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Re: gcc miscompiling duff's device (probaby two different bugs)
On Tue, 2010-03-02 at 11:27 +0100, Richard Guenther wrote:
> On Tue, Mar 2, 2010 at 10:38 AM, Peter Kourzanov
> <peter.kourzanov@gmail.com> wrote:
> >
> > Hi guys,
> >
> > I have the following variation on Duff's device that seems to
> > mis-compile on all GCC versions I can access within a minute (that
> > is gcc-3.{3,4}, gcc-4.{1,2,3,4} on x86 and gcc-4.3.2 on x86_64). The
> > symptoms are as follows:
> >
> > $ gcc-4.4 -o duffbug duffbug.c ; ./duffbug
> > { heïï3)
> > { hello world ! }
> >
> > As you can observe in the difference between duff4_works() and
> > duff4_fails(), apparently due to for-loop initializer being externalized
> > vs. specified as the first for-loop expression. It doesn't matter if the
> > 'case 0' is labeling the for-loop, or the first statement in the
> > for-loop in case of duff4_works() of course. However, older gcc-3.x do
> > give a warning though if the 'case 0' labels the first statement for
> > duff4_fails(), since the first expression in the for-loop is then
> > inaccessible. All gcc-4.x versions don't warn, even when supplied with
> > the -Wall flag (which is wrong, hence this *first* bug):
> >
> > $ gcc-4.4 -Wall -o duffbug duffbug.c ; ./duffbug
> > $ gcc-3.4 -Wall -o duffbug duffbug.c ; ./duffbug
> > duffbug.c: In function `duff4_fails':
> > duffbug.c:28: warning: unreachable code at beginning of switch statement
> >
> > I think the compiler is generating wrong code for duff4_fails() when
> > 'case 0' labels the for-loop. It somehow skips the first for-loop
> > expression, just as if 'case 0' pointed to the first statement in the
> > for-loop (hence this *second* bug). Haven't checked the assembly
> > though...
>
> The routines are not equivalent. in _works you unconditionally
> do dst += n while in _fails you only do it for rem == 0.
That's right. And about missing warning?
>
> Richard.
>
> > Kind regards,
> >
> > Pjotr Kourzanov
> >
>