Hello,
In brief, I am trying to call into a shared library from python, more specifically, from numpy. The shared library is implemented in C using sse2 instructions. Enabling optimisation, i.e. building the library with -O2 or –O1, I am facing strange segfaults when calling into the shared library via ctypes. Disabling optimisation (-O0), everything works out as expected, as is the case when linking the library to a c-program directly (optimised or not). Attached you find a snipped which exhibits the delineated behaviour on my system. With optimisation enabled, gdb reports a segfault in __builtin_ia32_loadupd (__P) at emmintrin.h:113. The value of __P is reported as optimised out.
test.c:
#include <emmintrin.h>
#include <complex.h>
void test(const int m, const double* x, double complex* y) {
int i;
__m128d _f, _x, _b;
double complex f __attribute__( (aligned(16)) );
double complex b __attribute__( (aligned(16)) );
__m128d* _p;
b = 1;
_b = _mm_loadu_pd( (double *) &b );
_p = (__m128d*) y;
for(i=0; i<m; ++i) {
f = cexp(-I*x[i]);
_f = _mm_loadu_pd( (double *) &f );
_x = _mm_loadu_pd( (double *) &x[i] );
_f = _mm_shuffle_pd(_f, _f, 1);
*_p = _mm_add_pd(*_p, _f);
*_p = _mm_add_pd(*_p, _x);
*_p = _mm_mul_pd(*_p,_b);
_p++;
}
return;
}
Compiler flags:
gcc -o libtest.so -shared -std=c99 -msse2 -fPIC -O2 -g -lm test.c
test.py:
import numpy as np
import os
def zerovec_aligned(nr, dtype=np.float64, boundary=16):
'''Create an aligned array of zeros.
'''
size = nr * np.dtype(dtype).itemsize
tmp = np.zeros(size + boundary, dtype=np.uint8)
address = tmp.__array_interface__['data'][0]
offset = boundary - address % boundary
return tmp[offset:offset + size].view(dtype=dtype)
lib = np.ctypeslib.load_library('libtest', '.' )
lib.test.restype = None
lib.test.argtypes = [np.ctypeslib.ctypes.c_int,
np.ctypeslib.ndpointer(np.float64, flags=('C', 'A') ),
np.ctypeslib.ndpointer(np.complex128, flags=('C', 'A', 'W') )]
n = 13
y = zerovec_aligned(n, dtype=np.complex128)
x = np.ones(n, dtype=np.float64)
# x = zerovec_aligned(n, dtype=np.float64)
# x[:] = 1.
lib.test(n,x,y)
My system:
Ubuntu Linux i686 2.6.31-22-generic
Compiler: gcc (Ubuntu 4.4.1-4ubuntu9)
Python: Python 2.6.4 (r264:75706, Dec 7 2009, 18:45:15) [GCC 4.4.1]
Numpy: 1.4.0
I have taken provisions (cf. python code) that y is aligned and the alignment of x should not matter (I think; explicitly aligning x does not solve the problem though).
Note also that i use _mm_loadu_pd instead of _mm_load_pd when loading b and f. For the C-only version _mm_load_pd works (as expected). However, when calling the function via ctypes using
_mm_load_pd always segfaults (independent of optimisation).
I have tried several days to sort out this issue without success ... and I am on the verge beating my monitor to death. Any input welcome.
Daniel
