Module Name: src
Committed By: jruoho
Date: Sat Mar 19 04:59:44 UTC 2011
Modified Files:
src/distrib/sets/lists/comp: mi
src/share/man/man3: Makefile attribute.3
Log Message:
Briefly document __predict_true() and __predict_false().
To generate a diff of this commit:
cvs rdiff -u -r1.1601 -r1.1602 src/distrib/sets/lists/comp/mi
cvs rdiff -u -r1.69 -r1.70 src/share/man/man3/Makefile
cvs rdiff -u -r1.9 -r1.10 src/share/man/man3/attribute.3
Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.
Modified files:
Index: src/distrib/sets/lists/comp/mi
diff -u src/distrib/sets/lists/comp/mi:1.1601 src/distrib/sets/lists/comp/mi:1.1602
--- src/distrib/sets/lists/comp/mi:1.1601 Fri Mar 18 00:57:19 2011
+++ src/distrib/sets/lists/comp/mi Sat Mar 19 04:59:43 2011
@@ -1,4 +1,4 @@
-# $NetBSD: mi,v 1.1601 2011/03/18 00:57:19 joerg Exp $
+# $NetBSD: mi,v 1.1602 2011/03/19 04:59:43 jruoho Exp $
#
# Note: don't delete entries from here - mark them as "obsolete" instead.
#
@@ -4977,6 +4977,8 @@
./usr/share/man/cat3/__insn_barrier.0 comp-c-catman .cat
./usr/share/man/cat3/__noinline.0 comp-c-catman .cat
./usr/share/man/cat3/__packed.0 comp-c-catman .cat
+./usr/share/man/cat3/__predict_true.0 comp-c-catman .cat
+./usr/share/man/cat3/__predict_false.0 comp-c-catman .cat
./usr/share/man/cat3/__pure.0 comp-c-catman .cat
./usr/share/man/cat3/__read_mostly.0 comp-c-catman .cat
./usr/share/man/cat3/__section.0 comp-c-catman .cat
@@ -11108,6 +11110,8 @@
./usr/share/man/html3/__insn_barrier.html comp-c-htmlman html
./usr/share/man/html3/__noinline.html comp-c-htmlman html
./usr/share/man/html3/__packed.html comp-c-htmlman html
+./usr/share/man/html3/__predict_true.html comp-c-htmlman html
+./usr/share/man/html3/__predict_false.html comp-c-htmlman html
./usr/share/man/html3/__pure.html comp-c-htmlman html
./usr/share/man/html3/__read_mostly.html comp-c-htmlman html
./usr/share/man/html3/__section.html comp-c-htmlman html
@@ -17039,6 +17043,8 @@
./usr/share/man/man3/__insn_barrier.3 comp-c-man .man
./usr/share/man/man3/__noinline.3 comp-c-man .man
./usr/share/man/man3/__packed.3 comp-c-man .man
+./usr/share/man/man3/__predict_true.3 comp-c-man .man
+./usr/share/man/man3/__predict_false.3 comp-c-man .man
./usr/share/man/man3/__pure.3 comp-c-man .man
./usr/share/man/man3/__read_mostly.3 comp-c-man .man
./usr/share/man/man3/__section.3 comp-c-man .man
Index: src/share/man/man3/Makefile
diff -u src/share/man/man3/Makefile:1.69 src/share/man/man3/Makefile:1.70
--- src/share/man/man3/Makefile:1.69 Wed Dec 22 19:35:42 2010
+++ src/share/man/man3/Makefile Sat Mar 19 04:59:44 2011
@@ -1,4 +1,4 @@
-# $NetBSD: Makefile,v 1.69 2010/12/22 19:35:42 jruoho Exp $
+# $NetBSD: Makefile,v 1.70 2011/03/19 04:59:44 jruoho Exp $
# @(#)Makefile 8.2 (Berkeley) 12/13/93
MAN= _DIAGASSERT.3 __CONCAT.3 __UNCONST.3 CMSG_DATA.3 \
@@ -29,7 +29,9 @@
attribute.3 __aligned.3 \
attribute.3 __section.3 \
attribute.3 __read_mostly.3 \
- attribute.3 __cacheline_aligned.3
+ attribute.3 __cacheline_aligned.3 \
+ attribute.3 __predict_true.3 \
+ attribute.3 __predict_false.3
MLINKS+=bitstring.3 bit_alloc.3 \
bitstring.3 bit_clear.3 \
bitstring.3 bit_decl.3 \
Index: src/share/man/man3/attribute.3
diff -u src/share/man/man3/attribute.3:1.9 src/share/man/man3/attribute.3:1.10
--- src/share/man/man3/attribute.3:1.9 Sun Dec 19 08:10:09 2010
+++ src/share/man/man3/attribute.3 Sat Mar 19 04:59:44 2011
@@ -1,4 +1,4 @@
-.\" $NetBSD: attribute.3,v 1.9 2010/12/19 08:10:09 jruoho Exp $
+.\" $NetBSD: attribute.3,v 1.10 2011/03/19 04:59:44 jruoho Exp $
.\"
.\" Copyright (c) 2010 The NetBSD Foundation, Inc.
.\" All rights reserved.
@@ -56,6 +56,10 @@
.Ic __read_mostly
.Pp
.Ic __cacheline_aligned
+.Pp
+.Fn __predict_true "exp"
+.Pp
+.Fn __prefict_false "exp"
.Sh DESCRIPTION
The
.Tn GNU
@@ -261,6 +265,39 @@
and
.Ic __read_mostly
are only available for the kernel.
+.It Ic __predict_true
+A branch is generally defined to be a conditional execution of a
+program depending on whether a certain flow control mechanism is altered.
+Typical example would be a
+.Dq if-then-else
+sequence used in high-level languages or
+a jump instruction used in machine-level code.
+A branch prediction would then be defined as an
+attempt to guess whether a conditional branch will be taken.
+.Pp
+The macros
+.Fn __predict_true
+and
+.Fn __predict_false
+annotate the likelihood of whether
+a branch will evaluate to true or false.
+The rationale is to improve instruction pipelining.
+Semantically
+.Ic __predict_true
+expects that the integral expression
+.Fa exp
+equals 1.
+.It Ic __predict_false
+The
+.Ic __predict_false
+expands to an attribute that instructs the compiler
+to predict that a given branch will be likely false.
+As programmers are notoriously bad at predicting
+the likely behavior of their code, profiling and
+empirical evidence should precede the use of
+.Ic __predict_false
+and
+.Ic __predict_true .
.El
.Sh SEE ALSO
.Xr gcc 1 ,
