-Wstack-usage and alloca in loops

[Dmitry Antipov]

For the following translation unit:

#include <stdlib.h>

int
foo (unsigned n)
{
  int *p;

  if (n < 1024)
    p = alloca (n * sizeof (int));
  else
    p = malloc (n * sizeof (int));

  return g (p, n);
}

int
bar (unsigned n)
{
  int x, i, *p;

  for (x = 0, i = 0; i < n; i++)
    {
      if (n < 1024)
        p = alloca (n * sizeof (int));
      else
        p = malloc (n * sizeof (int));

      x += h (p, n);

      if (n >= 1024)
        free (p);
    }

  return x;
}

compiling with -Wstack-usage=32 produces (as of 4.9.1):

test.c: In function 'foo':
test.c:14:1: warning: stack usage might be unbounded [-Wstack-usage=]
 }
 ^
test.c: In function 'bar':
test.c:35:1: warning: stack usage might be unbounded [-Wstack-usage=]
 }
 ^

1) I'm just curious why it's unbounded for foo().  It shouldn't be too
hard to find that alloca() is never requested to allocate more than
1024 * sizeof (int), and never called more than once, isn't it?

2) In bar(), stack usage is unbounded unless bar() is always inline with
a compile-time constant argument N.

IIUC good detection of 2) is much harder to implement, but is it
reasonable/possible to make -Wstack-usage more accurate in 1)?

Dmitry