I'd say the problem comes from the
int retval = retval;
in isr_tr_complete_low, which is supposedly inlined into the function 
containing err = istr_tr_complete_low (mEp);
At least replacing that with retval = 0; makes the warning go away.

Reduced testcase for -Os -Wall:
struct A { struct A *a, *b; };
struct D { struct A e; };
struct E { unsigned char f; struct { struct A e; } g; };
struct F { struct E i[32]; };
extern int fn0 (void);
extern int fn1 (struct E *, struct D *);
static inline __attribute__ ((always_inline)) int
fn2 (const struct A *x)
{
  return x->a == x;
}
static int
fn3 (struct E *x)
{
  struct D *l, *m;
  int retval = retval;
  if (fn2 (&x->g.e))
    return 0;
  for (l = (struct D *) x->g.e.a, m = (struct D *) l->e.a; &l->e != &x->g.e; l 
= m, m = (struct D *) m->e.a)
    retval = fn1 (x, l);
  return retval;
}
void
fn4 (struct F *x, unsigned k)
{
  unsigned i;
  for (i = 0; i < k; i++)
    {
      struct E *y = &x->i[i];
      int err = -22;
      err = fn3 (y);
      if (y->f == 0)
        {
          if (err > 0)
            err = fn0 ();
          if (err < 0)
            fn0 ();
        }
    }
}

I believe if fn2 returns zero, then the for body will be run at least
once, but perhaps it isn't simplified as such early enough.

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https://bugs.launchpad.net/bugs/1175744

Title:
  [4.8 Regression] wrong -Wmaybe-uninitialized warning with -Os

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