[Bug c/59590] New: gcc produces an infinite loop on O2 optimization

[Bug c/59590] New: gcc produces an infinite loop on O2 optimization

[cottrell at wfu dot edu]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59590

            Bug ID: 59590
           Summary: gcc produces an infinite loop on O2 optimization
           Product: gcc
           Version: 4.8.2
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: cottrell at wfu dot edu

Created attachment 31508
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=31508&action=edit
minimal test case

I'm getting an infinite loop with -O2, though the code is compiled
correctly with just -O.

I'm attaching a minimal test case -- but please see also the real 
function that exposes the problem: the following is the real
counterpart to fake_gradient() in the minimal case:

static int richardson_gradient (double *b, double *g, int n,
                BFGS_CRIT_FUNC func, void *data)
{
    double df[RSTEPS];
    double eps = 1.0e-4;
    double d = 0.0001;
    double v = 2.0;
    double h, p4m;
    double bi0, f1, f2;
    int r = RSTEPS;
    int i, k, m;
    int err = 0;

    for (i=0; i<n; i++) {
    bi0 = b[i];
    h = d * b[i] + eps * (b[i] == 0.0);
    for (k=0; k<r; k++) {
        b[i] = bi0 - h;
        f1 = func(b, data);
        b[i] = bi0 + h;
        f2 = func(b, data);
        if (na(f1) || na(f2)) {
        b[i] = bi0;
        return 1;
        }            
        df[k] = (f2 - f1) / (2.0 * h); 
        h /= v;
    }
    b[i] = bi0;
    p4m = 4.0;
    for (m=0; m<r-1; m++) {
        for (k=0; k<r-m; k++) {
        df[k] = (df[k+1] * p4m - df[k]) / (p4m - 1.0);
        // if (k == r-m-1) break;
        }
        p4m *= 4.0;
    }
    g[i] = df[0];
    }

    return err;
}

[Bug c/59590] gcc produces an infinite loop on O2 optimization

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59590

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
df[k+1] reads past the bounds of df as k is 0...RSTEPS-1 so k+1 is 1...RSTEPS
and the bounds of df is 0...RSTEPS-1.

[Bug c/59590] gcc produces an infinite loop on O2 optimization

[cottrell at wfu dot edu]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59590

--- Comment #2 from Allin Cottrell <cottrell at wfu dot edu> ---
OK, you're right, there's an off-by-one bug in the second
k-loop.

But it's not very nice that gcc takes that as a license to
produce an infinite loop. However, I guess that makes this
report a duplicate of some others that have made the same 
observation.