From: Jamie Lokier <jamie@shareable.org>
To: gcc@gcc.gnu.org
Subject: Documentation bug for __builtin_choose_expr
Date: Mon, 22 Nov 2004 08:06:00 -0000 [thread overview]
Message-ID: <20041122042652.GA26998@mail.shareable.org> (raw)
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.
But the `? :' operator does not evaluate the expression that was not
chosen either. If the condition evaluates to true, EXP2 is not
evaluated in `COND ? EXP1 : EXP2'.
It would be better to replace that paragraph with:
This built-in function is analogous to the `? :' operator in C,
except that the expression returned has its type unaltered by
promotion rules.
If there is some other effect of __builtin_choose_expr, for example if
it were to inhibit compilation or warnings in the unselected branch in
some way, it would be useful to mention that. I don't know of any
such effects. (Inhibiting warnings in the unselected branch and
promising to not expand large inline functions in that branch would be
a useful addition imho).
-- Jamie
next reply other threads:[~2004-11-22 4:26 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2004-11-22 8:06 Jamie Lokier [this message]
2004-11-22 10:41 ` Joseph S. Myers
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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