On Tue, Mar 2, 2010 at 12:00 PM, Pjotr Kourzanov
<peter.kourza...@xs4all.nl> 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.
Richard.
>>
>> Andrew.
>>
>
>
>
