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