Infinite number of iterations in loop [v850, mep]

["Paulo Matos" <pmatos at broadcom dot com>]

Hi,

I am seeing a strange phenomenon on GCC 4.8 which I can't understand. I tested this with v850 and mep, obtaining the same results (compiling with -O2).
With loop-1.c:
extern int *c;

void fn1 (unsigned int b)
{
  unsigned int a;
  for (a = 0; a < b; a++)
    *c++ = 0;
}


both v850 (requires -mloop and -mv850e3v5) and mep generate a doloop_end pattern.

For loop-2.c:
extern int *c;
extern unsigned int b;
void fn1 (void)
{
  unsigned int a;
  for (a = 0; a < b; a++)
    *c++ = 0;
}

None of v850 nor mep generate doloop_end pattern. doloop complains loop is not simple and may have infinite iterations:
Loop 1 is not simple.
Doloop: Possible infinite iteration case.
Doloop: The loop is not suitable.

I cannot understand GCC's reasoning that the second loop is not simple. The only source code difference is that unsigned int b is extern. However, it will always be higher than 'a' (unsigned int) and the loop can't possibly be infinite.

Does anybody know why GCC is behaving this way?

Paulo Matos