDR (args))
+ || !scm_is_null (SCM_CDDR (args))))
scm_wrong_num_args (proc);
else
RETURN (SCM_SUBRF (proc) (arg1, SCM_CAR (args), SCM_CADR (args)));
@@ -1777,7 +1778,7 @@
RETURN (SCM_SUBRF (proc) (SCM_UNBNDP (arg1) ? SCM_EOL : scm_cons (arg1,
args)));
#endif
case scm_tc7_lsubr_2:
- if (!scm_is_pair (args))
+ if (SCM_EXPECT_FALSE (!scm_is_pair (args)))
scm_wrong_num_args (proc);
else
RETURN (SCM_SUBRF (proc) (arg1, SCM_CAR (args), SCM_CDR (args)));
@@ -1809,7 +1810,7 @@
#else
arg1 = (SCM_UNBNDP (arg1) ? SCM_EOL : scm_cons (arg1, args));
#endif
- if (scm_badargsp (SCM_CLOSURE_FORMALS (proc), arg1))
+ if (SCM_EXPECT_FALSE (scm_badargsp (SCM_CLOSURE_FORMALS (proc), arg1)))
scm_wrong_num_args (proc);
/* Copy argument list */
--- orig/libguile/numbers.c
+++ mod/libguile/numbers.c
@@ -3950,16 +3950,16 @@
SCM
scm_sum (SCM x, SCM y)
{
- if (SCM_UNBNDP (y))
+ if (SCM_EXPECT_FALSE (SCM_UNBNDP (y)))
{
if (SCM_NUMBERP (x)) return x;
if (SCM_UNBNDP (x)) return SCM_INUM0;
SCM_WTA_DISPATCH_1 (g_sum, x, SCM_ARG1, s_sum);
}
- if (SCM_I_INUMP (x))
+ if (SCM_EXPECT_TRUE (SCM_I_INUMP (x)))
{
- if (SCM_I_INUMP (y))
+ if (SCM_EXPECT_TRUE (SCM_I_INUMP (y)))
{
long xx = SCM_I_INUM (x);
long yy = SCM_I_INUM (y);
@@ -4144,7 +4144,7 @@
SCM
scm_difference (SCM x, SCM y)
{
- if (SCM_UNBNDP (y))
+ if (SCM_EXPECT_FALSE (SCM_UNBNDP (y)))
{
if (SCM_UNBNDP (x))
SCM_WTA_DISPATCH_0 (g_difference, s_difference);
@@ -4173,9 +4173,9 @@
SCM_WTA_DISPATCH_1 (g_difference, x, SCM_ARG1, s_difference);
}
- if (SCM_I_INUMP (x))
+ if (SCM_EXPECT_TRUE (SCM_I_INUMP (x)))
{
- if (SCM_I_INUMP (y))
+ if (SCM_EXPECT_TRUE (SCM_I_INUMP (y)))
{
long int xx = SCM_I_INUM (x);
long int yy = SCM_I_INUM (y);
@@ -4388,7 +4388,7 @@
SCM
scm_product (SCM x, SCM y)
{
- if (SCM_UNBNDP (y))
+ if (SCM_EXPECT_FALSE (SCM_UNBNDP (y)))
{
if (SCM_UNBNDP (x))
return SCM_I_MAKINUM (1L);
@@ -4398,7 +4398,7 @@
SCM_WTA_DISPATCH_1 (g_product, x, SCM_ARG1, s_product);
}
- if (SCM_I_INUMP (x))
+ if (SCM_EXPECT_TRUE (SCM_I_INUMP (x)))
{
long xx;
@@ -4411,7 +4411,7 @@
case 1: return y; break;
}
- if (SCM_I_INUMP (y))
+ if (SCM_EXPECT_TRUE (SCM_I_INUMP (y)))
{
long yy = SCM_I_INUM (y);
long kk = xx * yy;
@@ -4611,7 +4611,7 @@
{
double a;
- if (SCM_UNBNDP (y))
+ if (SCM_EXPECT_FALSE (SCM_UNBNDP (y)))
{
if (SCM_UNBNDP (x))
SCM_WTA_DISPATCH_0 (g_divide, s_divide);
@@ -4671,10 +4671,10 @@
SCM_WTA_DISPATCH_1 (g_divide, x, SCM_ARG1, s_divide);
}
- if (SCM_I_INUMP (x))
+ if (SCM_EXPECT_TRUE (SCM_I_INUMP (x)))
{
long xx = SCM_I_INUM (x);
- if (SCM_I_INUMP (y))
+ if (SCM_EXPECT_TRUE (SCM_I_INUMP (y)))
{
long yy = SCM_I_INUM (y);
if (yy == 0)
--- orig/libguile/validate.h
+++ mod/libguile/validate.h
@@ -3,7 +3,7 @@
#ifndef SCM_VALIDATE_H
#define SCM_VALIDATE_H
-/* Copyright (C) 1999,2000,2001, 2002, 2004, 2006 Free Software Foundation,
Inc.
+/* Copyright (C) 1999,2000,2001, 2002, 2004, 2006, 2007 Free Software
Foundation, Inc.
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
@@ -99,8 +99,10 @@
#define SCM_OUT_OF_RANGE(pos, arg) \
do { scm_out_of_range_pos (FUNC_NAME, arg, scm_from_int (pos)); } while (0)
-#define SCM_ASSERT_RANGE(pos, arg, f) \
- do { if (!(f)) scm_out_of_range_pos (FUNC_NAME, arg, scm_from_int (pos)); }
while (0)
+#define SCM_ASSERT_RANGE(pos, arg, f) \
+ do { if (SCM_EXPECT_FALSE (!(f))) \
+ scm_out_of_range_pos (FUNC_NAME, arg, scm_from_int (pos)); } \
+ while (0)
#define SCM_MUST_MALLOC_TYPE(type) \
((type *) scm_must_malloc (sizeof (type), FUNC_NAME))
- Using branch prediction hints (or not),
Ludovic Courtès <=