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Re: Loop optimization bug with Ada front end on PPC (and probably Alpha)


I've committed the following simplification as execute/20011126-2.c.


r~



/* Problem originally visible on ia64.

   There is a partial redundancy of "in + 1" that makes GCSE want to
   transform the final while loop to 

     p = in + 1;
     tmp = p;
     ...
     goto start;
   top:
     tmp = tmp + 1;
   start:
     in = tmp;
     if (in < p) goto top;

   We miscalculate the number of loop iterations as (p - tmp) = 0
   instead of (p - in) = 1, which results in overflow in the doloop
   optimization.  */

static const char *
test (const char *in, char *out)
{
  while (1)
    {
      if (*in == 'a')
	{
	  const char *p = in + 1;
	  while (*p == 'x')
	    ++p;
	  if (*p == 'b')
	    return p;
	  while (in < p)
	    *out++ = *in++;
	}
    }
}

int main ()
{
  char out[4];
  test ("aab", out);
  return 0;
}


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