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+.\" $NetBSD: __builtin_constant_p.3,v 1.2 2010/12/19 10:08:44 jruoho Exp $
+.\"
+.\" Copyright (c) 2010 Jukka Ruohonen <jruohonen@iki.fi>
+.\" All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
+.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
+.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+.\" PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
+.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+.\" POSSIBILITY OF SUCH DAMAGE.
+.\"
+.Dd December 19, 2010
+.Dt __BUILTIN_CONSTANT_P 3
+.Os
+.Sh NAME
+.Nm __builtin_constant_p
+.Nd GNU extension to determine compile time constants
+.Sh SYNOPSIS
+.Ft int
+.Fn __builtin_constant_p "value"
+.Sh DESCRIPTION
+The
+.Fn __builtin_constant_p
+is a
+.Tn GNU
+extension for determining whether a value
+is known to be constant at compile time.
+The function is closely related to the concept of
+.Dq constant folding
+used by modern optimizing compilers.
+.Pp
+If the
+.Fa value
+is known to be a compile-time constant, a value 1 is returned.
+If
+.Fn __builtin_constant_p
+returns 0, the
+.Fa value
+is not a compile-time constant in the sense that
+.Xr gcc 1
+was unable to determine whether the value is constant or not.
+.Sh EXAMPLES
+A typical example of the use of
+.Fn __builtin_constant_p
+involves a situation where it may be desirable to
+fold the computation if it involves a constant,
+but a function call is needed otherwise.
+For instance,
+.Xr bswap16 3
+is defined in
+.Nx
+as:
+.Bd -literal -offset indent
+#define bswap16(x) \\
+        (__builtin_constant_p((x)) ? \\
+         __byte_swap_u16_constant(x) : __BYTE_SWAP_U16_VARIABLE(x))
+.Ed
+.Sh SEE ALSO
+.Xr gcc 1 ,
+.Xr __builtin_object_size 3 ,
+.Xr __builtin_return_address 3
+.Sh CAVEATS
+This is a non-standard, compiler-specific extension.