From: "Joseph S. Myers" <joseph@codesourcery.com>
To: Jamie Lokier <jamie@shareable.org>
Cc: gcc@gcc.gnu.org
Subject: Re: Documentation bug for __builtin_choose_expr
Date: Mon, 22 Nov 2004 10:41:00 -0000 [thread overview]
Message-ID: <Pine.LNX.4.61.0411221004590.24998@digraph.polyomino.org.uk> (raw)
In-Reply-To: <20041122042652.GA26998@mail.shareable.org>
On Mon, 22 Nov 2004, Jamie Lokier wrote:
> The documentation for __builtin_choose_expr says:
>
> -- Built-in Function: TYPE __builtin_choose_expr (CONST_EXP, EXP1, EXP2)
> You can use the built-in function `__builtin_choose_expr' to
> evaluate code depending on the value of a constant expression.
> This built-in function returns EXP1 if CONST_EXP, which is a
> constant expression that must be able to be determined at compile
> time, is nonzero. Otherwise it returns 0.
>
> This built-in function is analogous to the `? :' operator in C,
> except that the expression returned has its type unaltered by
> promotion rules. Also, the built-in function does not evaluate
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> the expression that was not chosen. For example, if CONST_EXP
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> evaluates to true, EXP2 is not evaluated even if it has
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> side-effects.
> ^^^^^^^^^^^^^
>
> The way this is written implies that the underscored behaviour is
> different from the `? :' operator in C.
I'd think the idea is actually to say that it is different from normal
functions (built-in and otherwise), which inevitably evaluate all their
arguments before calling the function. (Although the syntax is that of a
built-in function, the implementation is that of a new keyword
__builtin_choose_expr handled specially in the parser.) Perhaps it should
say "Unlike most built-in functions, but like the ? : operator, ...".
--
Joseph S. Myers http://www.srcf.ucam.org/~jsm28/gcc/
jsm@polyomino.org.uk (personal mail)
joseph@codesourcery.com (CodeSourcery mail)
jsm28@gcc.gnu.org (Bugzilla assignments and CCs)
next prev parent reply other threads:[~2004-11-22 10:07 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2004-11-22 8:06 Jamie Lokier
2004-11-22 10:41 ` Joseph S. Myers [this message]
2004-11-22 10:42 ` Jamie Lokier
2004-11-22 12:21 ` Andreas Schwab
2004-11-22 14:44 ` Jamie Lokier
2004-11-22 15:02 ` Andreas Schwab
2004-11-22 18:58 ` Robert Dewar
2004-11-22 20:45 ` Kai Henningsen
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