[src/trunk]: src/sys/sys Add __predict_true() and __predict_false() macros to...

[thorpej <thorpej%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/cea7e8ac17fb
branches:  trunk
changeset: 485904:cea7e8ac17fb
user:      thorpej <thorpej%NetBSD.org@localhost>
date:      Mon May 08 18:36:00 2000 +0000

description:
Add __predict_true() and __predict_false() macros to access the
explicit branch prediction available on GCC 2.96 and later.  See
the comment regarding usage.

diffstat:

 sys/sys/cdefs.h |  32 +++++++++++++++++++++++++++++++-
 1 files changed, 31 insertions(+), 1 deletions(-)

diffs (44 lines):

diff -r e9d395d65f6e -r cea7e8ac17fb sys/sys/cdefs.h
--- a/sys/sys/cdefs.h   Mon May 08 18:31:10 2000 +0000
+++ b/sys/sys/cdefs.h   Mon May 08 18:36:00 2000 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: cdefs.h,v 1.32 2000/05/05 00:19:04 thorpej Exp $       */
+/*     $NetBSD: cdefs.h,v 1.33 2000/05/08 18:36:00 thorpej Exp $       */
 
 /*
  * Copyright (c) 1991, 1993
@@ -194,4 +194,34 @@
 #define        __RENAME(x)     no renaming in kernel or standalone environment
 #endif
 
+/*
+ * GNU C version 2.96 adds explicit branch prediction so that
+ * the CPU back-end can hint the processor and also so that
+ * code blocks can be reordered such that the predicted path
+ * sees a more linear flow, thus improving cache behavior, etc.
+ *
+ * The following two macros provide us with a way to utilize this
+ * compiler feature.  Use __predict_true() if you expect the expression
+ * to evaluate to true, and __predict_false() if you expect the
+ * expression to evaluate to false.
+ *
+ * A few notes about usage:
+ *
+ *     * Generally, __predict_false() error condition checks (unless
+ *       you have some _strong_ reason to do otherwise, in which case
+ *       document it), and/or __predict_true() `no-error' condition
+ *       checks, assuming you want to optimize for the no-error case.
+ *
+ *     * Other than that, if you don't know the liklyhood of a test
+ *       succeeding from empirical or other `hard' evidence, don't
+ *       make predictions.
+ */
+#if __GNUC_PREREQ__(2, 96)
+#define        __predict_true(exp)     __builtin_expect(((exp) != 0), 1)
+#define        __predict_false(exp)    __builtin_expect(((exp) != 0), 0)
+#else
+#define        __predict_true(exp)     ((exp) != 0)
+#define        __predict_false(exp)    ((exp) != 0)
+#endif
+
 #endif /* !_SYS_CDEFS_H_ */