Date: Mon, 11 Apr 2005 03:55:31 +1000 (EST) From: Bruce Evans <bde@zeta.org.au> To: Chuck Swiger <cswiger@mac.com> Cc: freebsd-current@freebsd.org Subject: Re: smbfs bug introduced at smbfs_vnops.c:1.58 Message-ID: <20050411032601.S55302@delplex.bde.org> In-Reply-To: <42595E04.60705@mac.com> References: <200504100251.j3A2pLEH055107@sana.init-main.com> <1892195662.20050410140423@andric.com><42595E04.60705@mac.com>
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On Sun, 10 Apr 2005, Chuck Swiger wrote:
> Daniel Ellard wrote:
>> On Sun, 10 Apr 2005, Dimitry Andric wrote:
> [ ... ]
>> At least the gcc folk now do detect this old chestnut:
>>
>> {
>> int a;
>>
>> a /= 0;
>> }
>>
>> which was used to provoke arguments in compiler
>> classes for many years. (Optimized, nothing happens.
>> Unoptimized, a division-by-zero error happens...)
>
> Great example.
>
> If the optimized code fails to generate a division-by-zero error here, the
> optimizer is buggy. (I won't quote Aho, Sethi, and Ullman again.... :-)
No, the behaviour is undefined. The compiler can do anything. gcc now
emits a warning even with -O0.
A similar example with "double a;" is more interesting. This also
gives undefined behaviour (C99 6.5.5[#5]). However, this is bogus if
there is a floating point infinity. C99 has support for IEEE floating
point and it is clearly intended that the behaviour of 1.0/0.0 is to
give +Inf and raise the divide-by-zero exception, but I couldn't see
anywhere in the C99 draft n869.txt where this is spelled out (raising of
the divide-by-zero exception is spelled out for lots of math functions
but doesn't seem to be mentioned for plain division).
Also, in C99 with IEEE FP support, "#pragma STDC FENV_ACCESS *" should
affect the behaviour. I'm not sure of the details, but think that
programs can only depend on getting the divide-by-zero exception if
FENV_ACCESS is ON. gcc still doesn't support this pragma, so it does
several wrong things with FENV_ACCESS ON:
- for "a = 1.0 / 0.0;", it evaluates 1.0 / 0.0 at compile time (even
with -O0) so it never raises a divide-by-zero exception.
- "a /= 0.0;" where "a" is not otherwise used is not dead code, since
it should have the side effect of raising the exception, but gcc
considers this code to be dead.
Bruce
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