Re: Where does the C standard describe overflow of signed integers?

[Georg Bauhaus <bauhaus at futureapps dot de>]

Paul Schlie wrote:
> > From: Robert Dewar <dewar@adacore.com>
> >
> > > Paul Schlie wrote:
> >
> > > I don't contest that it may, I simply don't believe it should.
> >
> > you can't seriously mean that with respect to uninitialized
> > variables. this would mean you could not put local variables in
> > registers. the effect on code quality woul be awful!
>
>
> Why would anyone care about the performance of an access to an
> un-initialized variable? [..] although do
> see substantial value in producing compiled code which is strictly
> consistent with the specified program and native target behavior,
> regardless of its portability.

You can have both, correctness and uninitialised local
variables. For an impression of the difference in performance,
and for a way to ensure correctness, I tried this
(switch register/volatile in the declaration lines in comp
and r to see the effects).

#include <stddef.h>
#include <assert.h>


#define BUFFER_SIZE 1000  // must be > 0
#define ITERATIONS 100000  // must be > 0

static inline int comp(const short, const short, const short);

/* pre: a has elements, that is hi > 0. Frequently called */
int r(short a[], size_t hi)
{
 //register int x, y, z;
 volatile int x=1, y=2, z=3;

 assert(hi > 0);

 for (size_t c=0; c < hi + 2; ++c) {
   if (a[c]) {
   jfssoae:
     x = c + 3, y = z = a[c];
     if (comp(x, y, z)) z = x - y;
   }
 }
 return x + y + z;
}

static inline int comp(const short x, const short y, const short z)
{
 //register int result = (x + y) == (x +  z);
 volatile int result = (x + y) == (x +  z);
 return result;
}

int main()
{
 short buffer[BUFFER_SIZE];
 int result;

 assert(ITERATIONS > 0);

 for (int runs = 0; runs < ITERATIONS; ++runs) {
   result = r(buffer, BUFFER_SIZE);
 }
 return result;
}