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...
>
> Andrew.
>
