On Tue, 2010-03-02 at 12:26 +0100, Richard Guenther wrote:
> On Tue, Mar 2, 2010 at 12:00 PM, Pjotr Kourzanov
> <[email protected]> wrote:
> > On Tue, 2010-03-02 at 10:47 +0000, Andrew Haley wrote:
> >> On 03/02/2010 10:34 AM, Pjotr Kourzanov wrote:
> >>
> >> >> int duff4_fails(char * dst,const char * src,const size_t n)
> >> >> {
> >> >> const size_t rem=n % 4, a=rem + (!rem)*4;
> >> >> char * d=dst+=a;
> >> >> const char * s=src+=a;
> >> >> /* gcc bug? dst+=n; */
> >> >>
> >> >> switch (rem) {
> >> >> case 0: for(dst+=n;d<dst;d+=4,s+=4) {
> >> >> /*case 0:*/ d[-4]=s[-4];
> >> >> case 3: d[-3]=s[-3];
> >> >> case 2: d[-2]=s[-2];
> >> >> case 1: d[-1]=s[-1];
> >> >> }
> >> >> }
> >> >> return 0;
> >> >> }
> >> >> The first time around the loop the initializer (d+=n) is jumped around,
> >> >> so
> >> >> d == dst. At the end of the loop, d+=4, so d > dst. Therefore the loop
> >> >> exits.
> >> >
> >> > And its wrong since it shouldn't jump around the initializer.
> >>
> >> Sure it should. On entry to that loop, rem == 3.
> >
> > I agree, this is one of the places where referential transparency
> > breaks in C. I wouldn't have expected that the compiler could or
> > would put the first expression before the switch in this case:
> >
> > switch (rem) {
> > for(dst+=n;d<dst;d+=4,s+=4) {
> > case 0: d[-4]=s[-4]; ...
> > }}
> >
> > However, the warning is still due, since a combination of a switch with a
> > for loop results in code that is completely ignored, i.e., is inaccessible.
> > As I said, gcc-3.x used to issue a warning for this one...
>
> Neither 2.95 nor 3.3.6 or 3.4.6 warn for me.
That's weird. I am having:
gcc-3.3 (GCC) 3.3.6 (Ubuntu 1:3.3.6-15ubuntu6)
Configured with: ../src/configure -v --enable-languages=c,c++
--prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info
--with-gxx-include-dir=/usr/include/c++/3.3 --enable-shared
--enable-__cxa_atexit --with-system-zlib --enable-nls
--without-included-gettext --enable-clocale=gnu --enable-debug
i486-linux-gnu
Thread model: posix
gcc-3.4 (GCC) 3.4.6 (Ubuntu 3.4.6-8ubuntu2)
Configured with: ../src/configure -v --enable-languages=c,f77
--prefix=/usr --libexecdir=/usr/lib
--with-gxx-include-dir=/usr/include/c++/3.4 --enable-shared
--with-system-zlib --enable-nls --without-included-gettext
--program-suffix=-3.4 --enable-__cxa_atexit --with-tune=pentium4
i486-linux-gnu
Thread model: posix
Are you sure you test the following variant?
switch (rem) {
for(dst+=n;d<dst;d+=4,s+=4) {
case 0: d[-4]=s[-4]; ...
}}
>
> Richard.
>
> >>
> >> Andrew.
> >>
> >
> >
> >
>
