gcc optimization breaks NumPy?
John Fisher
jfisher at are.berkeley.edu
Mon Aug 23 22:27:34 CEST 1999
Hey folks,
I was getting consistent but inexplicable segmentation faults with the
following code, using NumPy's C API to move data between Python and C
(for sparse matrix multiplication; the data structure is from Meschach,
in case that's relevant). I found the problem to be caused by gcc's
optimization options.
static PyObject * mul(PyObject *self, PyObject *args) {
...Snip declarations...
...Snip working code. Function ends with:
return MakeFromMes(product);
}
which is defined as:
PyObject * MakeFromMes(SPMAT *in) {
int i, j, m, n, nzs, count, pos, therow, thecol;
int dummy[0];
double elem;
PyArrayObject *pr, *ir, *jc;
m = in->m; n = in->n; nzs = 0;
/* Count the number of nonzero elements of in
ISZERO is a macro to test for "good enough" floating point 0 */
for(i = 0; i < m; i++) {
for(j = 0; j < n; j++) {
if(!ISZERO(sp_get_val(in, i, j))) nzs++;
}
}
dummy[0] = nzs;
if(!( (pr = PyArray_FromDims(1, dummy, PyArray_DOUBLE)) &&
(ir = PyArray_FromDims(1, dummy, PyArray_INT)) &&
(jc = PyArray_FromDims(1, dummy, PyArray_INT)) )) {
fprintf(stderr, "MakeFromMes: Could not create output arrays.\n");
return Py_BuildValue("O", Py_None);
}
count = pos = 0;
/* Fill column-wise */
for(j = 0; j < n; j++) {
for(i = 0; i < n; i++) {
elem = sp_get_val(in, i, j);
if(!ISZERO(elem)) {
therow = pos % n;
thecol = (int)(pos / n);
*(double *)(pr->data + count * pr->strides[0]) = elem;
*(int *)(ir->data + count * ir->strides[0]) = therow;
*(int *)(jc->data + count * jc->strides[0]) = thecol;
count++;
}
pos++;
}
}
return Py_BuildValue("OOO", pr, ir, jc);
}
The segmentation fault occured immediately after MakeMes() returned.
Were I to change the mul() function to end with
PyObject *out = MakeFromMes(prod);
printf("Hello.");
return out;
}
the error would occur even before execution passed to the printf.
The problem occurs only with an -O option in compilation. Other
specific -f optimization options do not cause the seg fault. So my
question is -- because I've let gcc optimize other extensions to Python
I've written without incident -- what was it here that caused this
behavior? I'd appreciate any ideas and speculations, because I'm still
baffled by this, though the problem seems to be solved.
Thanks,
John
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