From: Paul Eggert <eggert@cs.ucla.edu>
To: Dmitry Antipov <dmantipov@yandex.ru>,
Stefan Monnier <monnier@IRO.UMontreal.CA>
Cc: Emacs development discussions <emacs-devel@gnu.org>
Subject: Re: Subtle error defining VALMASK?
Date: Wed, 10 Sep 2014 07:58:29 -0700 [thread overview]
Message-ID: <54106715.4010005@cs.ucla.edu> (raw)
In-Reply-To: <541058E7.2000700@yandex.ru>
Dmitry Antipov wrote:
> In:
>
> #define VALMASK_val (USE_LSB_TAG ? - (1 << GCTYPEBITS) : VAL_MAX)
>
> shouldn't it be
>
> #define VALMASK_val (USE_LSB_TAG ? - (1L << GCTYPEBITS) : VAL_MAX)
>
> or, if --with-wide-int on a 32-bit system:
>
> #define VALMASK_val (USE_LSB_TAG ? - (1LL << GCTYPEBITS) : VAL_MAX)
There's no error here. All three definitions are equivalent because -
(1 << GCTYPEBITS) equals -8, which sign-extends to the width of VAL_MAX.
When it doesn't matter, it's better to avoid length suffixes on
integer constants.
There is another programming style, where authors take care to write
exact suffixes on constants, e.g., 'lseek (fd, 0L, SEEK_SET)' when
lseek's 2nd argument is 'long'. In the old days before function
prototypes this style was often necessary but that need went away long
ago, and nowadays this other style is typically more trouble than it's
worth. For example, the 'lseek' call would be "wrong" now because
lseek's argument type was changed from 'long' to 'off_t', and so a
maintainer would have to change the call to 'lseek (fd, (off_t) 0,
SEEK_SET)', whereas the simpler style 'lseek (fd, 0, SEEK_SET)' would
survive the API change without needing maintenance.
next prev parent reply other threads:[~2014-09-10 14:58 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-09-10 13:57 Subtle error defining VALMASK? Dmitry Antipov
2014-09-10 14:58 ` Paul Eggert [this message]
2014-09-10 15:30 ` Dmitry Antipov
2014-09-10 16:54 ` Paul Eggert
2014-09-10 17:25 ` More issues with r117851 [Was: Re: Subtle error defining VALMASK?] Dmitry Antipov
2014-09-10 20:57 ` Paul Eggert
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